Sulfur BacteriaA Camacho, University of Valencia, Burjassot, Spain

ã 2009 Elsevier Inc. All rights reserved.

Introduction

What are Sulfur Bacteria?

Sulfur is an essential component of organic matter. It isincluded in some protein-forming amino acids thathave an important role in configuration of proteins.Sulfur is also present as other biological molecules inreduced forms. Consequently, all living organisms needsulfur as an elemental component, and uptake of sulfurfor use in anabolic processes is therefore inherent to alllife. Sulfur is acquired either in form of sulfate, withfurther reduction to sulfide to form the sulfhydrylgroup, or, inmost heterotrophs, directly from the sulfurcontained in the organic matter that they consume.The average sulfur content of organisms is only

0.2% of dry weight. However, some organisms alsouse sulfur compounds, in much higher amounts thanfor anabolic processes, as a source of energy, reducingpower (electron donor), or as electron acceptor. Sincesulfur can be oxidized or reduced, redox reactionsinvolving exchanges of energy and electrons canoccur in nature. The release of energy and the capa-city of accepting or transferring electrons make sulfuran element suitable for energetic metabolism. Thistype of metabolism, where most of the sulfur is notused to built up biomass, but to obtain energy or toact as mediators of redox reactions, implies the use ofa much larger amount of the element, sulfur, thanneeded simply for assimilation.Sulfur bacteria are defined based on the way that

they use sulfur compounds in their energy conserva-tion metabolic pathways, and there are three mainoptions in their usage of sulfur. First, reduced sulfurcompounds can provide the main electron donor forphotoautotrophic growth. Second, sulfur-oxidationcan give growth energy. Third, oxidized sulfur com-pounds can be used as electron acceptors for anaero-bic respiration of organic matter. These three meansof sulfur use are additional to uptake for anabolicprocesses, and consequently sulfur bacteria are con-sidered as those prokaryotes using the inorganic sul-fur compounds for energetic processes in chemicalreactions that change the oxidation status of sulfur.Nowadays, the classification of the prokaryotes is

mainly based on the analysis of the 16S rRNA genesequence. Increasing chemotaxonomic and phylo-genetic data has led to a redefinition of bacterial tax-onomy. Grouping within the higher taxa, however,should remain more stable than in recent yearsbecause classification based on phylogenetic traits is

currently available for most groups of knownprokaryotes. With this point of view, the taxonomicassignments given in this chapter should be consideredas the current state of knowledge, with likely futurechanges provided in forthcoming versions of Bergey’sManual of Systematic Bacteriology. In additionto Bacteria, some Archaea, a phylogenetic domaindistinct from Bacteria, can also dissimilatorily useinorganic sulfur compounds. These archaea will con-sequently be considered, although briefly, in thischapter.

Dissimilatory Metabolism in Sulfur Bacteria

Sulfur compounds are used in different ways by sulfurbacteria. Because of a variety of metabolisms noted inthese bacteria, concepts regarding these metabolismsshould be clarified. Definition of metabolism is basedon four aspects: (1) the source of energy, (2) the sourceof reducing power, (3) the source of carbon, and(4) the electron acceptor used for respiration. Photo-trophic organisms gain energy from light, whereas inchemotrophic (i.e., chemosynthetic) organisms energyis obtained from chemical substances. The latter canbe differentiated as chemolithotrophic when thesource is an inorganic substance and as chemoorgano-trophic when the energy sources are organic com-pounds. The flow of reducing equivalents (electrons)is commonly associated with the energy conservationprocess. Reducing power and energy (ATP) genera-tion can be jointly considered when referring to theterminology for the use of chemical electron donorsand energy sources. For instance, the aerobic oxidationof elemental sulfur (S0) to sulfate yields energy, part ofwhich can be used as energy source by chemolitho-trophic aerobic sulfur-oxidizing bacteria. Regardingreducing power, inorganic chemical compounds arethe source of electrons (commonly coupled to energygeneration) driving metabolic activities in lithotrophicorganisms, whereas organic compounds act as electrondonors in organotrophic organisms.With respect to thecarbon source, autotrophs use CO2 as source of car-bon, while heterotrophs obtain carbon from organicmatter. Finally, aerobic organisms use oxygen as aterminal electron acceptor for respiration, whereasanaerobic organisms use compounds other thanmolecular oxygen as terminal acceptors, such as sulfatefor sulfate-reducing bacteria. Additionally, some com-pounds can be fermented, meaning that they aredegradedwith an endogenous organic electron acceptor

261

262 Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria

without using an external electron acceptor. Fermenta-tion yields less energy in comparison with that yieldedby a compound oxidized using an external terminalacceptor. The metabolism of an organism can be conse-quently defined regarding these four physiologicalaspects, namely the sources of reducing power, ofenergy, of carbon, and the terminal acceptor for respi-ration. For example, a plant would then be regarded asan aerobic (oxygen used for respiration) photo- (energyfrom light) litho- (reducing power from water) auto-troph (carbon from CO2). A plant is then an aerobicphotolithoautotrophic organism. Most types of combi-nations are found among bacteria, but additionally,many bacteria, including some sulfur bacteria, are met-abolically versatile, allowing them to profit from differ-ent environmental conditions. In this chapter, whenpossible, the different taxa will be defined by metabolictype recognizing these assignments should be consid-ered with precaution, because some taxa could faculta-tively follow other metabolic pathways than thosereported here under certain conditions.In aquatic inland environments, sulfide can come

from abiotic sources, such as sulfide-rich geologicalemanations, or from biotic origins, usually from thereduction of sulfate or from the decomposition ofproteins containing sulfur-rich amino acids. Amongthe biogenic sources, the amount of sulfide producedby anaerobic oxidation of organic matter with sulfateas electron acceptor is much larger (�567 gH2S per kgof organic matter), compared with the much smalleramount of sulfur released by decomposition usingother electron acceptors, where sulfide originate justfrom the sulfhydryl groups of the proteins (�10 gH2S per kg of organic matter). Because sulfate is themost abundant form of sulfur in the biosphere, micro-bial sulfate-reduction is a key process that drives thebiogeochemical sulfur cycle.Sulfide has a considerable impact on the water

and sediment chemistry as well as on the biota ofinland aquatic ecosystems. Although used by severalsulfur bacteria, sulfide is toxic for many organisms,because it readily reacts with cytochromes, haemo-proteins, and other iron-containing compounds.Thus, the presence of sulfide restricts the compositionof biological communities. Beyond certain concentra-tions, this compound is toxic even for sulfur bacteria.Relative tolerance varies among different sulfur-bacterial groups; of these groups, higher tolerance isexhibited by sulfate-reducers. These microorganismscan typically tolerate sulfide up to 10mM, which ismuch higher than that commonly experienced innature. From the chemical point of view, sulfur inter-acts strongly with the cycles of iron and phosphorus.Sulfur can determine the presence of different ironforms and, partly, phosphorus availability because

the formation or dissolution of iron phosphatesdepends on the presence of sulfide, thereby influen-cing eutrophication.

Dissimilatory use of sulfur compounds by prokary-otes follow a diverse set of well known metabolicpathways associated with higher taxonomic units.A diversity of dissimilatory sulfur metabolism is evi-dent evenwithin taxa, as some bacteria havemetabolicflexibility to perform different types of dissimilatorysulfur metabolism depending on environmental condi-tions. For example, some photolithotrophs can func-tion in darkness as sulfur-oxidizing chemolithotrophsunder aerobic conditions. It is therefore inadequate toestablish exclusive definitions of physiological types ofsulfur bacteria. However, to offer an overview, organ-isms are grouped by the ways through which sulfurcompounds are used to obtain reducing power andenergy, or as a respiratory electron acceptor (Figure 1).

Anoxygenic photosynthesis Anoxygenic photosyn-thetic sulfur bacteria use light as energy source for fixinginorganic carbon into organic matter, mainly withreduced sulfur compounds and especially H2S as elec-tron donor. Because water, from which oxygen is pro-duced in the oxygenic photosynthesis, is not used in thiscase as an electron donor, this photosynthetic process isanoxygenic. Instead, sulfide or other reduced sulfurcompounds act as electron donor for photosyntheticcarbon fixation and oxidized sulfur compounds suchas elemental sulfur or sulfate are produced, as follows:

2H2Sþ CO2 ! CH2OþH2Oþ 2S0

H2Sþ 2CO2 þ 2H2O ! 2CH2OþH2SO4

Sulfide-dependent anoxygenic photosynthesis is per-formed by two main groups of sulfur bacteria – thepurple and the green sulfur bacteria. These bacteria,which have photosynthetic pigments allowing light har-vesting, thrive in anaerobic environments where sulfideis available. Additionally, anoxygenic photosynthesiswith sulfide as electron donor has also been demon-strated for some cyanobacterial strains, although mostcyanobacteria commonly perform plant-type oxygenicphotosynthesis.

Chemotrophic sulfur oxidation Chemolithotrophicsulfur prokaryotes use inorganic reduced sulfur com-pounds as electron donors and energy sources fortheir metabolism. Although some photosyntheticsulfur bacteria can also show facultative chemolitho-trophic growth, the so-called colorless sulfur bacteriaare those sulfur-oxidizers not performing anoxygenicphotosynthesis, but instead obtaining energy from inor-ganic sulfur compounds (chemotrophic). The most

Sulfur-oxidizing prokaryotes Sulfate and sulfur reducing prokaryotes

Anoxygenic phototrophic sulfur bacteriaChlorobiaceaeAncalochloris

ChlorobaculumChlorobium

ChloroherpetonClathrochloris

Prosthecochloris

ChromatiaceaeChromatium

AllochromatiumHalochromatiumLamprobacterLamprocystis

MarichromatiumPfennigia

RhabdochromatiumThermochromatium

ThioalkalicoccusThiocapsaThiococcusThiocystis

ThiodictyonThioflavicoccusThiohalocapsaThiolamprovum

ThiopediaThiorhodococcusThiorhodovibirio

Thiospirillum

EctothiorhodospiraceaeEctothiorhodospira

HalorhodospiraThioalkalivibrioThiorhodospira

Sulfate reducing prokaryotesBacteria

DesulfovibrioDesulfoarculusDesulfobaccaDesulfobacter

DesulfobacteriumDesulfobotulusDesulfobulbusDesulfocapsaDesulfocella

DesulfococcusDesulfocellaDesulfofustis

DesulforhabdusDesulfohalobiumDesulfomicrobium

DesulfomonileDesulfonatronovibrio

DesulfonemaDesulforhopalusDesulfosarcina

DesulfosporosinusDesulfotomaculum

Thermodesulfovibrio

Archaea

Archaeoglobus

Bacteria Archaea

Colorless sulfur-oxidizing prokaryotes

ThiobacillusThiosphaeraThermothrixBeggiatoaThiothrixThioploca

ThiodendronThiobacteriumMacromonasAchromatium

ThiospiraThioalkalimicrobium

Thioalkalispira

SulfolobusAcidianus

Sulfur reducing prokaryotesBacteria Archaea

DesulfuromonasDesulfurella

DesulfomicrobiumDethiosulfovibrio

DesulfitobacteriumSulfospirillum

GeobacterPelobacter

DesulfurobacteriumAquifex

SulfolobusThermoproteus

Acidianus

Figure 1 Selected genera of sulfur prokaryotes (Bacteria and Archaea) thriving in inland waters environments. Current generic names

according to Bergey’s Manual of Systematic Bacteriology and to The Prokaryotes (see ‘Further Reading’), except for green sulfurbacteria, which is based on Imhof J (2003) Phylogenetic taxonomy of the family Chlorobiaceae on the basis of 16S rRNA and fmo

(Fenna–Matthews–Olson protein) gene sequences. International Journal of Systematic and Evolutionary Microbiology 53: 941–951.

Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria 263

characteristic reaction performed by chemolithotrophicsulfur-oxidizing bacteria is the aerobic oxidation ofhydrogen sulfide with oxygen as electron acceptor,

H2Sþ 1

2O2 ! S0 þH2O;

although the use of electron acceptors other than oxy-gen (e.g., nitrate) has also been reported. Since underaerobic conditions sulfide is chemically oxidized byoxygen, the coexistence of both oxygen and sulfide innature is restricted to environments where there is acontinuous supply of both substances, either by in situproduction or by external renewal. The elemental sulfurproduced by reducing sulfide can be accumulated insidethe bacterial cell or released from the cell. Elementalsulfur can be further oxidized to sulfate when sulfide isnot available

S0 þ 11

2O2 þH2O ! H2SO4

Sulfate and sulfur reduction Oxidized sulfur com-pounds can be used as terminal electron acceptor forthe anaerobic respiration of organic matter bysulfate- and sulfur-reducing bacteria producinghydrogen sulfide (H2S). These bacteria are commonlyheterotrophic, obtaining organic matter from exter-nal sources, although some strains can be facultativeautotrophs or mixotrophs (use of both organic andinorganic compounds as carbon source). To be usedas an electron acceptor, sulfur must have a certainlevel of oxidation. Among the sulfur compounds,sulfate is not the only one used as electron acceptors,but others such as sulfite, thiosulfate, hyposulfite andelemental sulfur may also be used. Sulfate and sulfur-reducing bacteria obtain energy by coupling the oxi-dation of organic compounds (or H2) to the reductionof sulfate or other sulfur compounds to sulfide, whichis released into the environment:

264 Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria

H2SO4 þ 2CH2O ! 2CO2 þ 2H2OþH2S

The process requires a nonlimiting electron donorand sufficient sulfate, usually in the range of severalmillimoles per liter. This dissimilatory sulfate reduction,which is characteristic of sulfate-reducing prokaryotes,differs from the assimilatory sulfate reduction, awide-spread capacity in prokaryotes and plants, which gen-erates reduced sulfur for biosynthesis without sulfidebeing released into the environment.

Inland Waters Environments Where

Sulfur Bacteria Thrive

The need for sulfur compounds in a certain redoxstatus determines the type of environment in whichvarious types of sulfur bacteria can thrive. Some ofthem have requirements of resources that commonlyhave opposite gradients in nature. For example, oxy-gen and sulfide react chemically. Sulfide and lightsources have opposite gradients in aquatic habitats,as light comes from the surface and sulfide from theanaerobic decomposition of organic matter mainlyoccurring in bottom layers and the sediments. Whenresources are supplied from different directions,microorganisms tend to accumulate at the interfaceoften showing very high metabolic rates enhancingthe chemical gradient. This is the reason why manysulfur bacteria are typically located in zones wherephysical and chemical gradients are steep. Contrast-ingly, other sulfur bacteria such as sulfate-reducersmostly require stable anaerobic conditions.Some signs of the suitability of an environment for

sulfur bacteria can be easily recognized with thenaked eye. The characteristic smell of sulfide indi-cates that this substance is available for sulfur-oxidizers and suggests active sulfate-reduction. Theblack color in the sediments is due to metal sulfidesformed from the biologically released sulfide andavailable metals such as iron. White patches of ele-mental sulfur resulting from the oxidation of sulfidein contact with the air also reflect the suitability of thehabitat, as well as the characteristics of the substratewhen it is rich in gypsum (calcium sulfate). Thesesuitable environments can be recognized in spite ofthe absence of signals of the presence of sulfur bacteria.Sometimes even the presence of sulfur bacteria can beeasily recognized, such as the case of red waters orsediment coloring by purple sulfur bacteria, or thewhitehair-shape structures formedby filamentous color-less sulfur bacteria. In the following sections, the maintypes of environments where sulfur bacteria can befound in inland water habitats will be examined, witha brief description of the features of these environmentsaffecting or being affected by sulfur bacteria.

Chemocline and hypolimnia of stratified lakes Thechem ocline of stra tified lakes (see Video Clip 1) is anenvironment typically occupied by massive develop-ments of photosynthetic sulfur bacteria. The chemicalgradient (so-called chemocline), representing thetransition from the well-oxygenated surface watersto the deep anoxic layers, is usually located at thebottom of the metalimnia or the uppermost hypolim-nia of thermally stratified lakes (Figure 2), and/or inthe top of the monimolimnia of meromictic lakes.At the chemocline, strong physical and chemical gra-dients are established during the stratification period,since the density gradient sustained by differencesin water temperature or salinity impedes turbulentdiffusion of chemical compounds. This density differ-ence promotes the separation of the top well-oxidizedphotic epilimnion from the bottom part of the meta-limnion and the hypolimnion, characterized by rela-tively low light availability and microaerobic oranoxic conditions. Anoxygenic photosynthetic bacte-ria are consequently restricted to a zone in the verticalprofile with a continuous supply of hydrogen sulfide.The relative stability of the thermal stratification inlakes, lasting usually for several months, allows thesebacteria to develop dense populations just below thedepth of oxygen exhaustion, where they can formabundance maxima (plates). Water in these layerscan be purple or green in color because of the bacte-rial photosynthetic pigments (Figure 2). Because ofdifferences in their main ecological requirements,light and sulfide, vertically stratified populations ofpurple sulfur bacteria, are usually located in shal-lower depths relative to green sulfur bacteria.

In lakes, the depth of the oxic–anoxic interfacelargely depends on the input of organic matter tobottom layers and the sediments. The higher theinput the greater the oxygen demand. Dead organicmatter sinks to deep layers and to the sediments,where it is decomposed and mineralized to inorganiccompounds. Sulfide can be derived from the degrada-tion of sulfur contained in this organic matter. Sincethe relative contribution of sulfur to organic matter islow, the amount of sulfide provided in thiswaydependson the amount of organic matter decomposed. Thehigher the trophic status (i.e., productivity) of thelake, the more organic matter sinks to bottom layersand the more sulfide is produced. However, when sul-fate is available in sufficient quantity it can be used asan electron acceptor under anaerobic conditions, and amuch higher amount (around 60 times more) can beproduced from the same amount of organic matter.In this case most of the sulfide is obtained from thereduction of sulfate instead of only from desulfurationof organic matter. This is the case of sulfate-rich lakes,such as brackish coastal lakes, lakes located on gypsum

Conductivity (mS cm−1)2.3

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2

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10.00 2 4 6 8

12 14 16 18 20

2.4 2.5 2.6 2.7 2.8 2.9 3.0Conductivity (mS cm−1)

2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0

Temperature (�C) Temperature (�C)

0 8.0

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10.00.0 0.4 0.8 1.2 1.6 2.0

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m)

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m)

Pigments (mg m−3)

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0.0 0.4 0.8 1.2Abundance (cell ml−1) � 106 Abundance (cell ml−1) � 106

1.6 2.0

Cond.Cond.

Temp.

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O2

H2S

H2S

Dep

th (

m)

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ChromatiumAmoebobacterThiocapsaPelodictyon

Chlorophyll aBacteriochlorophyll a

100 150 200 250 0 50 100 150 200 250

Inorganic C uptake(mg m−3 h−1)

Inorganic C uptake(mg m−3 h−1)

AnoxygenicOxygenicDark fixation

H2S (mM)0 0.5 1 1.5 2

H2S (mM)

Figure 2 Vertical profiles of (Top left) the main physical and chemical key variables; (Top right) photosynthetic (oxygenic and anoxygenic) and chemolithotrophic inorganic carbon

assimilation; (Bottom left) abundance of photosynthetic sulfur bacteria (names based on phenotypic features); and (Bottom right) concentrations of main chloro-pigments, in the sulfate-richLake Arcas (Cuenca, Spain) near the end of the stratification period. For each paired graph, the right graph represents a zoom of the left graph centered two meters around the oxic–anoxic

interface.

Pro

tists,Bacteria

andFungi:Planktonic

andAtta

ched_S

ulfu

rBacteria

265

1 000 000

100 000

10 000

1000

100ind

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w/w

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pigm

ents

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0 2 4 6 8 10 12 14 16 18 20 22 24

OxygenicAnoxygenicDark fixation

Rivularia sp.Diatoms

Pseudanabaena sp.

Planktothrix agardhii

Komvophoron sp.

Chromatium sp.Meters from spring

Spring source

pHEh (mV)

H2S (mM)

O2 (mg L−1) O2 (mg L−1)

O2 (mg L−1)

SO4 (mEq L−1) SO4 (mEq L−1)

SO4 (mEq L−1)

pHEh (mV)

H2S (mM)

(8-18 meters)from source

Average

Average

(0-8 meters)from source

(18-24 meters)from source

17.23.596.9

−1670.68

2.3

48.4

16.21.987.6−270.05

6.7

13.6

Average

Average

16.4

1.078.22910.00

10.84.3

17.83.616.9

−1420.41

2.3

45.1

Temperature (-°C)

Conductivity (mS cm−1)

pHEh (mV)

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Temperature (-°C)

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Temperature (-°C)

Conductivity (mS cm−1)

pHEh (mV)

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Temperature (-°C)

Conductivity (mS cm−1)

Thiothrix sp.

Rivularia sp.DiatomsPseudanabaena sp.

Planktothrix agardhiiKomvophoron sp.Chromatium sp.

Thiothrix sp.

(a) (e)

(c)

(d)

(h)

(f)

(g)

(j)(i)

(k)

(b)

Figure 3 Continued

266 Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria

Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria 267

karst, or saline endorheic lakes from sulfate-rich lands,where sulfate concentrations are high enough to ensureunlimited availability of this electron acceptor for theanaerobic degradation of organic matter. Sulfatereduction in lakes mainly occurs within or in closeproximity to sediments.

The water–sediment interface Another type ofgradient environment where some sulfur bacteriacan develop is the water–sediment interface. Here,oxygen-containing water contacts anaerobic sulfide-releasing sediments. In these environments, colorlesssulfide-oxidizing bacteria can find both substancesneeded for chemolithotrophic growth.

Sediments The sediments of aquatic habitats are asink for organic matter. Because oxygen diffusionthrough the sediment is very low, oxygen is primarilyconsumed by aerobic organisms, thus turning most ofthe sediments anaerobic. Depending on the supply oforganic matter and oxygen exchange with the over-lying waters, the anoxic part could be located up to afew centimeters below the sediment surface. Mostcommonly, oxygen penetrates only the top few milli-meters of the sediments. In anaerobic sediments elec-tron acceptors other than oxygen can be used for theanaerobic remineralization of the organic debris asfor example in sulfate reduction.Microbial mats are multilayered microbial com-

munities that can include layers of sulfur bacteria.In inland waters these communities are frequent inshallow sediments of saline lakes or brackish coastallagoons, and can harbor several types of photosyn-thetic organisms, such as diatoms, cyanobacteria,and/or photosynthetic sulfur bacteria, as well as othersulfur bacteria such as colorless sulfur-oxidizers andsulfate-reducers. These organisms vertically stratifyaccording to the physical and chemical gradients thatare created by their own metabolic activities. Remark-ably, the concentrations of oxygen and sulfide change

Figure 3 Ecological scheme of a sulfuretum (Fuente Podrida, ValencMacro- and microphotographs, track arrows: (a) source and the surro

(c) detail of a red patch, (d) microphotograph of the patch showing th

cyanobacterial and colorless filamentous sulfur-oxidizing bacterial mabacteria with sulfur granules, ( j) filamentous cyanobacterium, (i,k) Ch

filaments per gram of wet weight) and distribution of the dominant m

restricted to the anoxic zones of the sulfide-rich waters located close t

distributed through the zones of coexistence of oxygen and sulfide, Kresistant cyanobacteria, Rivularia is a cyanobacterium whose oxygen

inhibited eukaryotic algae. (Bottom left) Photosynthetic (oxygenic and

carbon assimilation rates in different zones of the sulfuretum, depend

Average values of selected physical and chemical variables for sprinwhere inorganic carbon assimilation wasmeasured. Further informatio

and Hahn M (2005) Spatial dominance and inorganic carbon assimilat

a cold sulfurous spring: The role of differential ecological strategies. M

through the vertical profile during the day, dependingon the balance between productive and consumptiveactivities bymicroorganisms, some of whichmay trackthese changes with diel vertical migrations.

The sulfuretum A sulfuretum is a peculiar type ofenvironment usually associated with a spring sourceof sulfide-richwaters (Figure 3). These waters are com-monly anoxic and contain sulfide, but they are releasedinto an oxygen-rich environment because of the con-tact with the atmosphere. Under these conditions oxy-gen and sulfide coexist, and chemical and biologicalsulfide oxidation reactions can occur, either by aerobicsulfur-oxidizing bacteria when both substances coexistor by photosynthetic sulfur bacteria where light andsulfide are available (see Video Clip 1). Commonly, therelative amount of sulfide and oxygen changes withthe distance to the source, with sulfide-rich anoxicwaters at the source point which are progressivelyenriched in oxygen and impoverished in sulfide awayfrom the source (Figure 3). Some of these environmentshave high temperatures (hot springs) underwhich char-acteristic thermophilic species of sulfur bacteria andarchaea can develop.

Saline and coastal lakes Waters of athalassohaline(inland saline) lakes and coastal lagoons are usuallyrich in sulfate as a result of salt accumulation inendorheic basins or by the influence of sea water,respectively. Active sulfate reduction can occur inthese lakes with sulfide release from sediments tothe water column, which in turn (together with thesurface sediments) can be colonized by sulfur-oxidizing photosynthetic or chemolithotrophic bacte-ria. To balance the outside osmotic pressure in salineenvironments bacteria accumulate osmotically-activemolecules that are compatible with molecular cellstructures and metabolic processes. Microbial mats,as described earlier, are usually also found in theseenvironments, especially in the saline inland lakes and

ia, Spain)(see also Video Clip 1) based on unpublished data. (Top)undings, (b) red patches corresponding to purple sulfur bacteria,

e accumulation of Chromatium sp. cells, (e) Detail of the

ts, (f–h) microphotographs of colorless filamentous gliding sulfurromatium spp. with sulfur granules. (Center) Abundance (cell or

icroorganisms, Chromatium sp. is a purple sulfur bacterium

o the spring, Thiothrix sp. is a colorless sulfur-oxidizing bacterium

omvophoron, Planktothrix, and Pseudanabaena are sulfide-ic photosynthesis is inhibited by sulfide, diatoms are sulfide-

anoxygenic) and chemolithotrophic (dark fixation) inorganic

ing from the distance to the sulfide-rich spring. (Bottom right)

g water and waters in several zones located far from the source,n can be found in Camacho A, Rochera C, Silvestre JJ, Vicente E,

ion by conspicuous autotrophic biofilms in a chemical gradient of

icrobial Ecology 50: 172–184.

268 Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria

alkaline soda lakes located in areas with dry climaticconditions that facilitate gradual salt accumulation indepressions. Saline inland lakes are often hypersaline(i.e., above the salinity of seawater). Soda lakes are aspecific type of salt lake with sodium bicarbonate asdominant salt causing high pH and, among sulfurbacteria, can host mainly alkaliphilic species of pur-ple sulfur bacteria and sulfate-reducers.

Small ponds rich in organic matter In small pondsaccumulating high amounts of organic matter, eitherfrom natural or anthropogenic origin, oxygen candeplete in quiet waters or in parts of the water bodywhere oxygenation is diminished. In these cases, whenenough sulfate is available, sulfate-reducers oxidizeorganic matter, producing hydrogen sulfide, whichin turn is used by photolithotrophic or chemolitho-trophic sulfur-oxidizing bacteria. Typically, these bac-teria form patches in the microenvironments wheretheir requirements are met, such as the lower face ofleafs deposited on the bottom of shallow ponds. Thesekinds of environments include natural habitats, suchas wind-sheltered ponds in deciduous forest areas,as well as human-made environments such as waste-water stabilization ponds.

Acidic aquatic environments Some species of color-less sulfur-oxidizing prokaryotes can live underextremely acidic conditions as low as a pH of 1.In inland waters, these environments are found, forexample, in acid mine drainage waters and in streamscoming from pyrite-rich areas where sulfuric acid isderived from the oxidation of sulfidic minerals.

Ecological Characterization of Inland Water

Sulfur-Using Prokaryotes

Sulfur bacteria are not a phylogenetically-relatedcluster, with taxa spread among most phylogeneticgroups. They also include a wide metabolic diversity.Some taxonomic groups, especially those sharing thesame type of basic metabolism, are often closelyrelated. However, even within metabolic types thereare polyphyletic origins. For example, anoxygenicphotosynthetic bacteria correspond to two differentphylogenetic clusters, with purple bacteria (Chroma-tiaceae) corresponding to the g-proteobacteria andthe green sulfur bacteria forming a separate phylum.Still, the treatment of such metabolic groups is themost appropriate approach to understanding the eco-logical significance of sulfur bacteria, as well as theirphysiological properties that explain their distribu-tion and role in inland aquatic environments. Conse-quently, a functional approach to illustrate the maintypes of sulfur bacteria will be used here.

Photosynthetic sulfur bacteria Photosynthetic sulfurbacteria are characterized by a metabolism basedon the photosynthetic bioconversion of inorganiccarbon into organic matter using reduced sulfur com-pounds, commonly sulfide, as electron donor, and lightas energy source. Light availability, both quantity andquality, and sulfide concentrations are the mainecological factors determining the distribution andgrowth of photosynthetic sulfur bacteria.

Photosynthetic sulfur bacteria can reach very highabundances in aquatic environments where theanoxic sulfide-containing waters overlap the photiczone. The contribution of photosynthetic sulfur bac-teria to primary production in lakes is related tolight availability, as well as to other resources suchas sulfide or inorganic nutrients. Inorganic carbonfixation rates (per volume of water) of photosyntheticsulfur bacteria are often much higher than those fromthe overlying phytoplankton (Table 1), but usuallyonly the uppermost part of the bacterial populationis photosynthetically active (Figure 2). Because of thenarrow depth range where these high photosyntheticrates occur, the contribution of the phototrophic bac-teria to lake primary production is usually modest,although in some lakes, at least during certain peri-ods, they can account for a majority of the planktonicprimary production.

The fate of most carbon fixation by photosyntheticsulfur bacteria is sediment deposition, since predationin the anoxic layers where these bacteria develop isgenerally quite low. However, diel fluctuations in theoxygen-sulfide interface allow some metazooplank-ton that occupy the microaerobic layers, such as somerotifers or microcrustaceans, to occasionally feedon purple sulfur bacteria. Additionally, anaerobicciliates in anoxic layers are also potential grazers onphototrophic bacteria, but the studies reported sofar on the impact of metazooplankton and protistangrazing on phototrophic bacterial populations havedemonstrated that loss rates to grazers are usuallylow. Other organisms, such as predatory bacteria orviruses, have also been reported to impact on photo-trophic sulfur bacteria.

Phototrophic sulfur bacteria are divided into twomain groups, the purple sulfur bacteria and thegreen sulfur bacteria, although some purple andgreen nonsulfur photosynthetic bacteria, which willnot be considered here, can also use H2S as an elec-tron donor as an alternative to organic donors.

Purple sulfur bacteria (PSB) Purple sulfur bacteria(the Chromatiales) are anoxygenic phototrophs thatmainly grow photolithoautotrophically in the lightusing sulfide or elemental sulfur (zero-valent sulfur),among other reduced sulfur compounds, as an electron

Table 1 Inorganic carbon photoassimilation rates of photosynthetic sulfur bacteria in several lakes

Lake % of inorganicC-photoassimilation

Main anoxygenic phototroph(s) Inorganic carbon photoassimilation byphotosynthetic sulfur bacteria

Source

mg Cm�3

h�1mg Cm�3

day�1mgCm�2

day�1gCm�2

year�1

Arcas 12 (d) Chromatium weissei 197 3

Banyoles 14 (d) Chlorobium phaeobacteroides, Chromatium minus 18 18Belovod 9 (d) Chromatium okenii 50–210 55 23

Big Soda 10 (a) Ectothiorhodospira vacuolata 11.4 110–210 50 5

Buchensee 4 (a) Amoebobacter purpureus, Pelodictyon phaeoclathratiforme,

Chloronema spp.

1–7 19

Cadagno 25 (d) Amoebobacter purpureus 51 300 4

Ciso 25 (a) Thiocystis (Chromatium) minus, Amoebobacter sp . 147 55.8 11, 12

92 (a) Chromatium minus 250 18Chernyi Kichier Pelodictyon luteolum 60 14

Dagow Pelochromatium roseum 3.7 13

Deadmoose 17.1 (a) Lamprocystis roseopersicina 6.8 – 63.4 14 16

Faro 45 (d) Chlorobium phaeovibrioides 60 75 24Fayetteville

Green

83 (a) Chlorobium phaeobacteroides 1628 2500 239 7

Fish 1.1 (d) Photosynthetic sulfur bacteria 13.4 21

Haruna 4.5 (a)–20 (d) Chromatium sp. 50 60 3.6 26Holmsjon Chromatium sp. 184 17

Kinneret 1–8 (d) Chlorobium phaeobacteroides 2

Kisaratsu 47–85 (d) Chromatium sp., Chlorobium sp. 154 1000 600 26Knaack 4.7 (a) Pelodictyon sp., Clathrochloris sp. 16.7 22

Konon’er Pelochromatium roseum, Amoebobacter roseus, Thiocapsa sp. 35 14

Kuznechikha 38 (d) Chlorochromatium aggregatum, Chloronema sp. 100 14

Mahoney 40 (d) Amoebobacter purpureus 14–168 33.5 15, 20Maral-Gel Thiocapsa sp., Chlorobium phaeobacteroides 120 14

Mary 0.26 (d) Pelodictyon sp., Clathrochloris sp. Chlorobium sp. 1.98 21

Medicine 43 (a)–55 (d) Lamprocystis roseopersicina 2000 190 110 cited in 1

Mirror 3.8 (d) Lamprocystis sp., Chromatium sp., Pelodictyon sp., Chlorobium sp. 59.8 21Mogil’noe Chlorobuim phaeovibrioides , Pelodictyon phaeum 380 14

Muliczne 9–34 (d) Thiopedia rosea 90 28–157 9

Paul 5.7 (d) Prostecochloris sp., Pelodictyon sp. 16.8 21

Peter 2.9 (d) Photosynthetic sulfur bacteria 10.5 21Plub see 25 (d) Ancalochloris sp., Pelodictyon sp. 35 cited in 1

Popowka Maia Chlorobium limicola 177 8

Pomyaretskoe 45 (d) Chlorobium sp. 90 14Repnoe 15 (d) Chlorobium phaeovibrioides 160 14

Rose 6.3 (d) Pelodictyon sp., Clathrochloris sp., Chlorobium sp. 32.9 21

Continued

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Table 1 Continued

Lake % of inorganicC-photoassimilation

Main anoxygenic phototroph(s) Inorganic carbon photoassimilation byphotosynthetic sulfur bacteria

Source

mg Cm�3

h�1mg Cm�3

day�1mgCm�2

day�1gCm�2

year�1

Smith Hole

(annual

average)

32 (a) Chromatium sp. 91 27

(maximum) 92 (d) 5960Solar 91 (d) Chromatium violascens, Prostecochloris aestuarii, (Oscillatoria

limnetica)

4960 8015 6

Suigetsu 20 (d) Chromatium sp., Chlorobium sp. 65 50 26

Valle de SanJuan

3 (a) Amoebobacter roseus, Pelodictyon phaeum 190 38 14 cited in 1

Vechten 3.9–17.5 (d), 3.6 (a) Chloronema sp., Thiopedia sp., Chromatium sp. 4.1 6 25

Veisovoe 20 (d) Pelodictyon phaeum 350 14Waldsea 46 (a) Chlorobium sp. 1320 32 16

Wadolek 15–62 (d) Chlorobium limicola 20 19–55 8

Zaca 8 (a) Thiopedia rosea 7.2 10

Percentage of contribution of inorganic C-photoassimilation by photosynthetic sulfur bacteria to overall lake primary production calculated on (a) annual or (d) daily basis. Taxonomic assignments and units for

photosynthetic rates are as given in the data sources.

Sources

1. Biebl H and Pfennig N (1979) Anaerobic CO2 uptake by phototrophic bacteria. A review. Archiv fur Hydrobiologie Beiheft. Ergebnisse der Limnologie 12: 48–58.

2. Butow B and Bergstein-Ben Dan T (1992) Occurrence of Rhodopseudomonas palustris and Chlorobium phaeobacteroides blomms in lake Kinneret. Hydrobiologia. 232: 193–200.

3. Camacho A and Vicente E (1998) Carbon photoassimilation by sharply stratified phototrophic communities at the chemocline of Lake Arcas (Spain). FEMS Microbiology Ecology 25: 11–22.

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4. Camacho A, Erez J, Chicote A, Florın M, Squires MM, Lehmann C, and Bachofen R (2001) Microbial microstratification, inorganic carbon photoassimilation and dark carbon fixation at the chemocline of the

meromictic Lake Cadagno (Switzerland) and its relevance to the food web. Aquatic Sciences 63: 91–106.

5. Cloern JE, Cole BE, and Oremland RS (1983) Autotrophic processes in meromictic Big Soda Lake, Nevada. Limnology and Oceanography 28: 1049–1061.

6. Cohen Y, Krumbein WE, and Shilo M (1977) Solar Lake (Sinai). 2. Distribution of photosynthetic microorganisms and primary production. Limnology and Oceanography 22: 609–620.

7. Culver DA and Brunskill GJ (1969) Fayetteville Green Lake, New York. V. Studies of primary production and zooplankton in a meromictic marl lake. Limnology and Oceanography 14: 862–873.

8. Czeczuga B (1968) An attempt to determine the primary production of the green sulphur bacteria, Chlorobium limicola Nads, (Chlorobacteriaceae). Hydrobiologia 31: 317–333.

9. Czeczuga B (1968) Primary production of the purple sulphuric bacteria Thiopedia rosea Winogr. (Thiorhodaceae). Photosynthetica 2: 161–166.

10. Folt CL, Wevers MJ, Yoder-Williams MP, and Howmiller RP (1989) Field study comparing growth and viability of a population of phototrophic bacteria. Applied and Environmental Microbiology 55: 78–85.

11. Garcıa-Cantizano J, Casamayor EO, Gasol JM, Guerrero R, and Pedros-Alio, C (2005) Partitioning of CO2 incorporation among planktonic microbial guilds and estimation of in situ specific growth rates.

Microbial Ecology 50: 230–241.

12. Gasol JM, Mas J, Pedros-Alio C, and Guerrero R (1990) Ecologıa Microbiana y Limnologıa en la Laguna Ciso: 1976–1989. Scientia Gerundensis 16: 155–178.

13. Glaeser J and Overmann J (2003) Characterization and in situ carbon metabolism of phototrophic consortia. Applied and Environmental Microbiology 69: 3739–3750.

14. Gorlenko VM, Dubinina GA, and Kuznetsov SI (1983) The ecology of aquatic microorganisms. Stuttgart: E. Schweizerbart’sche Verlagsbuchhandlung (Nagele U. Obermiller).

15. Hall KJ and Northcote TG (1990) Production and decomposition processes in a saline meromictic lake. Hydrobiologia 197: 115–128.

16. Lawrence JR, Haynes RC, and Hammer UT (1978) Contribution of photosynthetic green sulphur bacteria to total primary production in a meromictic saline lake. Verhandlungen Internationale Vereinung fur

Theoretishe und Angewandte Limnologie 20: 201–207.

17. Lindholm T and Weppling K (1987) Blooms of phototrophic bacteria and phytoplankton in a small brackish lake on Aland, SW Finland. Acta Academiae Aboensis 47: 45–53.

18. Montesinos E and van Gemerden H (1986) The distribution and metabolism of planktonic phototrophic bacteria. In: Megusar F and Gantar M (eds.) Perspectives in microbial ecology, pp. 349–359. Ljubljana:

Slovene Society for Microbiology.

19. Overmann J and Tilzer MM (1989) Control of primary productivity and the significance of photosynthetic bacteria in a meromictic lake, Mittlerer Buchensee, West-Germany. Aquatic Sciences 51: 261–278.

20. Overmann J, Beatty JT, Krouse HR, and Hall KJ (1996) The sulphur cycle of a meromictic salt lake. Limnology and Oceanography 41: 147–156.

21. Parkin TB and Brock TD (1980) Photosynthetic bacterial production in lakes: The effect of light intensity. Limnology and Oceanography 25: 711–718.

22. Parkin TB and Brock TD (1981) Photosynthetic bacterial production and carbon mineralization in a meromictic lake. Archiv fur Hydrobiologie 91: 366–382.

23. Sorokin YI (1970) Interactions between sulphur and carbon turnover in meromictic lakes. Archiv fur Hydrobiologie 66: 391–446.

24. Sorokin YI and Donato N (1975) On the carbon and sulphur metabolism in the meromictic Lake Faro (Sicily). Hydrobiologia 47: 241–252.

25. Steenbergen CLM and Korthals HJ (1982) Distribution of phototrophic microorganisms in the anaerobic and microaerophilic strata of Lake Vechthen (The Netherlands). Pigment analysis and role in primary

production. Limnology and Oceanography 27: 883–895.

26. Takahashi M and Ichimura KS (1968) Vertical distribution and organic matter production of photosynthetic sulfur bacteria in Japanese lakes. Limnology and Oceanography 13: 644–655.

27. Wetzel RG (1973) Productivity investigations of interconnected marl lakes (I). The eight lakes of Oliver and Walter chains, northeastern Indiana. Hydrobiological Studies 3: 91–143.

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272 Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria

donor for photosynthetic carbon fixation throughthe Calvin reductive pentose phosphate cycle. Manyspecies are strictly anaerobic and obligate phototrophs,whereas others also grow chemolithoautotrophicallyor chemoorganoheterotrophically. Purple sulfur bacte-ria (PSB) include two families of g-Proteobacteria – theChromatiaceae and the Ectothiorhodospiraceae. Apartfrom phylogenetical and other chemotaxonomic differ-ences, the main feature differentiating these familiesis that elemental sulfur coming from sulfide oxidationaccumulates inside the cells of Chromatiaceae and out-side the cells of Ectothiorhodospiraceae. PSB have bac-teriochlorophyll a (many also bacteriochlorophyll b)as the main photopigment, and these molecules havestrong absorption in the near infrared (Figure 4). How-ever these wavelengths, as well as others where bacte-riochlorophyll a could harvest photons, are stronglyabsorbed by water or by phytoplankton situated inthe overlying waters (Figure 5). Consequently, light-harvesting strategies based on bacteriochlorophyllare only successful in shallow water bodies mainlyallowing growth at the water–sediment interface. Inmicrobial mats located in shallow environments therelative availability of infrared wavelengths is muchhigher than in the anoxic layers of lakes. In sediments,however, light attenuation limits the habitat of photo-synthetic bacteria to the upper few millimeters ofthe anoxic zone. In contrast, most planktonic sulfurbacteria colonizing deep lake layers must use other

0.25

0.30

0.20

0.15

0.10

0.05

0.00500

Rel

ativ

e ab

sorb

ance

Wavelength (nm)

600

Chl a(680 nm)

BChl d(720 nm)

700

Figure 4 In vivo absorption spectra of selected samples from the mabsorption by chlorophyll a at 680nm in the epilimnetic sample (0.5m

hypolimnetic sample from 12.5m showing the near infrared absorptio

bacteria (photograph a: Lamprocystis purpurea, formerly Amoebobac

and green sulfur bacteria (photograph b: Chlorobium clathratiforme,bacteriochlorophyll d.

light-harvestingmolecules.Thecarotenoidswithabsor-ption maxima at 480–550nm are more efficient forlight-harvesting at the wavelengths dominating at thesedepths (Figure 5). Among these carotenoids, okenone isthe most efficient and is present in meta-hypolimneticspecies, although other carotenoids such as spirillox-anthin, lycopene, rhodopinal, or related molecules areproduced by various purple sulfur bacteria.

The family Chromatiaceae include species fromfreshwater environments (although most toleratemoderate salinities), as well as salt-requiring speciesdistributed in marine or saline inland waters environ-ments. In inlandwaters, these bacteria thrive in anoxicstagnant water bodies and/or sediments, whereenough light arrives to allow phototrophic growth.Some Chromatiaceae are adapted to high temperatureenvironments and cold habitats. The Chromatiaceaecommonly present peculiar cell inclusions or struc-tures, such as sulfur globules, gas vesicles, and storagepolymers of polysaccharides, volutine (polyphos-phate), and poly-b-hydroxybutyrate, which can alsoinfluence cell density. Sulfur accumulation by Chro-matiaceae represents a competitive advantage of thesebacteria over other anoxygenic photosynthetic bacteriathatdeposit elemental sulfuroutside the cells.The intra-cellular sulfur granules servenotonly as electrondonorsfor photosynthesis in the absence of dissolved sulfide,but are also used as electron acceptors for endogenousfermentation of stored carbohydrates under dark

BChl a(830 nm)

0.5 m12.5 m

800

(a)

(b)

eromictic Lake La Cruz (Cuenca, Spain). Note the strong) where only algae are present, compared with the

n maxima of bacteriochlorophylls from purple sulfur

ter purpureus) at 830 nm corresponding to bacteriochlorophyll a,

formerly Pelodictyon clathratiforme) at 720 nm corresponding to

3

2.5

2

1.5

1

0.5

0

400Wavelength (nm)

Irra

dian

ce (

W m

−2nm

−1)

500 600 700

Surface 1X1 m. 2X3 m. 4X7.5 m. 20X11.5 m. 120X15 m. 2000X18.5 m. 20 000X

800

Figure 5 Spectral distribution of light availability at different depths of the meromictic Lake La Cruz (Cuenca, Spain) measured

with a spectroradiometer. Note the magnification level for the scale at each depth.

Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria 273

anoxic conditions. Although some species are obligatephototrophs using sulfide or elemental sulfur as theonly electron donor, others have the capacity for com-plementary growth or maintenance strategies, whichprovides metabolic flexibility. These alternative meta-bolic strategies can serve to cope with the changingconditions at the oxic–anoxic interfaces of the envir-onmentswhere these bacteria develop. SomeChroma-tiaceae, mainly freshwater species, move by polarflagel la (see Video Clip 1), wher eas oth er plankt onicspecies can change their buoyancy by means of cellinclusions or structures.Motility or buoyancy canhelpchange their location for finding suitable conditions.The second phylogenetic group of PSB is the family

Ectothiorhodospiraceae. This family includes usuallyhalophilic and/or alkaliphilic purple sulfur bacteriathat also grow under anaerobic conditions in the lightwith reduced sulfur compounds as photosyntheticelectron donors. Although its main metabolic wayof life is photoautotrophic with deposition of elemen-tal sulfur globules outside the cell, some species canalso grow photoheterotrophically, or under micro-aerobic or aerobic conditions in the dark. Ectothior-hodospiraceae move usually with polar flagella, butonly one species, Ectothiorhodospira vacuolata, isknown to produce gas vesicles. Some species ofEctothiorhodospiraceae require very saline and/oralkaline growth conditions and thrive at high saltconcentrations; this is the case of Halorhodospirahalophila, which is the most halophilic eubacteriumknown, and can grow in salt-saturated solutions.

Among inland waters, the main habitats of Ectothior-hodospiraceae are anoxic waters with sulfide andlight and surface layers of anoxic sediments fromsaline and hypersaline environments with alkalinepH, such as salt and soda lakes. Soda lakes, fromwhere the alkaliphilic members of the genus Thioalk-alivibrio have been isolated, often show pH values of10–11. In saline lakes these bacteria accumulate com-patible solutes, such as glycine betaine, ectoine, tre-halose, and/or sucrose, to cope with the high osmoticpressure.

Green sulfur bacteria (GSB) Green sulfur bacteria(the family Chlorobiaceae) are anoxygenic photo-trophic bacteria that grow only under strictly anoxicconditions. They form a phylogenetically isolatedgroup within the domain Bacteria (Phylum Chlorobi),with freshwater strains clustering separately frommarine strains. They are obligate anaerobic photo-lithotrophs, that use sulfide or elemental sulfur and,in some species, thiosufate, molecular hydrogen orreduced iron, as electron donor for anoxygenic pho-tosynthesis. Some species can also photoassimilate afew organic substances. The electrons from thereduced sulfur compound are used for the assimila-tory reduction of CO2 via the reductive tricarboxylicacid cycle. Oxidation of sulfide results in the forma-tion of sulfur globules, which are deposited outsidethe cell.

Typically, taxa of GSB have been distinguishedaccording to biochemical or morphological features,

274 Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria

such as pigment composition, cell morphology, andthe capacity to form gas vesicles. However, the resultsof recent studies based on the 16S rRNA genesequence analysis indicate that most of these pheno-typic traits are of little phylogenetic significance,although other features such as the salt requirementsfor growth, type of cell fission, filamentous morpho-logy, and gliding motility might be traits of higherphylogenetic relevance.Chlorobiaceae use light-harvesting complexes

called chlorosomes, which are attached to the cyto-plasmic membrane carrying the photosynthetic pig-ments. Green-colored species (Figure 4) containbacteriochlorophyll c or d and the carotenoid chloro-bactene. Brown-colored strains contain bacterio-chlorophyll e and the carotenoids isorenieratene orb-isorenieratene. The type and intracellular concen-tration of pigments is of high ecological relevance,since Chlorobiaceae are distinguished from anyother phototrophic microorganism by strong adapta-tion to low light intensities, allowing them to colonizeanoxic deep waters or sediments with very low lightavailability. For instance, the amount of carotenoids

0.160Pigment

375−378, 590, 796−805, 820−898400, 605, 835−850, 980−1030745−755710−740700−710515−520435, 505459

BChl d1

BChl c1

BChl c2

B

in vivo absorption maxima

bcde

OkenoneIsorenierateneChlorobactene

aBacteriochlorophylls0.150

0.140

0.130

0.120

0.110

0.100

0.090

AU

0.080

0.070

0.060

0.050

0.040

0.030

0.020

0.010

0.0000.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.0

M

Figure 6 HPLC chromatogram of an acetone extract of photosynth

of the meromictic Lake La Cruz (Cuenca, Spain), the same sample a

pigments corresponding to purple sulfur bacteria (bacteriochlorophyll

homologues with different degree of alkylation and carotenoids chlorothe in vivo absorption maxima of these pigments.

in brown-colored strains of green sulfur bacteria canbe several times higher than those of their green-colored species. This is an important feature regard-ing the competition within Chlorobiaceae in naturalenvironments, because of the selective absorption ofcertain wavelengths in surface waters, light availabil-ity in deeper layers may correspond to a spectralrange that is more efficiently harvested by carote-noids (Figures 4 and 5). Other traits of ecologicalsignificance related to the light-harvesting and photo-synthetic efficiencies are the degree of alkylation ofbacteriochlorophylls, which increases as light avail-ability diminishes, leading to a shift of the position ofthe absorption maximum (Figure 6). The size of theantenna light harvesting complexes can be 10 timeslarger for GSB than for PSB. This, together with thehigher quantum yield (mol of C assimilated per molof quanta absorbed) for CO2 fixation that is almostdouble that of PSB, and the lower ATP requirementsper molecule of CO2 fixed by the tricarboxilic acidcycle (used by green sulfur bacteria) compared withthe Calvin cycle (used by purple sulfur bacteria),explains the dominance of GSB over PSB in low

BChl a

BChl d2

Okenone

Chlorophyll a

Chlorobactene

Isorenieratene

Chl d3

BChl d4

0

inutes

50.00 55.00 60.00 65.00 70.00 75.00 80.00 85.00 90.00 95.00

etic pigments from an hypolimnetic sample taken at 12.5m depth

s for the in vivo spectrum shown in Figure 4. Note the different

a and okenone) and to green sulfur bacteria (bacteriochlorophyll d

bactene and isorenieratene). Table on the top of the figure shows

Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria 275

light environments. In green sulfur bacteria, the bac-teriochlorophyll synthesis is strongly regulated bylight intensity, and both the concentrations of pig-ments and chlorosomes can be multiplied underlight-limiting conditions.GSB are found in various inland waters habitats

such as the anoxic hypolimni a of lakes (see VideoClip 1), the bottom layers of microbial mat s, or insulfur springs. Because of their limited physiologicalcapacities compared with purple bacteria, sincegrowth of most green sulfur bacteria depends solelyon anoxygenic photosynthesis, these bacteria developin the narrow zone of overlap between the opposinggradients of light and sulfide. Planktonic lacustrinehabitats are commonly colonized by species capableof forming gas vesicles, which aid buoyancy regula-tion. GSB lack flagellar motility, although glidingmotility has been observed for some benthic species.Sulfur springs, including hot springs, are another habi-tat where GSB occur.In addition to better performance under low light

comparedwith PSB, the affinity of green sulfur bacteriafor sulfide is much higher. Also, sulfide inhibition ofgrowth also occurs atmuch higher concentrations thanfor purple bacteria. These abilities explain either colo-nization of habitats with low but stable sulfide concen-tration or alternatively those with very high sulfideconcentrations by different GSB species. Sulfide con-sumption by GSB thriving underneath populations ofPSB may also limit sulfide diffusion from the sulfide-forming deep layers to the photic zone. This can act as adetoxification mechanism, but also could promote sul-fide limitation to the purple sulfur bacteria located inthe uppermost layers of the hypolimnia. Other interac-tions among PSB and GSB are determined by the selec-tive light absorption by the populations situated inshallower waters, commonly purple bacteria, sincelight penetrating the layers of purple sulfur bacteria ispredominantly violet-blue light coinciding with theshort wavelength absorption maximum of bacterio-chlorophylls c and d and of chlorobactene. As a conse-quence, green-colored forms of Chlorobiaceae, whichhold this pigment assemblage, are often found under-neath Chromatiaceae in stratified lakes, whereasbrown-colored species, with a light-harvesting strategymainly based on carotenoids, thrive in lakes or sedi-ment layers where the selective absorption in shallowerlayers by algae favors penetration of wavelengths thatcan be absorbed by isorenieratene.

Colorless sulfur-oxidizing bacteria Since the firstmicrobiologists, such as Winogradsky and Beijerinck,the name ‘the colorless (pigment lacking) sulfurbacteria’ has been used to designate prokaryotes thatare able to use reduced sulfur compounds as sources of

energy and reducing power for growth (thus chemo-lithotrophic sulfur bacteria). Because of their large size,gliding filamentous colorless sulfur bacteria, such asBeggiatoa and Thiothrix, (see Vi d e o C l i p 1 ) character-ized by the intracellular deposits of elemental sulfur,were first described in the nineteenth century. Colorlesssulfur bacteria comprise a heterogeneous diversityof prokaryotes with few phylogenetic relationshipsamong them, indicating an evolutionary convergence(instead of a divergence from a common ancestor) forthe possession of the relevant metabolic pathwayssupporting growth on reduced sulfur compounds.

The capacity to use reduced sulfur compounds assources of energy and/or reducing power is widelyspread among the different phylogenetic groups ofprokaryotes, including not only taxa from theDomain Bacteria but also the Domain Archaea.Although most colorless sulfur bacteria are Proteo-bacteria (alpha, beta, gamma and epsilon subdivi-sions); some are included in other bacterial highertaxa, and some, such as Sulfolobus, belong to theArchaea (phylum Crenarchaeota). To illustrate theheterogeneous phylogenetic distribution of the color-less sulfur bacteria, according to their 16sRNA genesequence, members classically belonging to the genusThiobacillus are now recognized to be widespreadamong proteobacteria; for instance, the formerT. novellus belonging to the alpha group (a-proteobac-teria), the genus-type species, T. thioparus, is a mem-ber of b-proteobacteria, whereas others, such asT. thiooxidans are members of the g-proteobacteria.Additionally to the commonly recognized as colorlesssulfur-oxidizers, there are several taxa that also havethe capacity for obtaining energy from the oxidation ofsulfur compounds as secondary metabolism (e.g., spe-cies of Pseudomonas, Alcaligenes, etc.).

A wide variety of metabolic possibilities is foundamong the colorless sulfur bacteria, including che-molithoautotrophy, chemolithoheterotrophy, and che-moorganoheterotrophy. Most colorless sulfur bacteriause oxygen as a terminal electron acceptor for respira-tion, and they are consequently aerobic, although someare also capable of growing or surviving anaerobically,for instance with nitrate as an electron acceptor (e.g.,Thiobacillus denitrificans) or by fermentation of intra-cellular stored carbohydrates. Aerobic sulfur-oxidizingbacteria thrive in gradient environments, where bothsulfide and oxygen are available, such as sulfur springsand sulfureta (see Vi d e o C l i p 1 ), which are probably themost characteristic environments for colorless sulfur-oxidizing bacteria such as the filamentous glidinggenera Beggiatoa and Thiothrix (Figure 3). Althoughmost colorless sulfur bacteria are mesophiles, some arethermophiles thriving in geothermal environments(hot springs) with sulfide emanations. Among them,

276 Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria

temperature tolerance is generally higher for theArchaeal taxa compared to Bacteria. Concerning pH,although most are neutrophiles, some are acidophiles,especially someThiobacilli, and the archaea SulfolobusandAcidianus. Interestingly,Thiobacillus ferrooxidanscan gain energy from the oxidation of iron as well asfrom sulfur oxidation. Other sulfur-oxidizing bacteriaare alkaliphiles, such as the g-proteobacteria Thioalk-alimicrobium, thriving in soda lakes.

Sulfate- and sulfur-reducers As for most of the otherfunctional groups of sulfur prokaryotes, sulfate andsulfur-reducing microorganisms can be grouped asfunctional groups which are not necessarily phylo-genetically consistent. Sulfate-reducers use sulfate aselectron acceptor for anaerobic respiration of a widevariety of organic compounds, mostly low molecularweight compounds or H2. Sulfur-reducers usereduced sulfur forms (or other low oxidation sulfurcompounds) as the main electron acceptor for therespiration (oxidation) of organic compounds or H2.Sulfide is the final product of sulfate or sulfur-reducing respiratory processes. Organic compoundsused as electron donors by sulfate-and/or sulfur-reducers include acetate, formate, propionate, buty-rate and other fatty acids, acetaldehyde, ethanol andother alcohols, malate, fumarate, succinate, aminoa-cids, methylated N- and S-compounds, and polararomatic substances. These organic compounds,many originating from fermentation performed byother bacteria, serve as electron donors for dissimila-tory sulfate-reduction, but also as carbon sources forgrowth. In the latter case, sulfate-reducing bacteriaact as terminal degraders of organic matter in anoxicenvironments, completing the decomposition processstarted by other organisms. Additional electronacceptors that can also be used are, for example,sulfite and thiosulfate.Among the domain Bacteria, most sulfate-reducers

belong to the d-proteobacteria, for example the typegenus Desulfovibrio. Other taxa belong to othergroups, such as the spore-forming Desulfotomacu-lum, branched with the Gram-positive bacteria, oreven other thermophilic genera recognized as deeplybranching lines within the domain Bacteria. Thedomain Archaea also includes thermophilic sulfate-reducers (e.g.,Archaeoglobus spp., phylum Euryarch-aeota) found in hot springs. Most sulfur-reducers alsobelong to the d-proteobacteria branching withsulfate-reducers (e.g., Desulfuromonas) althoughsome sulfur-reducers belong to the e-proteobacteriaor to Archaea (Desulfurolobus and Acidianus). Sul-fate and sulfur reducers group of prokaryotesincludes mostly mesophilic species but also archaeanthermophilic taxa and thermophilic bacteria, such as

the sulfate-reducer Thermodesulfobacterium and thesulfur-reducer Desulfurella. Some psychrophilicstrains have also been described.

It is commonly accepted that most sulfate-reducersare strict anaerobes. Some species, however, canthrive under oxic conditions for a certain time,whereas sulfur-reducing bacteria include both strictand facultative anaerobes. When considering expo-sure to oxygen, the existence of anoxic micronicheswithin the oxic environments harboring strict anae-robes has been argued to explain active sulfate-reduction in oxic environments. Alternatively, anintermediate metabolism of sulfur compounds avoid-ing the harmful effects of oxygen has also beendescribed for sulfate-reducers found in oxic environ-ments. However, there is also evidence that somesulfate-reducers are able to use oxygen as electronacceptors for the oxidation of hydrogen under micro-aerobic conditions.

In inland waters environments, the main habi-tats for sulfate and sulfur-reducing bacteria are thesediments of sulfate-rich water bodies. These sedi-ments are commonly anaerobic just below a fewmillimeters under the water–sediment interface. Inthe absence of oxygen and nitrate, sulfate yieldsenergy as an electron acceptor, and organic mattercan be remineralized mainly via sulfate-reduction,allowing for the release of sulfide that diffuses to thewater column. From the point of view of free energychanges involved in the respiratory energy generationprocesses, the energetic yield of anaerobic respirationof organic compounds with sulfate as an electronacceptor is much lower (around 5–10%) than thatof the aerobic respiration (oxygen as an electronacceptor). Around 90% of the organic substrate isdevoted to energy generation in sulfate reductionand only the remaining being used for biomass gener-ation, thus yielding low growth rates. However,energy and growth yields for sulfate-reduction arehigher than for other types of anaerobic respirationof organic matter, such as the methanogenesis. Thisexplains why, under anaerobic conditions in sulfate-rich aquatic environments, sulfate-reduction is a maindegradative process for organic compounds.

In sedimentary microbial communities such as themicrobial mats, sulfate-reducers are abundant espe-cially in deep layers below the phototrophic microor-ganisms, providing sulfide to the overlying populationsof sulfur-oxidizing bacteria (phototrophic and/orchemotrophic). Interestingly, sulfide diffuses upwards,and consumption by sulfur-oxidizers determines dielchanges in the concentration of sulfide to which theoverlying microorganisms are exposed. Sulfate andsulfur-reducing bacteria can also be commonly foundin other inlandwaters environments, such as the anoxic

Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria 277

hypolimnia of stratified lakes as well as flooded soilsuch as rice paddies.

Mutualistic Interactions among Sulfur Bacteria

Since sulfur bacteria, depending on the type, use oxi-dized or reduced sulfur compounds, it is obvious thatthey are indirectly related in nature by the biogeo-chemical transformation of these compounds. How-ever, direct interactions including physical contact orclose trophic relationships can also occur among sul-fur bacteria. An example are photosynthetic consor-tia, which are stable structural associations of greensulfur bacteria surrounding motile chemotrophicbacteria (thought to be sulfate or sulfur-reducers)situated in the central part of the consortium, whichare supposed to be capable of syntrophic growthbased on the exchange of inorganic sulfur and organiccompounds. This association represents the mostevolved symbiosis described so far between prokar-yotes. The bacterial partners differ greatly from theirfree-living counterparts. The green bacteria are phy-logenetically different from all known green sulfurbacteria whereas the central bacteria in the consortiastudied so far does not belong to the d-proteobacteriaas do the classical sulfate-reducers, but to b-proteo-bacteria. These consortia are commonly found in theplanktonic anaerobic environments of freshwaterhabitats. Flagellar motility provided by the centralcell may also be of ecological relevance allowing thephototrophic non-motile partners to reach well illu-minated water layers.

Applied Issues

Some sulfur bacteria are of applied interest. Purplesulfur bacteria and colorless sulfur-oxidizers com-monly appear in mixed microbial communities insewage treatment processes, and could also beused for sulfide removal. Sulfur bacteria can alsobe used for production of biopolymers such as poly-b-hydroxybutyrate, and of molecular hydrogen.Because of their high tolerance, Ectothiorhodospira-ceae can be used under alkaline and saline conditionsfor some of these purposes. In some cases, such asin the bulking processes of activated sludge in waste-water treatments plant, bacteria such as Thiothrixcause problems in the industrial process. In miningactivities acidophilic sulfur bacteria, such as Thioba-cillus ferrooxidans, T. thiooxidans, and T. acidophi-lus, are used in the recovery of metals by leachingfrom ores that are too poor for conventionalmetallurgical extraction. By this process, recoveriesof up to 70% of copper from low-grade ores are

possible. Some sulfate-reducers, such as Desulfovi-brio desulfuricans, are capable of reducing uraniumand can be used for the concentration and removal ofradioactive uranium.

See also: Algae; Archaea; Bacteria, Bacterioplankton;Bacteria, Distribution and Community Structure; TheBenthic Boundary Layer (in Rivers, Lakes andReservoirs); Biodiversity of Aquatic Ecosystems;Biological-Physical Interactions; Chemical Properties ofWater; Chemosynthesis; Comparative PrimaryProduction; Competition Among Aquatic Organisms;Competition and Predation; Currents in Stratified WaterBodies 2: Internal Waves; Cyanobacteria; DensityStratification and Stability; Diel Vertical Migration;Dissolved CO2; Ecological Zonation in Lakes; Iron andManganese; Lakes as Ecosystems; Light, BiologicalReceptors; Meromictic Lakes; Microbial Food Webs;Physical Properties of Water; Phytoplankton PopulationDynamics: Concepts and Performance Measurement;Phytoplankton Productivity; Protists; Redox Potential;Regulators of Biotic Processes in Stream and RiverEcosystems; Role of Zooplankton in Aquatic Ecosystems;Saline Inland Waters; Sediments of Aquatic Ecosystems;Small-scale Turbulence and Mixing: Energy Fluxes inStratified Lakes; Trophic Dynamics in Aquatic Ecosystems.

Further Reading

Abelson JN, Simon MI, Peck HD, and LeGall J (eds.) (1994)Methods in Enzymology, vol. 243: Inorganic Microbial SulfurMetabolism. New York: Academic Press.

Barton LL (ed.) (1995) Sulfate-Reducing Bacteria. New York:

Springer.Blankenship RE, MadiganMT, and Bauer CE (eds.) (1995) Anoxy-

genic Photosynthetic Bacteria. Dordrecht, The Netherlands:

Kluwer.

Boone DR and Castenholz RW (eds.) (2001) The Archaea and thedeeply branching and phototrophic bacteria. In: Garrity GM

(editor-in chief) Bergey’s Manual of Systematic Bacteriology,2nd edn., vol. 1. New York: Springer-Verlag.

Brenner DJ, Krieg NR, and Staley JT (eds.) (2005) The Proteobac-

teria. In: Garrity GM (editor-in chief), Bergey’s Manual of Sys-tematic Bacteriology, 2nd edn., vol. 2. New York: Springer-

Verlag.Drews G and Imhoff JF (1991) Phototrophic purple bacteria. In:

Shively JM and Barton LL (eds.) Variations in Autotrophic Life,pp. 51–97. London: Academic Press.

DworkinM, Falkow S, Rosenberg E, Schleifer KH, and StackebrandtE (eds.) (2006) The Prokaryotes: A Handbook on the biology ofBacteria, 3rd edn., 7 vols. New York: Springer-Verlag.

Glaeser J and Overmann J (2004) Biogeography, evolution, anddiversity of epibionts in phototrophic consortia. Applied andEnvironmental Microbiology 70: 4821–4830.

HolmerM and Storkholm P (2001) Sulphate reduction and sulphur

cycling in lake sediments: A review. Freshwater Biology 46:431–451.

MadiganM andMartinko J (2005) Brock: Biology of Microorgan-isms. Upper Saddle River, NJ: Pearson Education.

278 Protists, Bacteria and Fungi: Planktonic and Attached _ Sulfur Bacteria

Overmann J (1997) Mahoney Lake: A case study of the ecological

significance of phototrophic sulfur bacteria. In: Gwynfryn Jones J(ed.) Advances in Microbial Ecology, vol. 15, pp. 251–288.

New York: Plenum Press.

Overman J and van Gemerden H (2000) Microbial interactions

involving sulfur bacteria: implications for the ecology and evolu-tion of bacterial communities. FEMS Microbiology Reviews 24:591–599.

Peschek GA, Loffelhardt W, and Schmetterer G (1999) The Photo-trophic Prokaryotes. New York: Kluwer/Plenum.

Stal LJ and Caumette P (1994) Microbial Mats: Structure, Devel-opment and Environmental Significance. NATO ASI Series,vol. 35. Berlin: Springer.

van Gemerden H and Mas J (1995) Ecology of phototrophic sulfurbacteria. In: Blankenship RE, Madigan MT, and Bauer CE (eds.)

Anoxygenic Photosynthetic Bacteria, pp. 49–85. Dordrecht, The

Netherlands: Kluwer.

Relevant Websites

http://microbes.arc.nasa.gov – NASA microbiology website.

http://commtechlab.msu.edu/sites/dlc-me/ – Digital Learning Cen-ter for Microbial Ecology (DLC-ME), Michigan State University.

http://microbewiki.kenyon.edu/index.php/MicrobeWiki – Wiki

resource on microbes and microbiology at Kenyon College.http://www.bacterio.cict.fr/foreword.html – List of Prokaryotic

names with standing in nomenclature.

http://bip.cnrs-mrs.fr/bip09/index.html – Evolution of prokaryotic

electron transport chains.http://www.photosynthesisresearch.org/ – International Society of

Photosynthesis Research.

http://serc.carleton.edu/microbelife/index.html – Microbial Life –

Educational Resources. Science Education Resource Center Car-leton College, Northfield, MN.

http://www.pol-us.net/ – Photobiology online. Sponsored by Euro-

pean and American Societies for Photobiology.

http://www.sciencedirect.com/ – Science Direct, Elsevier website onresearch resources.

http://www.microbionet.com.au/ – Microbio Net. Sciencenet Mul-

timedia Publishing House Pty Limited.http://www.dsmz.de/ – German Collection of Microorganisms and

Cell Cultures.

http://www.cells.de/cellsger/1medienarchiv/Zellfunktionen/Memb_

Vorg/Photosynthese/Anoxigene_PST/index.jsp – Videos and othereducational resources on microbiology hosted by the German

Federal Ministry of Education and Research.