encyclopedia of inland waters || bacteria, bacterioplankton

Bacteria, BacterioplanktonR D Robarts, UNEP GEMS/Water Programme, Saskatoon, SK, CanadaG M Carr, UNEP GEMS/Water Programme, Gatineau, QC, Canada

ã 2009 Elsevier Inc. All rights reserved.

Introduction

To the old adage of ‘out of sight, out of mind’ could beadded ‘and unimportant.’ With respect to bacterio-plankton in inland waters this was true for manyscientists interested in aquatic ecology, but not all,and is certainly not true today. Bacteria have beenknown to inhabit natural waters since the seventeenthcentury although their numbers, biomass, distribution,and roles in these systems were poorly understooduntil the latter part of the twentieth century. Even inclassical textbooks of the 1930s, such as P.S. Welch’s‘Limnology,’ a whole chapter (albeit short) wasdevoted to bacteria and other microorganisms. Welchclearly had an intuition about the potential importanceof bacteria in lakes and lamented the fact that very fewlakes have received any study of their native bacteria.Indeed, E.P. Odum in his book ‘Fundamentals of Ecol-ogy’ in 1963 noted that, ‘‘Because of the technicaldifficulties of study, microbial ecology is, unfortu-nately, often completely omitted from the general col-lege course in ecology.’’The situation with respect to our knowledge and

understanding of bacterioplankton in inland waterschanged drastically beginning in the 1960s withpublications describing and applying radioactivelylabeled compounds to measure the turnover anduptake of specific dissolved organic compounds inlakes. Thereafter, new protocols were developed tomeasure in situ the rates of bacterioplankton produc-tion in terms of cell numbers, biomass, and carbon. Inaddition, the introduction of the use of fluorescentdyes in microscopy led to the routine measurement ofbacterial numbers and biomass and later to quantita-tive measurements of different aspects of microbialmetabolism such as respiration and enzymatic activ-ities. The methodology to study bacterioplankton hascontinued to evolve and thus there has been a revolu-tionary advance in our knowledge and understandingof the distribution and size of bacterioplankton popu-lations in lakes, reservoirs, wetlands, rivers, andgroundwaters and of their role in biogeochemicaland energy cycles in these systems over the pastthree decades. In addition, since microbes are thefirst link joining the biotic and abiotic componentsof aquatic systems, they are excellent and sensitiveindicators of human impacts on these systems.

Bacterial Numbers and Biomass

Early methods to estimate the size of bacterioplank-ton populations were based on plate culturing tech-niques. But it had been accepted for a long time thatthese methods greatly underestimated the size of thepopulations. A number of direct-count methods(counting of stained bacteria with a microscope)were devised in conjunction with the use of centri-fuges, chemical flocculation, or filtration in order toconcentrate bacterial cells. Limnologists in the 1920sbelieved that direct counting of bacteria could not beundertaken without concentrating cells because lakepopulations were too low for accurate enumeration.Such concentrating mechanisms also created pro-blems so there was some resistance to the use ofdirect counts. In the 1930s a concentration methodwas developed that involved evaporating watersamples under reduced pressure. It was claimedthat this method produced bacterial counts 2–4000times higher than those from culture plates. Forexample, researchers reported bacterial numbers of(1–6)� 106ml�1 in the Russian Lake Glubokoye. Ascan be imagined, such numbers for an unpolluted lakewere viewed as being overly high and would producea visible turbidity (which it did not), especially whenothers were reporting numbers of only 740–32 600cells ml�1 in lakes using a direct-count method ataround the same time. However, a survey of bacterialnumbers in Wisconsin lakes found that they rangedwidely from 19� 103 to 2� 106ml�1 using this newmethod.

Today with the widespread use of epifluorescencemicroscopy, and more recently with the use offlow cytometers, we know that bacteria occur inmany unpolluted inland waters at concentrations ofmillions per milliliter. Bacteria in most naturalaquatic systems range from 104 to 108 cells ml�1 ofwater, although cell concentrations as high as109ml�1 have been reported (Figure 1). In a 1984cross-system overview of bacteria in surface waters,cell concentrations of (0.1–13.4)� 106ml�1 werereported for a range of lakes. Even a very remoteand pristine Arctic lake, Lake Taymyr on the TaymyrPeninsula in Russia, has bacterial numbers that variedbetween 1.3� 106 and 4.8� 106 cells ml�1. More-over, there have been several studies that found that

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107 108

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Production (mg C m−3d−1)

101 102 103

Figure 1 Distribution of bacterioplankton production (top

panel) and abundance (bottom panel) measurements from inland

waters, as reported in the primary literature and summarized inCarr GM and Morin A (2002) Sampling variability and the design

of bacterial abundance and production studies in aquatic

environments. Canadian Journal of Fisheries and Aquatic

Sciences 59: 930–937. Box plots show median (mid-line) and25th and 75th percentiles. Whiskers extend to data points that fall

within 1.5 times the midrange. Asterisks denote data points that

extend beyond 1.5 times the midrange, and open circles are datapoints that extend beyond 3 times the midrange.

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10 100 1000

Figure 2 Range in published relationships between chlorophyll

a and bacterioplankton abundance. Adapted fromGasol, JM andDuarte CM (2000) Comparative analysis in aquatic microbial

ecology: how far do they go? FEMS Microbiology Ecology 31:

99–106, with permission.

194 Protists, Bacteria and Fungi: Planktonic and Attached _ Bacteria, Bacterioplankton

bacterial abundance in the anoxic hypolimnia of stra-tified lakes can be as much as twice that of the oxicstrata.Several studies have tried to identify limnological

parameters that regulate the variations in bacterialnumbers between different aquatic systems (Figure 2)and seasonally in a specific system. In one suchstudy, bacterial numbers were significantly correlated

(log-transformed data) with chlorophyll a concentra-tions for lakes inNorthAmerica and Europe. This wasnot surprising as it had been assumed for some timethat phytoplankton would be a major provider ofsubstrates for bacterial production and growth, eitherat the time of their senescence and death or by theextracellular release of dissolved organic compoundsduring photosynthesis. However, one of the discon-certing conclusions was that the increase in the num-ber of bacteria did not increase with the increase inchlorophyll a concentrations as lake trophy changed,since the slopes of the regressions were less than1. As a general rule of thumb, oligotrophic watershave bacterial concentrations <1.7� 106 cells ml�1,mesotrophic waters (1.7–6.5)� 106 cells ml�1, andeutrophic waters >6.5� 106 cells ml�1.

Results of within-system correlation analyses ofbacterial numbers have been variable. In some stud-ies, no correlations have been found whereas in otherssignificant correlations between bacterial numbersand water temperature, primary production, andchlorophyll a concentration have been reported.This should not be surprising as total counts of bac-terial cell numbers with epifluorescence microscopytechniques do not discriminate between differentphysiological groups, between live and dead cells,or between metabolically active and nonactive cells(Figure 3). Another factor that influences bacterio-plankton dynamics in aquatic systems is losses dueto processes such as grazing (protistan and inverte-brate), parasitism, and sedimentation. However, eventoday few lakes have been sufficiently studied to pro-vide firm conclusions on the relative importance ofenvironmental factors on population sizes.

Metabolically Active Bacterioplankton

Generally, the number of metabolically active bacte-ria is more variable between plankton communitiesthan the total number of bacteria (Figure 4). As withtotal bacterial concentrations, the number of activecells in bacterioplankton populations has been corre-lated to indicators of lake trophy such as total phos-phorus, chlorophyll, and dissolved organic carbon(DOC). However, the numbers of these two groupsdo not increase in the same way over enrichmentgradients, resulting in a proportionately higher num-ber of active cells in eutrophic versus oligotrophicwaters. The mean of published numbers of activebacteria in lakes is about 22% with a rare studyreporting as much as 100% active cells. For aquaticsystems, generally, the numbers range from <1% inoligotrophic waters to >90% for highly productivesystems. Differences in methodology may account for

Figure 3 Photomicrographs of bacterioplankton cells from

Wascana Creek, Saskatchewan, filtered onto 0.2-mm pore-sizecellulose nitrate filters: (a) stained with Syto 9 for total cell counts

(Molecular Probes Inc., Eugene, OR, USA; cells appear green),

(b) the same preparation stained with the commercial Live Deadstain for determination of ratio of live to dead cells, live cells

appear green while putative dead cells red, and (c) filtered cell

preparation stained with 5mmol l�1 cyanoditolyltetrazolium

chloride (Polysciences Inc, Warrington, PA, USA) incubated for60min and counterstained with Syto 9; yellow cells are active

while green cells are not metabolically active as determined by

this assay. Magenta cells and filaments are cyanobacteria

imaged using autofluorescence of their photosynthetic pigments.Imaging was carried out using the 522þ16, 598þ 16 excitation

lines of an MRC 1024 laser microscope (Zeiss, Jena, Germany).

(Photos: J.R. Lawrence and G.W. Swerhone, NWRI, EnvironmentCanada, Saskatoon, unpublished.)

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Figure 4 Variation in the total number of bacterial cells,metabolically active cells and the percentage of metabolically

active bacteria in 24 lakes of southern Quebec, Canada. The

average number of active cells in the lakes was 20.9%. Data arefrom del Giorgio P A and Scarborough G (1995) Increase in the

proportion of metabolically active bacteria along gradients of

enrichment in freshwater and marine plankton: implications for

estimates of bacterial growth and production rates. Journal ofPlankton Research 17: 1905–1924.

Protists, Bacteria and Fungi: Planktonic and Attached _ Bacteria, Bacterioplankton 195

some of this variance, but research, first in marinesystems and later in fresh waters, indicated that bac-terial grazers may selectively remove larger, activelygrowing cells from a population leaving it dominatedby small, slow-growing cells. Studies of the long-termseasonal variations in metabolically active bacteria


in a particular system have shown that their con-tribution to the total bacterioplankton populationcan also be large, but such studies are rare. Hopefully,with the recent use of flow cytometry to determinemetabolically active bacteria, more data will becomeavailable that will provide new insights on metaboli-cally active bacteria in inland waters and the factorsthat influence them. Such data have significant con-sequences for the measurement of bacterioplanktongrowth and production measurements, which willbe outlined here.

Bacterial Biomass

Bacterial Biovolumes

Spatial and temporal changes in bacterial cell volumehave been reported from a wide range of aquatichabitats, although the number of studies has beenlimited because of the difficulty in obtaining suchdata. To model energy and material flows, it is essen-tial to know the factors that influence bacterial cellvolume, as conversion of bacterial cell numbers tocarbon, nitrogen, or phosphorus units is dependenton the derivation of accurate ‘numbers to volume’conversion factors. Therefore, a major challenge foraquatic microbial ecologists is to identify the factorsthat control cell volume and hence bacterial biomass.Factors that have been identified include carbon sup-ply from algae, phosphorus concentration, predation,and water temperature.To derive a value for bacterioplankton biomass,

data are needed on the number of cells and their bio-volume. This is not as straightforward as it mightseem. The methods used to obtain biovolume includeepifluorescence microscopy, scanning electron micros-copy (SEM), scanning confocal laser microscropy (inconjunction with fluorescent dyes; Figure 3) and morerecently flow cytometry, also using fluorescent dyes toestimate cell dimensions. These methods have pro-blems ranging from halo effects caused by fluorescenceto cell distortions caused by sample preparation forSEM, all of which can alter the calculation of biovo-lumes, especially with the very small cells typical ofbacterioplankton. Median bacterial cell volumes usu-ally range between 0.013 and 0.200mm3. Althoughnot confirmed yet, several studies that have looked atbacterial cell size have reported that this decreases withincreasing trophic status. Furthermore, there have beensome studies comparing bacterial cells between theupper oxic and lower anoxic parts of lakes. Thesehave shown that bacteria from the anoxic hypolimnioncan be between 2 and 10 times larger than bacteria in

the oxic layer. The reasons for this difference and itsimplications on lake metabolism require furtherinvestigation.

Bacterial Biomass

To derive a carbon biomass value for bacterioplank-ton, most researchers have either used a constantvalue for a cell, usually between 10 and 20 fgC cell�1, or a volume conversion factor that has upto a fivefold range of a commonly used value of 121 fgC mm�3. Several studies have produced values in therange of 350–720 fg C mm�3, whereas others reportedmuch lower values of between 32 and 160 fg C mm�3.One reason for the higher values may be the under-estimation of cell volumes due to shrinkage effectscaused by fixatives and air drying, although this isnot applicable to all published values. Smaller cellstend to have higher carbon and lower water contentsthan larger cells. Therefore, what is clear is that theassumption of a constant ratio of carbon:biovolumeis not correct. Some studies have shown that thecarbon:biovolume ratio of bacterioplankton notonly varied with cell size but also had temporal andgeographical variations, indicating that factors suchas species composition, nutritional state, growth rate,and other factors played a role.

While it is now clear that bacterial biomass contri-butes a larger portion to the total planktonic biomassin unproductive freshwater systems than in more pro-ductive systems, what is not clear is why. The mostwidely accepted explanations include allochthonouscarbon inputs being important in oligotrophic sys-tems, decreased bacterivory, and bacterioplanktonaccess to nutrients that are not available to phyto-plankton (i.e., very low concentrations and organicforms) or a combination of these. Correct estimatesof bacterioplankton biomass distribution are a funda-mental requirement in aquatic microbial ecology, andmore data that lead to a sound understanding of thefactors that govern biomass in diverse systems areneeded.

Bacterial Production

The use of 14C-glucose in the early 1960s andsubsequent development of methods to determinerates of bacterial uptake, respiration, and turnoverof organic compounds at natural substrate concentra-tions later in the decade provided much data on thedecomposition and flow of organic carbon throughfood webs in a wide range of natural systems. Sincebacteria degrade a large number of organic sub-strates, further developments introduced the use of


labeled sugars, amino acids, organic acids, lignocellu-lose, and other dissolved and particulate substrates.These early studies demonstrated that bacteria wereactively metabolizing organic matter but did notprovide quantitative estimates of growth rates andproduction.While the growth of autotrophic bacteria that fix

carbon dioxide for their primary source of carbon canpotentially be measured using 14CO2, until about20 years ago there had not been a reliable method todetermine the growth of natural assemblages of het-erotrophic bacteria that utilize organic substrates fora carbon source. The two most commonly used meth-ods were the incorporation of [3H-methyl]thymidine([3H]TdR) into bacterial DNA, and the incorporationof 3H-leucine into bacterial protein. Labeled leucineis now generally more widely used than labeledthymidine because of its greater sensitivity and thesimpler assumptions and calculations required toderive cell and carbon production estimates.Bacterial production has been measured in a wide

range of aquatic systems from the Arctic to Antarctic.Not surprisingly, there is huge variation in the ratesreported (Figure 1). Some of this variation is due tothe factors used to convert the rates of label incor-poration into cell and carbon production units, par-ticularly with the more complicated conversion ofthymidine incorporation rates. Several studies haveconcluded that there is no significant difference inproduction rates determined with labeled leucineand thymidine, whereas others have concluded thatthere is a difference with thymidine-derived ratesbeing lower. In addition, there are potential problemsassociated with isotope dilution for both methods,which is usually not measured on the assumptionthat saturating concentrations of tracer have beenadded. If saturating concentrations are not usedthen production will be underestimated.Volumetric rates of bacterioplankton produc-

tion have been reported to range from 0.4 to>900mgC m�3 d�1 in hypertrophic lakes (Figure 1).Cross-system analysis of bacterial production hasfound significant correlations with phytoplanktonproduction, chlorophyll a concentration, bacterialnumbers, and total phosphorus concentration.Within-systems correlations have been found withwater temperature, and several studies have alsodemonstrated that bacterioplankton can be limitedby nutrients (phosphorus) and/or by the availabilityof dissolved organic substrates. Bacterial-specificgrowth rates vary between 0.017 and 8.7 day�1, pro-ducing doubling times ranging from hours to weeks.Values less than 0.01 day�1 have been reported forcold waters of <6 �C and under winter ice, giving

doubling times of several years. Specific growthrates have been correlated with water temperature,producing Q10 values (i.e., the rate of change ingrowth rate associated with a 10 �C increase in tem-perature) of 2–4 in various studies. However, specificgroups of aquatic bacteria can have much higher Q10

values so that even small water temperature changescan promote large increases in some bacterialprocesses.

To compare bacterial production and other data,e.g., on the uptake of organic substrates, from differ-ent systems or different parts of systems, microbialecologists normalize them by dividing by the totalcell abundance. This scaling process is intended topermit a more robust comparison of data generatedfrom waters with markedly different bacterioplank-ton population sizes. However, this calculation of cell-specific production, uptake, and specific growthrates (to give population doubling times) in mostinstances does not enhance data analysis but compli-cates it. This is because, as noted earlier, a varyingproportion of a bacterioplankton population is meta-bolically inactive and may account for a significantamount of the variation in these rates as well asproduce overestimates of bacterial turnover times.At least one study has concluded that if productionis scaled to the number of active bacteria in a popula-tion instead of the total population abundance,then specific production rates are fairly similarbetween systems and may, in fact, be higher in unpro-ductive systems that have the lowest bacterial densi-ties. As more data become available for a wide rangeof aquatic systems, the veracity and implicationsof this tentative conclusion to aquatic system func-tioning will become clearer.

In some lakes, rivers, and wetlands, daily heterotro-phic bacterial production exceeds daily autotrophicprimary production and such systems are consideredto be net heterotrophic. To calculate the ratio of thedaily rates of primary to bacterial production, theassumption is made that bacterial production is con-stant over a diel cycle, although studies have shownthis not to be the case. Since most studies of bacterialproduction do not include diel studies, hourly produc-tion rates are multiplied by 24. It is also necessary totake into account respiration losses as productionmeasured by labeled leucine or thymidine are consid-ered to be net production. This is done using a growthyield factor which, like many other conversion factorsin aquatic microbial ecology, has a very wide range of<0.15–0.9. These calculations provide an estimate ofthe amount of phytoplankton carbon production andtherefore an indication of whether a system is netheterotrophic or autotrophic.


The concept of net heterotrophic systems is still acontroversial topic amongst aquatic microbial ecolo-gists. However, evidence is mounting that net hetero-trophic systems are prevalent in most rivers and inoligotrophic to mesotrophic lakes, and that thesesystems act as a source of CO2 to the atmosphere.Moreover, the occurrence of net heterotrophy in somany systems implies even tighter connections betweenbiogeochemical processes in aquatic systems and adja-cent watersheds, as organic carbon from awatershed isneeded to fuel heterotrophic processes in the receivingwaters.

Role of Bacterioplankton in Inland Waters

The traditional role attributed to bacteria in aquaticecosystems was that of decomposer of organic matter.But bacteria not only remineralize nutrients back tothe water column through decomposition processes,they also store organic carbon, are a food source toother microbes, and are important in the cyclingof phosphorus and nitrogen. While early scientificpapers recognized the importance of the ‘microbialooze’ in the trophic dynamics of aquatic ecosystems,it is only recently that we have begun to be able toquantify the influence of microbes on biogeochemicalprocesses. In the last few decades, bacteria have beenviewed as important components of the microbialloop, which contains multiple trophic levels and isimportant in the cycling of matter and in dissipatingenergy through respiration. The contribution of bac-teria to biogeochemical cycles is only beginning to befully appreciated.

Decomposition

As decomposers, bacterioplankton degrade dissolvedorganic matter (DOM), assimilate the byproductsinto their cell (bacterial production), and remineralizecarbon and nutrients back to the water through respi-ration. Organic carbon that originates from phyto-plankton, either through cell death or excreted duringphotosynthesis, is usually assumed to be the primarysource of dissolved organic carbon (DOC) that fuelsbacterial metabolism. Organic carbon of algal originis typically composed of simple sugars and is rapidlyremineralized by bacteria. In contrast, organic matterthat enters from allochthonous sources is a complexmixture of highmolecular weight and lignified organiccompounds that are not as readily decomposed.Extracellular enzymes are required to mediate deg-

radation of polymeric organic macromolecules intosmaller compounds that can be assimilated by thecell. The products of the enzymatic reactions are

believed to limit rates of assimilation of organic mat-ter by bacteria and, hence, bacterial growth. As such,estimates of extracellular enzyme activity (EEA) maybe used to quantify decomposition rates. For example,studies of EEA and bacterial production found thathigh bacterial productivities were typically associatedwith the availability of simple sugars such as sacchar-ides and that during the summer bacterial productionwas fueled by algal lysates and exudates generatedthrough the microbial loop, whereas production inspring and autumn appeared to be fueled by alloch-thonous carbon. Such studies support the observationthat algal products are the ‘preferred’ organic carbonsource to fuel bacterial metabolism, but that externalcarbon sources may also fuel decomposition undercertain conditions.

Bacterial community structure has been shownto affect the rate of processing of dissolved organicmatter (DOM) in aquatic systems. There are differ-ences among major phylogenetic groups in terms ofutilization of high versus low molecular weight DOMand in terms of enzyme activities, and changes inphytoplankton community structure have also beenshown to produce a response in bacterioplanktoncommunity composition. Differences in bacterialcomposition between lakes have also been found tobe correlated to variables that reflect the relative load-ing of allochthonous versus autochthonous carbon,whereas seasonal changes in community compositionwithin lakes correlated to patterns in algal abundance,temperature, and DOC.

Although a common observation is that sedimentsoverlain by anoxic waters are rich in organic mattercontent, it is also generally thought that anoxiaslows decomposition rates in lakes. There are pub-lished studies both supporting and refuting this view.Examination of this contradiction across a series oflakes showed that bacterial production was in factgreater in the anoxic hypolimnion with lower watertemperatures than in the epilimnion. The mean ratioof anoxic production to aerobic production was 1.6for lakes that ranged from ultraoligotrophic to eutro-phic. The doubling time of bacteria in the colderanoxic waters was lower than in the warmer oxicwaters. This was offset by the greater bacterial abun-dance and biomass in the anoxic waters. The signifi-cance of these findings to not only other lakes but alsoto our general, and still poor, understanding of the roleof bacteria in anoxic zones in lake functioning is aresearch area requiring further investigation.

It is important to bear in mind that bacteria are notthe sole decomposers in aquatic systems: fungi areresponsible for the decomposition of high molecularweight and/or lignified compounds, whereas bacteriaare primarily responsible for the final decomposition


of lower molecular weight polymeric compoundsand polysaccharides. A recent study experimentallydemonstrated an association between bacteria andfungi, where bacterial decomposition of allochtho-nous organic matter was dependent on intermediatedecomposition products that were produced by fungi.Still, in other studies, fungi have been shown to be thedominant organisms in decomposition.

Nutrient Uptake and Cycling

Bacterioplankton are typically assumed to be limitedby the availability of organic carbon and, since algalcarbon appears to be the preferred substrate for bac-terial metabolism, bacterial production shouldbe closely linked to algal production and/or biomass.At coarse, cross-system scales, this relationshipdoes hold and, as noted above, reasonably good cor-relations have been detected between bacterial abun-dance and chlorophyll a and between bacterialproduction and net phytoplankton production. How-ever, there have been many studies that have demon-strated an uncoupling between phytoplankton andbacterioplankton abundance and production. The rea-sons for theuncoupling appear to rest, at least in part, onthe availability of inorganic nutrients, and of phos-phorus in particular.Bacteria are efficient competitors for inorganic

nutrients such as phosphorus and nitrogen (mostly asammonium) that limit algal growth, particularly inlow nutrient environments. It has been shown throughsize fractionation studies that bacteria are usuallyresponsible for at least 50%of the uptake of inorganicphosphorus in lakes. The actual proportion of theinorganic phosphorus pool that is taken up by bacteriadoes vary, probably in part as a function of phyto-plankton biomass and availability of organic carbon.The proportion of ammonium that is taken up bybacteria in freshwater systems has not been intensivelystudied, mostly because freshwater ecosystems areusually assumed to be phosphorus limited, but inmarine systems bacteria can be responsible for ~30%of ammonium uptake.Although bacteria may be able to incorporate phos-

phorus more rapidly than do algae, algal biomass ishigher than bacterial biomass in more enriched sys-tems and algae are better able to store phosphorus intheir cells as phospholipids. Thus, over longer timeperiods (days to weeks versus minutes and hours),algae may be able to incorporate more inorganicphosphorus than do bacteria. If bacterial biomassis high, as in oligotrophic systems, then algal growthmay be limited because of bacterial uptake of nutrients.However, grazing onbacteria by protozoa in themicro-bial loop will remineralize organic matter and release

inorganic P and N, making these elements once againavailable to phytoplankton. Also, short-term physical-forcing events can temporally reverse this situation ifnutrients are brought up into thewater column throughthe entrainment of P-rich bottom sediments and/orinterstitial water. Similarly, annual overturn in strati-fied lakes can introduce nutrients to the upper watercolumn.

In a cross-system comparison of lake bacterioplank-ton and phytoplankton, bacterial abundancewasmorestrongly correlated to total phosphorus than to algalabundance, suggesting a degree of uncoupling betweenalgal and bacterial growth. At least one study hasshown that inorganic phosphorus additions can stim-ulate bacterial but not algal production, whereas theaddition of both nitrogen and phosphorus increasedboth algal and bacterial production, suggesting thatbacteria are better scavengers for nutrients than arealgae when one or more nutrients are limiting.

Effect of Environmental Perturbationson Bacterioplankton

Bacterioplankton community composition, produc-tion, and abundance vary seasonally in most freshwa-ter systems where temporal patterns have beenexamined, and these variations appear to track cli-matic variations that affect numerous biogeochemicalprocesses. Because of their rapid turnover rates andcorrelation to numerous environmental variables,bacterioplankton can also be expected to be sensitiveto environmental change such as climate change andvariability, increases in nutrient concentrations or topollution by a wide range of human-made chemicals.

The impact of human-made chemicals on bacterio-plankton metabolism and diversity is not yet wellunderstood. The addition of herbicides to naturalecosystems, in conjunction with inorganic nutrients,has been shown to have a synergistic effect on bacte-rial production. Antibiotics commonly used to treatbacterial infections and that have increasingly beendetected in aquatic ecosystems at low concentrationscan have an inhibitory effect on bacterial production,even at very low concentrations. Long-term exposureto toxic metals can alter a bacterial communitytoward strains that are resistant to heavy metals.Interestingly, a number of bacterial communitiesthat are metal resistant have also been found to beresistant to antibiotics, suggesting an indirect selec-tion for antibiotic resistance in natural ecosystemsexposed to heavy metals. This type of indirect selec-tion for resistance is worrisome, given general publichealth concerns over the evolution of antibiotic resis-tant bacteria.


See also: Fungi; Microbial Food Webs.

Further Reading

Bird DF and Kalff J (1984) Empirical relationships between bacte-

rial abundance and chlorophyll concentration in fresh and ma-rine waters. Canadian Journal of Fisheries Aquatic Sciences 41:1015–1023.

Cole JJ (1999) Aquatic microbiology for ecosystem scientists: new

and recycled paradigms in ecological microbiology. Ecosystems2: 215–225.

Cole JJ and PaceML (1995) Bacterial secondary production in oxic

and anoxic freshwaters. Limnology and Oceanography 40:

1019–1027.Cole JJ, Findlay S, and Pace ML (1988) Bacterial production in

fresh and salt water ecosystems: a cross-system overview.MarineEcology Progress Series 43: 1–10.

Cotner JB and Biddanda BA (2002) Small players, large role: mi-

crobial influence on biogeochemical processes in pelagic aquatic

ecosystems. Ecosystems 3: 105–121.

Odum EP (1959) Fundamentals of ecology. Philadelphia: W.B.

Saunders Company.Robarts RD and Zohary T (1993) Fact or fiction—Bacterial growth

rates and production as determined by [methyl-3H]-thymidine?

Advances in Microbial Ecology 13: 371–425.

Stepanauskas R, Glenn TC, Jagoe CH, et al. (2005) Elevatedmicrobial tolerance to metals and antibiotics in metal-

contaminated industrial environments. Environmental Scienceand Technology 39: 3671–3678.

Verma B, Robarts RD, and Headly JV (2007) Impacts of tetracy-cline on planktonic bacterial production in prairie aquatic sys-

tems. Microbial Ecology 54: 52–55.

Welch PS (1935) Limnology. New York: McGraw-Hill Book

Company, Inc.White PA, Kalff J, Rasmussen JB, and Gasol JM (1991) The effect

of temperature and algal biomass on bacterial production and

specific growth rate in freshwater andmarine habitats.MicrobialEcology 21: 99–118.

encyclopedia of inland waters || bacteria, bacterioplankton

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