ARCHAEBACTERIA

CONTENTS

• Introduction• Archaeal cell walls• Archaeal lipids & membranes• Genetics & molecular biology• Metabolism• Archaeal taxonomy• Significance of Archaebacteria• References

Introduction

Introduction to the ArchaeaMorphology

•Stain either Gram +ve or Gram –ve

•Shapes – Spherical, rod, spiral, lobed, irregularly shaped or pleomorphic

•Single cells or filaments or aggregates, diameter from 0.1 to over 15 μm

•Multiplication – binary fission, budding, fragmentation or other mechanisms

Contd…Contd…

• Physiology•Aerobic, facultative and strictly

anaerobic

•Range from chemolithotrophs to organotrophs

•Mesophilic & hyperthermophiles

•Found in extreme environments

•Few are symbionts in animal digestive system

Contd…Contd…

Archaeal cell walls

Structure

Gram +ve Archaea•Single thick homogenous layer as in

Gram +ve bacteria

Gram +ve Archaea• lack outer membrane & complex

peptidoglycan network•have a surface layer of proteins or

glycoproteins

Contd…Contd…

Chemistry

Gram +ve Archaea•Variety of complex polymers

•Methanobacterium – pseudomurein

•Methanosarcina & Halococcus – complex polysaccharides similar to chondroitin sulphate of animal connective tissue

•Other heteropolysaccharides are also found Contd…Contd…

Gram -ve Archaea•Layer of protein or glycoprotein outside

their PM (20 to 40 nm thick)

•Methanolobus, Halobacterium & several extreme thermophiles (Sulfolobus, Thermoproteus & Pyrodictium) consist of glycoprotein units

•Methanococcus, Metahnomicrobium, Methanogenium & Desulfurococcus consist of protein units

Contd…Contd…

Archaeal Lipids & Membranes

•Branched chain hydrocarbon attached to glycerol by ether links rather than fatty acids joined by ester links

•Sometimes two glycerol groups – form an extremely long tetraether (40C)

•Length is adjusted by cyclizing the chain to form pentacyclic rings

•Polar lipids like phospholipids, sulfolipids & glycolipids are present

•7 to 30% are non polar lipidsContd…Contd…

• lipids can be combined in various ways – membranes of different rigidity

– C20 diethers – regular bilayer membrane– C40 tetraether – much more rigid

monolayer membrane

Contd…Contd…

Genetics & Molecular Biology

•Circular DNA – like bacteria

•Genomes are significantly smaller than bacteria.

– E. coli – 2.5 x 109 Daltons– T. acidophilum – 0.8 x 109 Daltons

•G + C content varies from 21 - 68 mol %

•mRNA – similar to bacterial mRNA

•TψC arm of tRNA – pseudouridine / 1-methyl pseudouridine

Contd…Contd…

•Sensitive to anisomycin & insensitive to chloramphenicol & kanamycin

•EF-2 reacts with diphtheria toxin

•Many methanogens - nucleosomes

•DNA dependent polymerases resemble eukaryotic enzymes rather than prokaryotic polymerases

Contd…Contd…

Metabolism

•CH metabolism is best understood

•6- phosphofructokinase is absent & E-M p/w is not followed

•Extreme halophiles & thermophiles – modified form of E-D p/w wherein initial intermediates are not phosphorylated

•Methanogens do not catabolise glucose

•Gluconeogenesis – reversal of E-M p/w

•All oxidize pyruvate – acetyl coA using pyruvate oxidoreductase

Contd…Contd…

•Biosynthetic p/w – similar to those in other organisms

» Some methanogens can fix atmospheric N2

» Few employ glycogen as the main reserve material

•Autotrophy is widespread among methanogens & extreme thermophiles

•Thermoproteus & Sulfolobus – fix CO2 by reductive TCA cycle

•Methanogens & extreme thermophiles –fix CO2 by reductive acetyl coA p/wContd…Contd…

Reductive TCA cycle

Contd…Contd…

Archaeal Taxonomy

Phylum Crenarchaeota•Most are extremely thermophilic &

many are acidophiles and S dependent

•S – as electron acceptor or electron source by lithotrophs

•Almost all are strict anaerobes & grow in geothermally heated water / soils that contain elemental sulfur

Contd…Contd…

•Divided into one class – Thermoprotrei & three orders :

» Thermoproteales» Sulfolobales» Desulfurococcales

•Contain 69 genera – two of the better studied genera are Thermoproteus & Sulfolobus

Contd…Contd…

Sulfolobus

• Gram –ve, aerobic, irregularly lobed spherical archaeons

• Optimum temp.– 70 to 80 0C & optimum pH 2 - 3hence also referred to as thermoacidophiles

• Cell wall – lipoprotein & CH, lacks peptidoglycan

• Grow lithotrophically on S granules in hot S springs oxidizing S to Sulfuric acid

• Oxygen Is the normal electron acceptor, Fe+3

may be usedContd…Contd…

• Sugars & amino acids (glutamate) also serve as C & energy sources

Contd…Contd…

Thermoproteus

• Gram –ve, strictly anaerobic, hyperthermophilic long thin rod, can be bent or branched

• Cell wall consists of glycoprotein

• Grows at temp. from 70 - 97 0C & pH 2.5 – 6.5

• Found in hot springs & other hot aquatic habitats rich in sulfur

• Can grow organotrophically & oxidize glucose, amino acids, alcohols & organic acids with S as TEA Contd…Contd…

An aquatic spring in Japan with Thermoproteus growth

• Grows chemolithtrophically using Hydrogen & S0

• CO & CO2 can serve as the sole C source

Thermoproteus

Contd…Contd…

Phylum Euryarchaeota

• Very diverse with 7 classes viz. Methanococcus, Methanobacteria, Halobacteria, Thermoplasmata, Thermococci, Archaeglobi & Methanopyri

• Consists of 9 orders & 15 families

• Includes methanogens, extreme halophiles, sulphate reducers & many extreme thermophiles with S dependent metabolism

Contd…Contd…

The Methanogens• Strict anaerobes that obtain energy by

converting CO2, H2, formate, methanol, acetate & other compounds to either CH4 or CH4 & CO2

• Autotrophic when growing on CO2 & H2

• 5 orders (Methanobacteriales, Methanococcales, Methanomicrobiales, Methanosarcinales & Methanopyrales) & 26 genera

• Metabolism is unusual• Unique cofactors like H4MPT, MFR, Coenzyme M

Coenzyme F420, Coenzyme F430

• Coenzyme F420 carries electrons and Hydrogen• Coenzyme F430 cofactor for methyl co-M methyl

reductase

Contd…Contd…

Methane synthesis from CO2 by Methanobacterium thermoautotrophicum

Contd…Contd…

• Some live autotrophically – acetyl coA from two molecules of CO2 & then converting acetyl coA to pyruvate & other products

• Found in anaerobic environment rich in organic matter

• Rumen & intestine of animals, fresh water & marine sediments, swamps & marshes, hot springs, anaerobic sludge digesters & anaerobic protozoa

• Ecological significance• Cow belches 200 – 400 ltrs CH4/day

• Source of energy for sewage treatment plantsContd…Contd…

E.g. Halobacterium salinarium

• Unusual type of photosynthesis

• On exposure to sunlight – purple membrane – bacteriorhodopsin

• Four types of rhodopsins• Bacteriorhodopsin – proton transport for ATP

synthesis• Halorhodopsin – uses light energy to transport

chloride ions into the cell & maintains KCl conc.

• Remaining two – photoreceptors, one for red light & one for blue

Contd…Contd…

• Ecological hazards• Methane gas – Greenhouse gas• Can oxidize Fe0

• May contribute to corrosion of buried or submerged iron pipes

Yellowstone National Park

M. thermoautotrophicum

Contd…Contd…

The Halobacteria• Class Halobacteria – 15 genera in one family,

the Halobacteriaceae

• Aerobic chemoheterotrophs with respiratory metabolism

• Non motile or motile by lophotrichous flagella

• Absolutely dependent on high NaCl conc.• At least 1.5 M NaCl, growth optimum at 3 – 4

M • Cell wall disintegrates below 1.5 M conc.• Grow only in high salinity habitats• Cause spoilage of salted fish Contd…Contd…

Purple lake in AustraliaH. salinarium

Contd…Contd…

The Thermoplasms

• Class Thermoplasmata

• Thermoacidophiles that lack cell wall

• Two genera, Thermoplasma & Pichrophilus

• Thermoplasma• Grows in refuse piles of coal mines• Temperatures from 55 – 590C & pH 1-2• Plasma membrane is strengthened by large

quantities of diglycerol, tetraethers, lipopolysaccharides & glycolipids

• DNA stabilized by nucleosomesContd…Contd…

• At 590C - irregularly shaped & at lower temperatures – spherical

• May be motile by flagella

Contd…Contd…

• Picrophilus• Lacks cell wall , has S layer outside PM• Aerobic, irregularly shaped cocci, 1 – 1.5 μm

in diameter• Temp. range 47 – 650C, optimum temp. 600C• pH below 3.5, optimum pH 0.7• Can grow at pH 0

Contd…Contd…

Extremely Thermophilic S0 Metabolizers

• Class Thermococci, order Thermococcales

• Strictly anaerobic

• Reduce sulfur to sulfide

• Motile by flagella

• Optimum growth temp. 88 –1000C

• Two genera, Thermococcus & PyrococcusContd…Contd…

Sulfate Reducing Archaea

• Class Archaeglobi, order Archaeoglobales

• Gram –ve, irregular coccoid cells

• Cell wall – glycoprotein subunits

• Electron sources – hydrogen, lactate & glucose, reduce sulfate, sulfite or thiosulfate to sulfide

• S is not used as electron acceptor

• Extremely thermophilic, optimum temp. around 830C, occur in hydrothermal vents

Contd…Contd…

Significance of Archaea

• Methanogens are used for the production of methane which is a rich source of energy

• Preparation of glycoproteins & proteins from archaeal cultures are used to increase body’s defense against infection

• Halophilic archaea are used to prescreen antitumor drugs active on eukaryotic proteins

• Thermophilic archaea are used in PCR

References

• Prescott, Lansing M.; Harley, John P. and Klein, Donald A.,2003. Microbiology, 5th edition. McGraw – Hill

• www.euarch.blogspot.com• www.filebox.vt.edu• www.nature.com/ntmicro/journal/v5/n4• www.fib_tab/nrmicro1619_F3.html• www.microbewiki.kenyon.edu/