Methanogens and homoacetogens - Uni Oldenburg15.11.2010! 1! Physiology and Diversity of Prokaryotes WS 2010/2011 ( Methanogens and homoacetogens Physiology and Diversity of Prokaryotes

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Physiology and Diversity of Prokaryotes WS 2010/2011 (www.icbm.de/pmbio/)

Methanogens and

homoacetogens


Two types of carbon respiration

- Methanogenesis - Homoacetogenesis

Low energy yield, but

…carbonate is almost everywhere abundant.

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Lake Maggiore (Italy) November 3, 1776

Observation of “combustible air”

Alessandro Volta (1745-1827)


Methanogens

BROCK Microbiology: Chapter 13, 17

CYPIONKA Grundlagender Mikrobiologie: Chapter 15

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Methanogens represent a large group of

microorganisms which share three features:

•! Formation of methane as the major product of their

energy metabolism

•! They are strict anaerobes

•! They are members of the domain Archaea


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The three domains of life


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Complex polymers

(polysaccharides, lipids, proteins)

Monomers

(sugars, fatty acids, amino acids)

Short chain fatty acids and alcohols

(lactate, butyrate, propionate, ethanol)

H2 + CO2

Formate

Acetate

Hydrolysis

Fermentation

CO2 + Methane Methanogenesis

Secondary fermentativ

bacteria

Anaerobic degradation of organic matter


Reaction !G°´ (kJ/mol of methane)

Carbonate respiration

4 H2 + CO2 ! CH4 + 2 H2O –135.6

4 Formate ! CH4 + 3 CO2 + 2 H2O –130.1

4 2-Propanol + CO2 ! CH4 + 4 Acetone + 2 H2O –36.5

2 Ethanol + CO2 ! CH4 + 2 Acetate –116.3

C1-Compounds

Methanol + H2 ! CH4 + H2O –112.5

4 Methanol ! 3 CH4 + CO2 + 2 H2O –104.9

4 Methylamine + 2 H2O ! 3 CH4 + CO2 + 4 NH4+ –75.0

2 Dimethylamine + 2 H2O ! 3 CH4 + CO + 2 NH4+ –73.2

4 Trimethylamine + 6 H2O ! 9 CH4 + 3 CO2 + 4 NH4+ –74.3

2 Dimethylsulfide + 2 H2O ! 3 CH4 + CO2 + H2S –73.8

Acetoclastic

Acetate ! CH4 + CO2 –31.0

3 types for methanogenesis

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Physiology and Diversity of Prokaryotes WS 2010/2011 (www.icbm.de/pmbio/) Physiology and Diversity of Prokaryotes WS 2006/2007 (www.icbm.de/pmbio/)

Methan (CH4)

Carbon dioxide (CO2) + IV

- IV

Oxidation state

Reduction

8 electrons (e-)

Formyl (-COH)

Methylene (=CH2)

Methyl (-CH3)


4 H2 + 1 CO2 ! CH4 + 2 H2O


Anaerobic Respiration (i.g.: Methanogenesis)

Hydrogen + Carbon dioxide Methane

H2 + CO2 CH4

Redox Balance

C +IV; H 0 C -IV; H 4(+I) 8 e-

4H2 + CO2 CH4

(e- donor) (e- acceptor)

Elemental Balance

8xH, 1xC, 2xO 4xH, 1xC

4H2 + CO2 CH4 + 2H2O

!G0 ’ = -50.75 + 2(-237.17) - (-394.4) = -130.7 kJ

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4 H2 + CO2 ! CH4 + 2 H2O

Methanobacterium

Methanomicrobium

Methanospirillum

MF: Methanofuran

MP: Methanopterin

CoM: Coenzyme M

(Mercaptoethansulfonat)

F420: Factor F420

F430: Factor F430


Methanophenazine

Electron

transport chain

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C1-Reaction

e.g. Methanol

Methanolobus

Methanosarcina

4 methanol ! 3 CH4 + CO2 + 2 H2O


Aceticlastic (acetate)

Reaction

Methanosaeta

Methanosarcina

Acetate ! CH4 + CO2

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acetate + SO42- ! 2 HCO3

- + HS-

!G0’ = -47.6 kJ/ mol acetate

acetate ! CH4 + CO2

!G0’ = -31.0 kJ/ mol acetate


Complex polymers


Monomers




H2 + CO2

Formate

Acetate

Hydrolysis

Fermentation

CO2 + Methane Methanogenesis

Secondary fermentativ

bacteria

Anaerobic degradation of organic matter

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Ethanol fermentation

Ethanol Acetate + Hydrogen

C2H6O C2H3O2- + H2

Ionic Balance

C2H6O C2H3O2- + H2 + H+

Elemental Balance

C2H6O + H2O C2H3O2- + 2H2 + H+

Redox Balance C 2(-II); H 6(+I); O (-II) C 2(0); H 3(+I); O 2(-II) + H (+I)

!G0 ’ = -369.41 + (-39.83) - [(-181.75) + (-237.17)] = 9.68 kJ


Effect of hydrogen partial pressure

on free-energies Ethanol fermentation:

ethanol + H2O acetate + 2H2 + H+

!G + = !G0 RT ln [C]c [D]d

[A]a [B]b

!G + = !G0 RT ln [H2]

2 [acetate] [H+ ]

[ethanol] [H2O]

!G = 9.68 + 2RT ln [10-4 ] = -36.03 kJ/mol

!G = !G0 + mRT ln [H2]

at 10-4 atm H2

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Syntrophic ethanol oxidation at

anaerobic conditions

2 ethanol + 2H2O 2 acetate + 4H2 + 2H+

Ethanol fermentation

Methanogenesis

4H2 + CO2 CH4 + 2H2O

Syntrophic coupled reaction

2 ethanol + CO2 2 acetate + CH4 + 2H+

!G0’ (kJ/reaction)

+ 19.4

- 130.7

- 111.3


Syntrophic co-culture “Methanobacillus omelianskii”

ethanol CO2

H2 H2 CH4

acetate

Strain S Strain MoH

“Methanobacillus omelianskii”

Interspecies Hydrogen-transfer

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Syntrophic co-cultures

Interspecies hydrogen transfer

Hydrogen-producer Hydrogen-consumer

Fermentation Anaerobic Respiration

fatty-acids CO2, SO4-2, NO3

-

(e.g., butyrate, propionate)

alcohols

(e.g.,ethanol)

acetate + CO2 acetate, methane, HS-, N2O, NO, N2

Syntrophomonas Methanogens

Syntrophobacter Sulfate-reducing bacteria

Denitrifyers

H2 H2


Habitats of Methanogens

-! Anoxic sediments: marsh, swamp,lake sediments etc.

-! Animal digestion tracts: rumen, large intestine of

monagastric animals (such as humans, swine, and dogs)

-! Geothermal sources, hydrothermal vents

-! Artifical biodegradation facilities: sewage sludge

-! Endosymbionts of various anaerobic protozoa

Methanogenic archaea are abundant in habitats where electron

acceptors such as O2, NO3–, Fe3+ and SO4

2– are limiting.

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Global methane emission from different habitats

in millionT/year

Animals (rumen) 80-100

Termites 25-150

Rice fields 70-120

Ocean & lakes 1-20

Biogenic 300-820

Abiogenic 48-155


Homoacetogens

BROCK Microbiology: Chapter 17

CYPIONKA Grundlagender Mikrobiologie: Chapter 15


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Complex polymers


Monomers




H2 + CO2

Formate

Acetate

Hydrolysis

Fermentation

Homoacetogenesis

Physiology and Diversity of Prokaryotes WS 2010/2011 (www.icbm.de/pmbio/) Physiology and Diversity of Prokaryotes WS 2006/2007 (www.icbm.de/pmbio/)

Acetate

(CH3COO-)

Carbon dioxide (CO2) + IV

Aver. 0

Oxidation state

Reduction

4 electrons (e-)


4 H2 + 2 CO2 ! CH3COO- + H+ + 2 H2O

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Moorella

thermoaceticum

Acetyl-CoA- or

Wood-Ljungdahl

Pathway

H2 Electrondonor

CO2 Electronenacceptor

The acetyl-CoA pathway


The acetyl-CoA pathway

-! In contrast to other carbon fixation pathways, not a

cycle

-! two linear reaction series resulting in A) a methyl- and

B) a carbonyl group

-! key enzyme: CO-DH (Carbon monooxide dehydrogenase/

acetyl-CoA synthase) CO2 + H2 ! CO + H2O

-! The CO2 reduction must be considered as bifunctional

pathway: A) energy metabolism B) C-fixation for

biosynthesis

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Homoacetogenic bacteria

- Extremely heterogenous group of strict anaerobes with

the ability to reduce CO2 to acetate for generation of

energy

-!The key enzyme is the carbonmonoxide dehydrogenase

-!Higher efficiency compared to fermentation (both acetyl-coA

derived from pyruvate can be used for energy conservation)

-!Electron donor: hydrogen and a variety of organic compounds (sugars, amino acids, carbonic acids)

-!Acetate is major end product (in contrast to other acetogenic

organisms)

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Habitats of homoacetogenic bacteria

Due to their diversity with respect to the substrate

spectrum homoacetogenic bacteria are abundant

in almost all nonmarine environments:

-! freshwater sediments

-! sewage sludge

-! rumen of cow, cheep etc.

-! biofilms


Organisms using the acetyl-CoA pathway

I. For energy metabolism (homoacetogenic bacteria)

Acetobacterium, Clostridium, Desulfotomaculum,

Eubacterium, Treponema

II. For carbon dioxide fixation

Autotrophic homoacetogenes, methanogenes, SRB

III. Acetate oxidation for energy metabolism

Complete-oxidizing SRB (w/o Desulfobacter), acetoclastic

methanogens (Methanosaeta, Methanosarcina)

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Methanogens and homoacetogens - Uni Oldenburg15.11.2010! 1! Physiology and Diversity of Prokaryotes WS 2010/2011 ( Methanogens and homoacetogens Physiology and Diversity of Prokaryotes