Sulfate and Sulfur Reducing Bacteria

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Sulfate, Sulfur Reducing Bacteria, and Homoacetic Acid Bacteria

We already considered S oxidation where reduced S was used as an electron donor forchemolithotrophic growth.

Here we consider organisms that reduce sulfate or sulfur during respiration. Sulfate or sulfur areused as terminal electron acceptors during Dissimilatory Sulfate Reduction

o Review assimilatory and dissimilatory processes

The use of S or SO4 as a terminal electron acceptor is an obligatory anaerobic form of metabolism

where the electron donor can come from a wide variety of substrates including organic compounds

(as organic acids, fatty acids, and alcohols) or the electron donor may be H2. The end product ofthis metabolism is H2S.

Use of carbohydrates is not common.

The organisms that can do this represent an extremely wide group whose majority falls within the

delta Proteobacteria. They are cosmopolitan and may be isolated from soils and anoxic mud where

sulfate is available (protein degradation).

o Review water heaters

Organisms may be characterized by three groupings

o Group I sulfate reducers Use lactate, pyruvate, many alcohols, and some fatty acids as electron donors

when converting SO4 to H2S Produce acetate as an end product of metabolism

o Group II sulfate reducers Use fatty acids (especially acetate) and oxidize substrate completely to CO2

while converting SO4 to H2S Some may grow chemoautotrophically using H2 as the electron donor (acetyl

CoA pathway)

o Dissimilatory sulfur reducers Organisms that reduce elemental sulfur to sulfide, but cannot reduce sulfate to

sulfide SO4 S H2S Use acetate and ethanol are common electron donors

Many facultative aerobes reduce So (and other S compounds as thiosulfate,sulfite, DMSO) to H2S (Proteus, Salmonella) BUT dissimilatory S reducers are

exclusively anaerobic and S is the only electron acceptor.

o A look at the organisms (Source of Table 12.21: Madigan et al. 2003)


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Physiology

The physiology of SO4 reduction takes place 1) in cytoplasm, where carbon metabolismtakes place and 2) at the cell membrane where the energetic reactions occur

Some notes:

o SO4 is a stable ion and must be activated before use. Activation takes place by

the enzyme ATP sulfurylase and uses ATP to create adenosine phosphosulfate

(APS). This requires the equivalent of 2 ATPs since the product of this reactionis AMP (Source of Figure 17.38: Madigan et al. 2003)

Carbon Flow

In the example we will follow, carbon flow proceeds from lactate to acetyl CoA throughpyruvate. Acetyl CoA produced may be (depending on the organism)

o completely oxidized to CO2,

o processed by a modified TCA pathway, or

o processed by the Acetyl CoA pathway

Reducing power generated in carbon flow is used in the energetics


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Energetics

o A sequence requiring 8

electrons (generated by carbon

flow)

o Several forms of bisulfite

reductase are known: as

desulfoviridin and

desulforubidin. They are

named according to theirabsorption spectrum

o A look at the energetics linkedto carbon flow (Figure 17.39

(adjacent) from Madigan et al.

2003)

Cyto c

8 H+8 e

2 NADH + H+

Energetics Carbon Flow2 Lactate

2 Pyruvate 2 Acetyl CoA 2 Acetyl P

2 Acetate

to...Acetyl CoA

Pathway

to Modified TCA

Pathway (G

pyruvate-ferrodoxin

oxidoreductasephosphotransacetylase

acetate kinase

Hydrognease

lactate dehydrogenase

Hmc

8 e

FeS

2 NADH + H+

4H2

2 e

H+

ATP

3 H2O

ATP sulfurylase

APS reductase

bisulfite reductase

SO4

APS

HSO3-

(bisulfite)

S3O6 (Trithionite)

S2O3 (Thiosulfate)

HS-

2 e

2 e

2 e

AMP

2 HSO3-

5 H+

HSO3-

HSO3-2 H+

H+

CoA

CoACO2

Pi

2 ADP2 ATP

(Group II)DesulfobacteriumDesulfotomaculum

roup I)Desulfobacter

Excreted(Desulfovibrio sesulfurincans)


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The Acetyl CoA pathway (Source of Figure 13.4: White 2000)

o This is an important pathway for many organisms and assumes as much importance as

the TCA cycle in others.

o The pathway is used to

oxidize acetate and for

autotrophic growth using CO2.

o We will consider this pathway in one form or another when looking at the sulfatereducers, the homoacetogens, and the methanogens.

In the Sulfate reduces we have two aspects of the cycle to consider

o Oxidation of acetate to CO2 and

o Autotrophic fixation of CO2

Oxidation of Acetate

-------------------- =

Production of acetateby non-acetate

oxidizing sulfate

reducers

-------------------- =

Production of CO2 byacetate oxidizing

sulfate reducers


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Autotrophic function (Source of Figure 13.4: White 2000)

A series of reactions result in the reduction of CO2 to CH3 bound to tetrahydrofolic acid (THF)

The CH3 is transferred to a corrinoid enzyme (CO-E) [Corrinoid enzymes are proteins containing

B12 derivatives as prosthetic groups. B12 is a cobalt containing coenzyme].

CH3 is transferred to carbon monoxide dehydrogenase (CODH) which also catalyses theconversion of CO2 to CO

A second CO2, which becomes the carbonyl carbon (CO) bound to the enzyme is reduced byCODH. This will eventually become the carboxyl carbon (COOH) of acetate.

Electrons to do this come from H2

CODH condenses CH3 and CO to form CH3COa bound acetyl group

Bound acetyl group reacts with CoA~SH to form acetyl CoA which will be acted upon by pyruvate

synthase (a ferredoxin-linked enzyme) which will add a CO2 to create pyruvate. The pyruvate canbe incorporated into cell material.


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Sulfur metabolism in those cells utilizing acetate by the TCA cycle (Desulfobacter) (Source of Figure13.9: White 2000)

Initial step is activation of acetate to acetyl CoA at the expense of succinyl CoA. This is anenergy saving stepnormally to make acetyl CoA from acetate requires ATP (acetate + ATP +

CoA Acetyl CoA)

Citrate lyase cleaves acetyl CoA and conserves the energy to ATP via substrate-level

phosphorylation

Acetate

CoA transferase


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The Acetogens (Source of Figure 13.4: White 2000)

These are obligate anaerobes utilizing CO2 as the terminal electron acceptor and theyproduce acetate as the only product of anaerobic respiration.

The overall reaction looks like4H2 + H

++ 2HCO3

- CH3COO

-+ 4H2O

Electrons for reduction of CO2 come from H2 (chemolithotrophic growth) or other C1compounds, sugars, organic acids, alcohols, amino acids, and certain nitrogen bases

(chemoorganotrophic growth).

CO2 is converted to acetate via the acetyl CoA pathway

A diverse group of organism are capable of this type of metabolism


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References:

Madigan, M.T., J.M. Martinko, and J. Parker. 2003. Brock: Biology of Microorganisms. Prentice Hall.

White, D. 2000. The physiology and biochemistry of prokaryotes. Oxford University Press, NewYork.

Sulfate and Sulfur Reducing Bacteria

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