Molecular and Cell Biology 112

Dissimilatory Sulfate- and Sulfur-Reducing Bacteria

November 9, 1998

Assimilative versus dissimilative sulfate reduction:
- Assimilative sulfate reduction: Sulfate serves as sulfur source for biosynthesis
(many organisms including higher plants, algae, fungi, and most prokaryotes).
- Dissimilative sulfate (and sulfur) reduction: Sulfate (sulfur) serves as terminal
electron acceptor (only sulfate- and sulfur-reducing bacteria).

The sulfate anion (SO42-) is chemically very stable. Dissimilative sulfate reduction is,
therefore, the main source of sulfide (S2-) in marine environments. This process is
carried out by sulfate-reducing bacteria.

Global sulfur emissions = 1.5 x 1014 g/year.
25% arises from the biological reduction of sulfate.

4H2 + SO42- + H+ -> HS- + 4H2O

DG0' = -158 kJ/reaction

H2 + S0 -> HS- + H+

DG0' = -28 kJ/reaction

 

Table1. Oxidation states of some inorganic sulfur compounds. (See Brock, p.505, Table
13.5 for a complete list.)

Inorganic sulfur compound

Oxidation state

Sulfide: H2S -> HS- -> S2-

-2

Elemental Sulfur: S0

0

Thiosulfate: S2O32-

+2

Sulfite: SO32-

+4

Sulfate: SO42-

+6

 

" Obligate anaerobes

" In anaerobic environments that are rich in sulfates (seewater environments:
[SO42-] = 28 mM; compare freshwater environments: [SO42-] = 0.02 mM),
sulfate reducing bacteria usually outcompete methanogens as terminal organisms.
Thermodynamics favors the sulfate reducing bacteria (see lecture notes on
Methanogens; Nov.2). Sulfate reducing bacteria also have higher affinities
and lower thresholds for key substrates (Table 2).

Table 2. Substrate affinities and thresholds of Sulfate Reducers and Methanogens.

 

Acetate, Km (mM)

H2 Uptake, Km (ÁM)

H2 Threshold (ppm)

Sulfate Reducers

0.2

0.7-1.9

5-24

Methanogens

3.0

2.5-13

25-100

" Many substrates serve as electron donors for sulfate reduction:
H2; volatile fatty acids (formate, acetate, propionate, butyrate, etc.); branched-chain
fatty acids; other mono-, di-, and tricarboxylic acids (lactate, pyruvate, succinate,
etc.); alcohols; aromatic compounds (benzoate and derivatives); heterocyclic
compounds (indole, etc.); amino acids; glycerol; sugars (a few).

" The sulfate reducers are placed in two physiological subgroups according to the compounds they utilize as energy and carbon sources (Table 3).
Group I Sulfate Reducers: Non-acetate Oxidizers:
utilize lactate, pyruvate, ethanol, certain fatty acids.
Group II Sulfate Reducers: Acetate Oxidizers:
specialize in the oxidation of fatty acids, particularly acetate.

Table 3. Some sulfate- and sulfur-reducing bacteria. (See Brock, p.672, Table 16.12 for a
complete list.)

Genus

Energy Metabolism

Major C-Source

Group I Sulfate Reducers: Non-acetate Oxidizers:

   

Desulfovibrio
(Delta Purple Bacteria)

Incomplete oxidation
(incomplete TCA Cycle)

Organic compounds
(Pyruvate Synthase Pathway)

Thermodesulfobacterium
(Bacteria)

Incomplete oxidation
(incomplete TCA Cycle)

Organic compounds
(Pyruvate Synthase Pathway)

Desulfotomaculum
(Gram-positive Bacteria)

Complete oxidation
(reverse Acetyl-CoA Pathway)

CO2
(Acetyl-CoA Pathway)

Archaeoglobus
(Euryarchaeota)

Complete oxidation
(reverse Acetyl-CoA Pathway)

CO2
(Acetyl-CoA Pathway)

Group II Sulfate Reducers: Acetate Oxidizers:

   

Desulfobacter
(Delta Purple Bacteria)

Complete oxidation
(modified TCA Cycle)

CO2
(reverse TCA Cycle)

Desulfobacterium
(Delta Purple Bacteria)

Complete oxidation
(reverse Acetyl-CoA Pathway)

CO2
(Acetyl-CoA Pathway)

Sulfur Reducer:

   

Desulfuromonas
(Gamma Purple Bacteria)

Complete oxidation
(modified TCA Cycle)

CO2
(reverse TCA Cycle)

 

Energy metabolism:

" Reverse acetyl-CoA pathway.

" Modified TCA cycle.

" Pathway of dissimilatory sulfate reduction. (see also Brock, p. 506,
Figures 13.31 and 13.32)

Important electron carriers included in the electron transport chain:
- Cytochrome c3: a low potential cytochrome (E0’ = -250mV to -340mV).
- Ferredoxins, rubredoxins, flavodoxins, etc.

" Disproportionation of inorganic sulfur compounds (Desulfovibrio sulfodismutans).
A) Disproportionation of thiosulfate:

 

S2O32- + H+ + 2e- -> SO32- + HS-

 
 

SO32- + H2O -> SO42- + 2H+ + 2e -

 

Sum:

S2O32- + H2O -> SO42- + HS- + H+

DG0' = -21.9 kJ/reaction

B) Disproportionation of sulfite:

 

3SO32- + 3H2O+ -> 3SO42- + 6H+ + 6e -

 
 

SO32- + 6e - + 7H+ -> 3H2O + HS-

 

Sum:

4SO32- + H+ -> 3SO42- + HS-

DG0' = -58.9 kJ/reaction

 

" Pathways of carbon assimilation:

Pyruvate Synthase Pathway:
Acetate + ATP + CO2 + 2[H] -> Pyruvate + ADP + Pi

Acetyl-CoA Pathway:
3CO2 + ATP + 10[H] -> Pyruvate + ADP + Pi

Reverse TCA Cycle:
3CO2 + 3ATP + 10[H] -> Pyruvate + 3ADP + 3Pi

 

Pfennig N. (1989) Antonie Van Leeuwenhoek 56:127-38.
"Metabolic diversity among the dissimilatory sulfate-reducing bacteria.
Albert Jan Kluyver memorial lecture."

Thauer, R.K. (1988) Eur. J. Biochem. 176:497-508.
"Citric-acid cycle, 50 years on. Modifications and an alternative pathway in anaerobic
bacteria"

Bak, F. and N. Pfennig (1987) Arch. Microbiol. 147:184-9.

Bak, F. and H. Cypionka (1987) Nature 326:891-2.

Schauder, R. et al. (1986) Arch. Microbiol. 145:162-72.

Odom, J.M. and H.D. Peck, Jr. (1984) Annu. Rev. Microbiol. 38:551-92.
"Hydrogenase, electron-transfer proteins, and energy coupling in the sulfate-reducing
bacteria Desulfovibrio"

Brandis-Heep, A. et al. (1983) Arch. Microbiol. 136:222-9.

Widdel, F. and N. Pfennig (1977) Arch. Microbiol. 112:119-22.
"A new anaerobic, sporing, acetate-oxidizing, sulfate-reducing bacterium,
Desulfotomaculum (emend.) acetoxidans."

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