Hydrogen Sulfide: A Toxic Gas Produced by Dissimilatory Sulfate and Sulfur Reduction and Consumed by Microbial Oxidation - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 241

1.1.3 Sulfur-, Sulfite-, and Thiosulfate-Disproportionating Bacteria

The disproportionation (or dismutation) of inorganic sulfur intermediates (also
called “inorganic sulfur compound fermentation”) at moderate temperatures (0-
80 C) consists of a microbiologically catalyzed chemolithotrophic process in
which compounds such as sulfite, thiosulfate, and S 0 serve as both electron donor
and acceptor to produce sulfate plus sulfide [ 44 ]. Reactions involving dispropor-
tionation of thiosulfate, sulfite, and S 0 are listed in equations ( 1 ), ( 2 ), and ( 3 ),
respectively.

S 2 O 2 3 SO 2 4 HS þ H þ G 0' 9 kJ mol 1 S 2 O 2 3 þ H 2 O ! þ ʔ ¼ 21 : ½

ð

41

1 Þ

4SO 2 3 H þ ! 3SO 2

4

HS G 0' 9 kJ mol 1 SO 2 3 þ þ ʔ ¼ 58 : ½

ð

41

2 Þ

4S 0 þ 4H 2 O ! SO 2

4

þ 3HS þ 5H þ G 0' ¼ þ 10 : 2 kJ mol 1 S 0 4½

ð

ʔ

3 Þ

Desulfovibrio sulfodismutans DSM 3696 was the first bacterium isolated able to
carry out the disproportionation of sulfite and thiosulfate to sulfide plus sulfate
[ 45 ]. Desulfocapsa sulfoexigens was the first microorganism able to disproportion-
ate S 0 to sulfate and sulfide [ 46 ]. The disproportionation of sulfur compounds is
associated with only small free energy changes [ 47 ]. Only three Desulfocapsa
species are able to disproportionate S 0 , sulfite, and thiosulfate with growth.
The capacities of dissimilatory sulfate reduction and of sulfite and thiosulfate
disproportionation are constitutively present in D. desulfuricans CSN (DSM 9104)
and D. sulfodismutans . In contrast, ATP sulfurylase and sulfite oxidoreductase
activities were not detected in these last two strains. During sulfite and thiosulfate
dismutation, sulfate is formed via APS reductase and ATP sulfurylase, but not by
sulfite oxidoreductase [ 47 ]. Elemental sulfur-disproportionating bacteria can be
traced back in time as long as 3.5 billion years indicating that elemental sulfur
dismutation would be one of the oldest biological processes on Earth [ 44 , 48 ].

1.1.4 Sulfide-, Sulfur-, Sulfite-, and Thiosulfate-Oxidizing Bacteria

Biological oxidation of hydrogen sulfide to sulfate is one of the major reactions of the
biological sulfur cycle (Figure 1 ). Chemotrophic sulfur-oxidizing bacteria (SOB) are
found in four classes of the Proteobacteria (Alphaproteobacteria, Betaproteobacteria,
Gammaproteobacteria, and Epsilonproteobacteria) whereas anoxygenic phototrophic
sulfur bacteria are only present in Gammaproteobacteria [ 49 ].