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 238

sulfate reducers. Dissimilatory sulfate reduction in Desulfovibrio species is linked
to electron transport-coupled phosphorylation because substrate level phosphory-
lation is inadequate to support their growth [ 18 ]. The SRB belonging to the genus
Desulfovibrio possess a number of unique physiological and biochemical charac-
teristics such as the requirement for ATP to reduce sulfate [ 18 ], the cytoplasmic
localization of two key enzymes [adenosine 5'-phosphosulfate (APS) reductase and
dissimilatory-type sulfite reductase] involved in the pathway of dissimilatory sul-
fate reduction [ 19 , 20 ], the periplasmic localization of some hydrogenases [ 21 , 22 ],
and the abundance of multiheme c -type cytochromes [ 15 , 16 , 23 ].
A set of unique membrane-bound respiratory complexes are involved in sulfate
respiration. The Dsr (dissimilatory sulfite reductase) MKJOP and the Qmo
(quinone-interacting membrane oxidoreductase) ABC complexes are present in
all SRP and are deemed essential for dissimilatory sulfate reduction [ 24 , 25 ]. A
group of other complexes (Hmc, high-molecular weight cytochrome; Nhc,
nineheme cytochrome; Ohc, octaheme cytochrome; Qrc, quinone reductase com-
plex; Tmc, tetraheme membrane cytochrome) are present only in sulfate reducers
that are characterized by a high content of multiheme cytochromes c (mainly the
deltaproteobacterial SRB) [ 24 ]. A model reflecting organization of membrane-
bound respiratory complexes associated with electron transport and cell energetics
in Desulfovibrio species is given in Figure 2 [ 25 ].

1.1.2 Sulfur-Reducing Bacteria and Archaea

Compared with SRP, the environmental distribution of elemental sulfur reducers,
and their quantitative role in carbon cycling, is poorly understood. The mechanism
of elemental sulfur reduction (characterization of enzymes and electron carriers)
has been much less studied than that of dissimilatory sulfate reduction [ 25 - 30 ].

1.1.2.1 Eubacterial Sulfur Reduction

Elemental sulfur is probably the most widespread sulfur species in sediments and
geological deposits. Many biological and chemical oxidation processes of H 2 Sdo
not directly produce sulfate but rather elemental sulfur, which may accumulate
[ 3 ]. Elemental sulfur is relatively reactive and in contrast to sulfate, it requires no
energy-dependent activation before a reduction can take place. The problem in the
utilization of elemental sulfur mainly concerns the low solubility of sulfur flower in
water (0.16 mole per liter at 25 C) [ 31 ]. The so-called “hydrophilic sulfur” is

probably the form available in aqueous medium; it consists of elemental sulfur
associated with small portions of oxocompounds such as polythionates.
Several genera of the domain Bacteria are able to grow by a dissimilatory
reduction of elemental sulfur to sulfide in a respiratory type of metabolism [ 2 , 3 ,
15 , 27 , 29 , 30 , 32 - 36 ]. The facultative sulfur-reducing eubacteria, such as the SRB,
utilize elemental (or colloidal) sulfur as a respiratory substrate in the absence of

μ