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 265

Many PSB and GSB can oxidize externally supplied solid elemental sulfur.
Utilization of solid S 0 must include binding and/or activation of the sulfur as well
as transport inside the cells. In the case of cyclo-octasulfur, this activation process
could be an opening of the S 8 ring by nucleophilic reagents, leading to the
formation of linear organic or inorganic polysulfanes. It may be speculated that
“sulfur chains” rather than the more stable “sulfur rings” are the microbiologically
preferred form of S 0 for most SOB. It has been proposed in Alc. vinosum that the
stored sulfur has to be reductively activated to the oxidation state of sulfide in order
to serve as a substrate for sulfite reductase operating in reverse performing the
six-electron oxidation of sulfide to sulfite [ 159 ].
4.2.3 Oxidation of Stored Sulfur to Sulfite
The mechanism by which the periplasmically stored sulfur is made available to the
cytoplasmic dissimilatory sulfite reductase is still unclear. The only gene region
known so far to be essential for the oxidation of stored sulfur is the dsr operon. The
reverse DsrAB of Alc. vinosum is encoded together with 13 other proteins in the dsr
operon, dsr ABEFHCMKLJOPNRS . A model of the Alc. vinosum sulfur oxidation
pathway has been proposed [ 160 ]. It is suggested that the sulfur is reductively
activated, transported to and further oxidized in the cytoplasm, since the proteins
encoded at the dsr locus are either membrane-bound or cytoplasmic and cannot act
directly on the extracytoplasmic sulfur globules [ 160 ].
4.2.4 Oxidation of Sulfite to Sulfate
Two different pathways for sulfite oxidation are known in phototrophic and
chemotrophic SOB: (a) Indirect, AMP-dependent sulfite oxidation via APS. Sulfite
is oxidized by APS reductase in a cytoplasmic reaction that consumes sulfite and
AMP and produces APS and reducing equivalents. This oxidative pathway occurs
exclusively in members of the Chromatiaceae and in some GSB. (b) Direct sulfite
oxidation by sulfite dehydrogenase (EC 1.8.2.1), typically a molybdenum-containing
protein belonging to the sulfite oxidase family of molybdoenzymes [ 161 ].
4.2.5 Oxidation of Thiosulfate
In phototrophic and chemotrophic SOB that do not form sulfur deposits a periplasmic
thiosulfate-oxidizing multienzyme complex (Sox complex) is responsible for forma-
tion of sulfate from thiosulfate. Alc. vinosum can pursue two different thiosulfate
oxidation pathways, first the complete thiosulfate oxidation to sulfate, and second
formation of tetrathionate.