If

Sulfate reducers

Sulfite reducers

Thio-sulfate

Thiosulfate reducers

Sulfide Oxidizers

Sulfite

Elemental sulfur oxidizers

SO42-Sulfate

S2O32-

SO32"

Figure 13.25 Biochemical sulfur transformations.

archaea associated with some of the nonassimilative sulfur reductions (use as an electron acceptor) and oxidations (use as an energy source) are listed in Table 13.5.

Also as with nitrogen, mineralization of organic sulfur does not involve oxidation or reduction of the sulfur, which is released as sulfide. However, assimilation of sulfide is uncommon, largely because of its toxicity. Instead, most organisms utilize sulfate (requiring a reduction) and/or organic sulfur.

13.3.1 Sulfur Reduction

Some bacteria and archaea, including chemoorganotrophs and chemolithotrophs, use sulfate or other oxidized forms of sulfur as an electron acceptor for anaerobic respiration, producing further reduced intermediates (sulfite, thiosulfate, elemental sulfur) or fully reduced sulfide. In addition, many organisms can assimilate sulfate through a reductive process.

Dissimilatory Sulfate and Sulfur Reduction The complete reduction of sulfate to hydrogen sulfide, during its use as an electron acceptor for anaerobic respiration, can be achieved by specialized but common and widespread groups of strictly anaerobic bacteria, referred to functionally as sulfate-reducing bacteria (SRBs). The best known of these are Desulfovibrio, a proteobacteria, and Desulfotomaculum, a gram-positive endo-sporeformer, although other genera are now recognized as well. Interestingly, one archaea, Archaeoglobus, is also known that can carry out this reaction. Some other anaerobic bacteria and archaea can reduce elemental sulfur to sulfide.

It was initially believed that SRBs could only utilize a few fermentation products, such as H2, lactate, and pyruvate. However, it is now realized that some can utilize acetate and

TABLE 13.5 Examples of Prokaryotes Able to Oxidize or Reduce (Dissimilatory) Inorganic Sulfur

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