General Microbiology - Schlegel H. 1987

Electron transport under anaerobic conditions
Hydrogen sulfide production during sulfur reduction

It has long been known that a Yeast suspension fermenting sugar pulls added flowers of sulfur into a "whirlpool of biological reductions," resulting in The formation of hydrogen sulfide. However, it was only recently discovered that certain Bacteria can also grow in the presence of elemental sulfur, using it as a hydrogen acceptor in anaerobic electron transport. In the process, sulfur is reduced to hydrogen sulfide. This process can be termed sulfur Respiration.

Currently, the species Desulfuromonas acetoxidans is well studied. This bacterium, which moves via a laterally positioned flagellum, can be isolated from seawater to establish an enrichment culture in a medium containing mineral salts, acetate, and flowers of sulfur under strict anaerobic conditions. D. acetoxidans oxidizes acetate or ethanol to СO₂. By virtue of its ability to reduce sulfur and completely oxidize organic substrates, this species differs from most members of the genus Desulfovibrio, which can reduce sulfate or sulfite but not sulfur, and oxidize lactate only to acetate, for example. D. acetoxidans contains a low-redox-potential cytochrome c₇ as well as a protein containing four Fe atoms and four S atoms. In addition, this bacterium possesses the Enzymes of The Tricarboxylic Acid Cycle.

D. acetoxidans can live in a syntrophic association with phototrophic green sulfur bacteria, which oxidize hydrogen sulfide to sulfur in the light and fix СO₂ (Fig. 9.5).

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Fig. 9.5. A mixed culture of sulfur-reducing bacteria (Desulfuromonas acetoxidans) and green phototrophic bacteria (Chlorobiaceae) on an Agar medium. In the large central colony of D. acetoxidans, acetate oxidation and sulfur reduction take place. Hydrogen sulfide diffuses into the surrounding agar and serves as a hydrogen donor for Chlorobium Cells. The sulfur released by these cells diffuses (likely as polysulfide) back to the central colony, acts as a hydrogen acceptor, and is reduced once again. The Symbiosis between these two bacteria is a striking example of a syntrophic association with bidirectional substrate transfer.