Dissimilatory sulfate- and sulfur-reducing prokaryotes

Abstract

This chapter provides an overview of prokaryotes that reduce oxygenated sulfate or elemental sulfur in their energy metabolism. Sulfate-reducing bacteria gain energy for cell synthesis and growth by coupling the oxidation of organic compounds or molecular hydrogen (H 2) to the reduction of sulfate (SO 4 2-) to sulfide (H 2 S, HS -). Sulfur-reducing strains reduce elemental sulfur (or other lower oxidation states of this element, S 0, S 8) but not sulfate. The electron transport to the inorganic electron acceptors is associated with a mode of energy conservation that may be regarded as an anaerobic analogue to respiration with O 2. Among the anaerobic respirations, the reduction of sulfur species is most noteworthy because it gives rise to a toxic end product, hydrogen sulfide (H 2 S). Within the sulfur cycle, this end product serves as electron donor for a great diversity of aerobic chemotrophic and anoxygenic phototrophic microorganisms that may form visible blooms in sulfidic habitats. The sulfate- and sulfur-reducing prokaryotes do not form a phylogenetically coherent group, but members are found in several phyla within the domains Archaea and Bacteria. Energy generating mechanisms as well as other physiological, biochemical, and molecular biological aspects of this ancient group of organisms are described in detail.