BACTERIAL SULFUR RESPIRATION

The reduction of elemental sulphur to HS- is coupled to the phosphorylation of ADP with inorganic phosphate in the catabolism of certain anaerobic bacteria. This process is called sulphur respiration. Since the discovery of sulphur respiration with the isolation of Desulfuromonas acetoxidans (Pfennig and Biebl 1976), many other bacteria have been found that catalyze the reduction of elemental sulphur (for review see Widdel 1988; Stetter et al. 1990; Adams 1990 ; Fauque et al. 1991). Most of these organisms belong to the group of extremely thermophilic archaebacteria. Also some species of Thermotoga, the earliest branch of the eubacterial domain. as well as the methanogens catalyze sulphur reduction. These organisms are thought to be more closely related to the common ancestor than any other living organism. Therefore, sulphur respiration may represent one of the first means of biological energy conservation in evolution (Stetter and Gaag 1983). For bacteria that grow with H-2 or formate as electron donors and sulphur as acceptor it is evident from growth that sulphur reduction is coupled to phosphorylation, since the growth reactions do not allow substrate level phosphorylation. In contrast, the metabolic significance of sulphur reduction is uncertain in other bacteria. After a brief discussion of the biology of sulphur reducers, this review will focus on the chemistry, enzymology and bioenergetics of catabolic sulphur reduction. Sulphur reduction serving anabolic purposes will not be discussed here (for review see Le Faou et al. 1990).