Anaerobic oxidation of sulphur compounds as electron donors for bacterial photosynthesis

Abstract

Most phototrophic bacteria use reduced inorganic sulphur compounds as electron donors during anoxygenic photosynthesis. Principally, sulphide is oxidized via sulphite to sulphate. Elemental sulphur may appear as intermediary storage product (inside: Chromatium, Thiocapsa ; outside: Chlorbium, Ectothiorhodospira ; not in : Rhodopseudomonas sulfidophila ). Adenosine phosphosulphate is an intermediate in sulphite oxidation by Chromatium, Thiocapsa and Chlorobium . Thiosulphate undergoes splitting to sulphide (or elemental sulphur) and sulphite, or is oxidized to tetrathionate. Sulphide may be oxidized to elemental sulphur by cytochrome c or to thiosulphate (perhaps sulphite?) by flavocytochrome c , or to sulphite by a reverse (sirohaem) sulphite reductase. The latter enzyme also oxidizes polysulphides and probably elemental sulphur. Sulphite is either oxidized by APS reductase to form adenosine phosphosulphate - from which sulphate is released by ADP sulphurylase - or by sulphite: acceptor oxidoreductase directly to sulphate. The electron acceptor of most of these oxidative enzymes are cytochromes or non-haem iron-sulphur proteins. The pathways of photolithotrophic sulphur oxidation in Chlorobiaceae, Chromatiaceae and Rhodospirillaceae are separately compiled under evaluation of the presently available data.