Dissimilatory Sulphur Metabolism in Phototrophic ‘Non-sulphur’ Bacteria

SUMMARY: Four species of the purple ‘non-sulphur’ bacteria (Rhodospirillaceae) were examined with respect to their dissimilatory sulphur metabolism. Under anaerobic conditions with CO2 as sole carbon source, cell suspensions of Rhodopseudomonas sulfoviridis, Rhodobacter veldkampii and Rhodobacter adriaticus, all dependent on reduced sulphur compounds for growth, oxidized sulphide to an intermediate compound (elemental sulphur, possibly polysulphides) that was converted either partly (R. veldkampii) or completely into sulphate, whereas thiosulphate oxidation occurred without detectable intermediates. In contrast, Rhodobacter sulfidophilus oxidized sulphide as well as thiosulphate to sulphate. Both oxidations occurred with simultaneous excretion of sulphate and sulphite; the latter was subsequently also transformed into sulphate. In cell-free extracts the presence of a reverse sulphite reductase could not be proven. An adenosine 5-phosphosulphate (APS) reductase could not be detected either, but all strains contained a membrane-bound sulphite oxidoreductase that is obviously responsible for sulphate production. This enzyme was solubilized and partly purified from R. adriaticus, Rps. sulfoviridis and R. sulfidophilus.