Stable Sulfur Isotope Fractionation by the Green Bacterium Chlorobaculum parvum During Photolithoautotrophic Growth on Sulfide

Growing cultures of the green obligate photolithotroph, Chlorobaculum parvum DSM263(T) (formerly Chlorobium vibrioforme forma specialis thiosulfatophilum NCIB 8327), oxidized sulfide quantitatively to elemental sulfur, with no sulfate formation. In the early stages of growth and sulfide oxidation, the sulfur product became significantly enriched with (34)S, with a maximum delta(34)S above +5 parts per thousand, while the residual Sulfide was progressively depleted in (34)S to delta(34)S values greater than -4 parts per thousand. As oxidation proceeded, the delta(34)S of the sulfur declined to approach that of the initial sulfide when most of the substrate sulfide had been converted to sulfur in this closed culture system. No significant formation of sulfate occurred, and the substrate sulfide and elemental sulfur product accounted for ill the sulfur provided throughout oxidation. The mean isotope fractionation factors (epsilon) for sulfide and sulfur were equivalent at c values of -2.4 parts per thousand and +2.4 parts per thousand respectively. The significance of the experimentally-observed fractionation to the (34)S/(32)S ratios seen in natural sulfur-containing minerals is considered.