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

Growing cultures of the green obligate photolithotroph, Chlorobaculum parvum DSM 263T (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 34S, with a maximum delta34S above +5 per thousand, while the residual sulfide was progressively depleted in 34S to delta34S values greater than -4 per thousand. As oxidation proceeded, the delta34S 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 all the sulfur provided throughout oxidation. The mean isotope fractionation factors (epsilon) for sulfide and sulfur were equivalent at epsilon values of -2.4 per thousand and +2.4 per thousand respectively. The significance of the experimentally-observed fractionation to the 34S/32S ratios seen in natural sulfur-containing minerals is considered.