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 δ³⁴S above +5‰, while the residual sulfide was progressively depleted in ³⁴S to δ³⁴S values greater than -4‰. As oxidation proceeded, the δS 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 (e) for sulfide and sulfur were equivalent at e values of -2.4‰ and +2.4‰ respectively. The significance of the experimentally-observed fractionation to the ³⁴S/³²S ratios seen in natural sulfur-containing minerals is considered.
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