A Combined Pathway of Sulphur Compound Disproportionation by a Sulphate-Reducing Bacterium (Desulfovibrio Desulfuricans): Evidence from Stable Sulphur Isotopes
Elemental sulphur and thiosulphate are important intermediates in the sulphur cycle of marine and lirnnic sediments. They are mainly formed during reoxidation of sulphide, the product of dissimilatory sulphate reduction, and can undergo various transformations, i.e. oxidation, reduction or disproportionation (Jorgensen, 1990; Canfield and Thamdrup, 1994) . It has been shown that in sediments a large part of sulphide is recycled via thiosulphate or sulphur disproportionation to sulphide and
... o sulphide and sulphate (Jorgensen, 1990; Canfield and Thamdrup, 1994) . Large 34S depletions are found in sedimentary sulphides, which are much higher then fractionations observed in pure culture studies on dissimilatory sulphate reduction. It has been considered that sulphur isotope fractionation during disproportionation processes contributes to the additional isotope fractionation observed in nature (Jorgensen, 1990), and it was shown experimentally for disproportionation of elemental sulphur (Canfield and Thamdrup, 1994) and sulphite (Habicht, 1997) . In our present study we have followed the fate of the inner (sulphonate) and outer (sulphane) sulphur atoms during thiosulphate disproportionation by Desulfovibrio desulfuricans. Experimental methods Sulphur isotope fractionation during disproportionation was investigated with growing batch cultures of Desulfovibrio desulfitricans CSN (DSM9104) at 30 C. Sulphide concentration was determined potentiometrically using an Ag/Ag2S electrode. Sulphate and thiosulphate were determined by ion chromatography. Thiosulphate was either oxidized for analysis of the total thiosulphate sulphur or selectively decomposed for separation of sulphonate and sulphane sulphur (Smock et al., 1998) . The sulphur isotopic composition was measured by combustion isotope-ratio-monitoring mass spectrometry (B6ttcher et al., 1998). The isotopic composition is expressed as 834S relative to the V-CDT. Results and discussion In our present study we have followed the fate of the sulphonate and sulphane sulphur atoms during thiosulphate disproportionation by washed cells of Desulfovibrio desulfi~ricans. This bacterium has a constitutive thiosulphate reductase and a constitutive capacity of thiosulphate disproportionation. We expected that disproportionation is catalyzed by thiosulphate reductase and some of the enzymes involved in sulphate reduction. Thiosulphate should be reduced to sulphide and sulphite in a first step, followed by oxidation of sulphite to sulphate, which is necessary to regain the electrons consumed for thiosulphate reduction. For the complete disproportionation we therefore expected to find the isotope signature of the inner sulphur in the produced sulphate and the signature of the outer sulphur in the produced sulphide. During disproportionation, 32S-thiosulphate was preferentially metabolized, and the residual thiosulphate became enriched in 34S (Fig.