Sulfur Biogeochemistry—Past and Present

4 V. BrüchertFigure 1. Phylogenetic distribution of sulfur isotope fractionations based on 16S rDNA sequence similarities (modified from Detmers et al., 2001a).Fractionations for all shown strains were determined on growing batch cultures with excess substrate under optimum growth conditions.3. Isotope fractionations of specific sulfate-reducing bacteriavary in some but not all cases as a function of cell-specificsulfate-reduction rate (Kaplan and Rittenberg, 1964; Kemp andThode, 1968; Detmers et al., 2001a; Böttcher et al., 1999). Likewise,temperature, a fundamental regulator of sulfate reductionrate, does not affect isotope fractionation in a unique way. Isotopefractionation increased with decreasing temperature in experimentswith Desulfovibrio desulfuricans (Kaplan and Rittenberg,1964); however, this strain does not grow at the lowest experimentaltemperatures used in this particular study and more likelyexhibited a stress response. In more recent studies with strainscapable of growth at all experimental temperatures, temperaturehad no effect on isotope fractionation (Brüchert et al., 2001).4. There appears to be a close relationship between thepathway of organic electron donor and hydrogen oxidation andthe isotope fractionation. Fractionations of