Diversity of sulfur isotope fractionations by sulfate-reducing prokaryotes

Batch culture experiments were performed with 32 different sulfate-reducing prokaryotes to explore the diversity in sulfur isotope fractionation during dissimilatory sulfate reduction by pure cultures. The selected strains reflect the phylogenetic and physiologic diversity of presently known sulfate reducers and cover a broad range of natural marine and freshwater habitats. Experimental conditions were designed to achieve optimum growth conditions with respect to electron donors, salinity, temperature, and pH. Under these optimized conditions, experimental fractionation factors ranged from 2.0 to 42.0‰. Salinity, incubation temperature, pH, and phylogeny had no systematic effect on the sulfur isotope fractionation. There was no correlation between isotope fractionation and sulfate reduction rate. The type of dissimilatory bisulfite reductase also had no effect on fractionation. Sulfate reducers that oxidized the carbon source completely to CO2 showed greater fractionations than sulfate reducers that released acetate as the final product of carbon oxidation. Different metabolic pathways and variable regulation of sulfate transport across the cell membrane all potentially affect isotope fractionation. Previous models that explained fractionation only in terms of sulfate reduction rates appear to be oversimplified. The species-specific physiology of each sulfate reducer thus needs to be taken into account to understand the regulation of sulfur isotope fractionation during dissimilatory sulfate reduction. The stable sulfur isotope ratio between 32S and 34S of solid and dissolved sulfur compounds is widely used as a marker for bacterial sulfate reduction and bacterial processes associated with the recycling of sulfide (5, 8, 18). The reduction of sulfate by sulfate reducers is coupled to a pronounced enrichment of 32S in the produced sulfide. However, the extent of the isotope enrichment remains a matter of ongoing debate. Results from batch-culture, continuous-culture, and resting-cell experiments suggested that the isotope enrichment is inversely proportional to sulfate reduction rates (9, 22, 23). Furthermore, below a threshold concentration of sulfate, the discrimination against 34S apparently decreases (21). Previous experimental studies of the isotope fractionation were conducted with only a few selected species that were known at that time, mainly Desulfovibrio spp. and two Desulfotomaculum spp. (9, 15, 22, 28). Moreover, since most of these species were isolated from freshwater environments, they are not necessarily of ecological importance in marine environments. The different electron donors used in these early pure-culture studies included ethanol, lactate, acetate, pyruvate, glucose, yeast extract, and hydrogen (22, 23). Today, a number of sulfate reducers are known that can metabolize a wide range of substrates including long-chain fatty acids, alcohols, and even aromatic compounds that represent relevant substrates for natural environments (33, 45,47). Hydrogen, propionate, butyrate, and acetate appear to be the most important electron donors for sulfate reducers in natural marine environments (31, 40), but propionate and butyrate have never been used as electron donors in sulfur isotope fractionation experiments. There is a need to expand the existing database of sulfur isotope fractionations by sulfate reducers with organisms that are important in natural environments and to conduct experiments with additional relevant electron donors. For this reason, we included microorganisms that cover the total temperature range of environments from which sulfate reducers have been isolated. Furthermore, we used a variety of likely natural substrates and conducted experiments at different salinities and pHs to cover as broad a range of natural conditions as possible