Diversity of sulfur isotope fractionations by sulfate-reducing prokaryotes

Abstract

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 per thousand. 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.

FIG. 1

Phylogenetic affiliation and sulfur isotope fractionation factors of investigated sulfate-reducing microorganisms. Neighbor-joining tree based on 1,308 positions of nearly full-length 16S rRNA sequences from 30 bacteria. Archaeoglobus fulgidus was taken to root the tree. Trees constructed with other tree reconstruction algorithms (maximum likelihood and parsimony) resulted in general in the same overall tree topology. The bar indicates 10% sequence divergence.

FIG. 2

Relationship between sulfate reduction rates in the mid-exponential growth phase (femtomoles of sulfate reduced per cell per day) and isotope fractionation. Each data point represents a different culture. The different substrates used are shown by the different symbols. Growth conditions were optimized for each culture so that the sulfate reduction rates were presumably close to the maximum potential rates for each organism.