Abstract
Studies of microbial sulfate reduction have suggested that the magnitude of sulfur isotope fractionation varies with sulfate concentration. Small apparent sulfur isotope fractionations preserved in Archean rocks have been interpreted as suggesting Archean sulfate concentrations of <200 μm, while larger fractionations thereafter have been interpreted to require higher concentrations. In this work, we demonstrate that fractionation imposed by sulfate reduction can be a function of concentration over a millimolar range, but that nature of this relationship depends on the organism studied. Two sulfate-reducing bacteria grown in continuous culture with sulfate concentrations ranging from 0.1 to 6 mm showed markedly different relationships between sulfate concentration and isotope fractionation. Desulfovibrio vulgaris str. Hildenborough showed a large and relatively constant isotope fractionation ((34) εSO 4-H2S ≅ 25‰), while fractionation by Desulfovibrio alaskensis G20 strongly correlated with sulfate concentration over the same range. Both data sets can be modeled as Michaelis-Menten (MM)-type relationships but with very different MM constants, suggesting that the fractionations imposed by these organisms are highly dependent on strain-specific factors. These data reveal complexity in the sulfate concentration-fractionation relationship. Fractionation during MSR relates to sulfate concentration but also to strain-specific physiological parameters such as the affinity for sulfate and electron donors. Previous studies have suggested that the sulfate concentration-fractionation relationship is best described with a MM fit. We present a simple model in which the MM fit with sulfate concentration and hyperbolic fit with growth rate emerge from simple physiological assumptions. As both environmental and biological factors influence the fractionation recorded in geological samples, understanding their relationship is critical to interpreting the sulfur isotope record. As the uptake machinery for both sulfate and electrons has been subject to selective pressure over Earth history, its evolution may complicate efforts to uniquely reconstruct ambient sulfate concentrations from a single sulfur isotopic composition.
Highlights
Studies of microbial sulfate reduction have suggested that the magnitude of sulfur isotope preserved in Archean rocks have been interpreted as suggesting Archaean sulfate concentrations sulfate concentrations
The ratio of MM relationships for sulfate and electron donor uptake produces the relationships relationship with growth rate. Geological samples, understanding their relationship is critical to interpreting the sulfur isotope selective pressure over Earth history, its evolution may complicate efforts to uniquely reconstruct seen in experimental studies: a MM relationship with sulfate concentration, and a hyperbolic
We examine pure strains complexities introduced by multiple competing strains, each with potentially different rather than enrichment cultures or diverse sedimentary communities in order to avoid sulfate affinities and transport kinetics
Summary
Surface oxidation state through time (Berner and Canfield, 1989; Canfield, 2004). Today,. Low K s values have been observed more frequently in freshwater demonstrate that individual microbial strains within a community can have different apparent K s , of a given environment This is consistent with genomic analyses (Hauser et al, 2011; sulfate transporters, possibly of varying sulfate K s and V max (maximal transport rate). V max /K s (Aksnes and Egge, 1991; Button, 1985; Healey, 1980; Smith et al, 2009) This term strains with a higher A s are able to import sulfate more efficiently into the cell, the opportunity for isotope fractionation should increase; at low transport velocities (i.e. sulfate import rates), transported sulfate is likely to be quantitatively reduced to sulfide, which due to mass balance. We present the factors that can explain the observed differences, and discuss the ramifications of these experimental design and results, consider potential physiological and environmental data on interpretations of the geological sulfur isotope record
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