Microscale reservoir effects on microbial sulfur isotope fractionation

Abstract

Microbial sulfate reduction can impart strong sulfur isotope fractionation by preferentially using the lighter 32SO42- over the heavier 34SO42-. The magnitude of fractionation depends on a number of factors, including ambient concentrations of sulfate and electron donors. Sulfur isotope compositions in sedimentary rocks thus facilitate reconstruction of past environmental conditions, such as seawater sulfate concentrations, primary productivity, organic carbon burial, and sulfur fluxes into or out of the ocean. Knowing the processes that regulate the magnitude of sulfur isotope fractionation is necessary for the correct interpretation of the geological record, but so far theoretical work has focused mostly on internal cellular processes. In sulfate-limited environments, like low sulfate lakes and the Archean ocean, microbial sulfate reduction can lead to sulfate depletion in the water column and an enrichment in isotopically heavy sulfate. This reservoir effect in turn mutes the fractionation expressed in the water column and ultimately preserved in sediments relative to the biologically induced fractionation. Here we use mathematical modeling to show that similar reservoir effects can also appear at the microscale in close proximity to sulfate-reducing cells. These microscale reservoir effects have the potential to modulate sulfur isotope fractionation to a considerable degree, especially at low (micromolar) sulfate concentrations. As a result, background sulfate concentrations, sulfate reduction rates, and extracellular ion diffusion rates can influence the fractionation expressed even if the physiologically induced fractionation is constant. This has implications for the interpretation of biogenic sulfur isotope fractionations expressed in the geological record, because the correct estimation of the environmental conditions that would promote these fractionations requires consideration of microscale reservoir effects. We discuss these implications, and demonstrate the integration of microscale reservoir effects into geobiological models for low sulfate marine water columns, as perceived for the Archean ocean.