MICROBIAL CONTROLS ON SULFUR ISOTOPE PARTITIONING UNDER LOW-NUTRIENT CONDITIONS

Microbial reduction of sulfate is ubiquitous in anoxic environments across the geosphere. Respiration rates of sulfate reducing bacteria (SRB) are commonly inferred in natural, nutrient-limited environments using the characteristic partitioning of stable S isotopes. However, most laboratory experiments use artificially high nutrient (eutrophic) conditions to constrain the fractionation factor through substantial S isotope partitioning. We hypothesize that under natural, nutrient-poor (oligotrophic) conditions, S isotope partitioning carried out by SRB will produce distinct S partitioning when compared to these artificially enhanced conditions. Natural conditions also promote a constant microbial population helping to simplify the δ³⁴S signal.

Our method incorporates eight anoxic bioreactors charged with natural concentrations of nutrients and a living culture of desulfovibrio vulgaris, a common SRB. The reactions are driven by additions of an electron donor, formate, continuously added at a range of input concentrations in order to precisely control respiration rates in each community. Concentrations of sulfate and formate are monitored via ion chromatography and used to determine sulfate reduction rates. Comparing the δ³⁴S of the residual sulfate pool to the δ³⁴S of the initial sulfate pool yields the S fractionation factor carried out by desulfovibrio vulgaris.

Based on our results, we hypothesize that mass dependent sulfate reduction under oligotrophic conditions is coupled to a mass dependence on the half-saturation constant exhibited by SRB. Such a relationship could provide pertinent information about a given microbial community using routine field sampling techniques paired with S isotope analysis. Knowledge of the subsurface microbial community and its ability to reduce chemical species is critical for safe storage of contaminants as well as stewardship of newly discovered and preexisting hydrologic regimes.