Distinct Microbial Behavior of Re Compared to Tc: Evidence Against Microbial Re Fixation in Aquatic Sediments

Abstract

Rhenium is enriched in suboxic and anoxic sediments relative to oxic sediments, a characteristic that is being exploited in its use as a paleoredox indicator. Rhenium is fixed at sediment depths where iron reduction and sulfate reduction are the dominant microbial terminal electron-accepting processes. In order to explore mechanisms of its fixation, we investigated perrhenate behavior in pure, batch cultures of two dissimilatory sulfate-reducing strains (Desulfovibrio desulfuricans subsp. desulfuricans and Desulfovibrio desulfuricans ND132) and two iron-reducing strains (Geobacter metallireducens GS-15 and Shewanella oneidensis MR-1). Perrhenate concentrations tested ranged from 0.04 to 12 μM, roughly 4 to 7 orders of magnitude larger than seawater Re concentrations. Within this broad concentration range, none of the organisms tested actively removed Re from solution during one week's growth to stationary phase. Despite these results, the sulfate-reducing cultures appeared to have reached supersaturation relative to ReS_2(s), and the iron-reducing cultures may have reached supersaturation relative to ReO_2(s). We conclude that neither direct nor short-term indirect microbial processes involving these bacteria are likely to explain Re fixation in sediments. Our results cannot exclude the possibility that microbial metabolites, such as Fe(II) or sulfide, do drive abiotic Re fixation over longer periods of time. The lack of perrhenate reduction by dissimilatory sulfate-reducing bacteria and iron-reducing bacteria contrasts with published reports of pertechnetate behavior. Despite many qualitative similarities between Re and Tc, it is clear that these two elements are quantitatively dissimilar, with Re fixation requiring more intensely reducing conditions.

Keywords:

• Desulfovibrio
• Geobacter
• Rhenium
• Shewanella
• Technetium

This research was supported by a Smithsonian Institution Predoctoral Fellowship to the corresponding author. The authors also greatly appreciate the assistance of Gerhardt (Fritz) Riedel, Georgia Riedel, Tyler Bell and Nise Butera of the Smithsonian Environmental Research Center and William McDonough and Richard Ash of the University of Maryland Department of Geology.

Notes

∗Data for uninoculated culture medium with and without TiNTA

∗∗TiNTA was added only to active cultures of SRB