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 ReS2(s), and the iron-reducing cultures may have reached supersaturation relative to ReO2(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.
Document Type: Research Article
Affiliations: 1: Department of Chemistry and Biochemistry, University of Maryland, College Park, MD, USA,Smithsonian Environmental Research Center, Edgewater, MD, USA 2: Smithsonian Environmental Research Center, Edgewater, MD, USA 3: Department of Chemistry and Biochemistry, University of Maryland, College Park, MD, USA
Publication date: October 1, 2009