The ability to predict the success of the microbial reduction of soluble U(VI) to highly insoluble U(IV) as an in situ bioremediation strategy is complicated by the wide range of geochemical conditions at contaminated sites and the strong influence of aqueous uranyl speciation on the bioavailability and toxicity of U(VI) to metal-reducing bacteria. To determine the effects of aqueous uranyl speciation on uranium bioreduction kinetics, incubations and viability assays with Shewanella putrefaciens strain 200 were conducted over a range of pH and dissolved inorganic carbon (DIC), Ca 2+ , and Mg 2+ concentrations. A speciation-dependent kinetic model was developed to reproduce the observed time series of total dissolved uranium concentration over the range of geochemical conditions tested. The kinetic model yielded the highest rate constant for the reduction of uranyl non-carbonate species (i.e., the 'free' hydrated uranyl ion, uranyl hydroxides, and other minor uranyl complexes), indicating that they represent the most readily reducible fraction of U(VI) despite being the least abundant uranyl species in solution. The presence of DIC, Ca 2+ , and Mg 2+ suppressed the formation of more bioavailable uranyl non-carbonate species and resulted in slower bioreduction rates. At high concentrations of bioavailable U(VI), however, uranium toxicity to S. putrefaciens inhibited bioreduction, and viability assays confirmed that the concentration of non-carbonate uranyl species best predicts the degree of toxicity. The effect of uranium toxicity was accounted for by incorporating the free ion activity model of metal toxicity into the bioreduction rate law. Overall, these results demonstrate that, in the absence of competing terminal electron acceptors, uranium bioreduction kinetics can be predicted over a wide range of geochemical conditions based on the bioavailability and toxicity imparted on U(VI) by solution composition. These findings also imply that the concentration of uranyl non-carbonate species, despite being extremely low, is a determining factor controlling uranium bioreduction at contaminated sites.

Reactive Fe(III) minerals can influence methane (CH ) emissions by inhibiting microbial methanogenesis or by stimulating anaerobic CH oxidation. The balance between Fe(III) reduction, methanogenesis, and CH oxidation in ferruginous Archean and Paleoproterozoic oceans would have controlled CH fluxes to the atmosphere, thereby regulating the capacity for CH to warm the early Earth under the Faint Young Sun. We studied CH and Fe cycling in anoxic incubations of ferruginous sediment from the ancient ocean analogue Lake Matano, Indonesia, over three successive transfers (500 days in total). Iron reduction, methanogenesis, CH oxidation, and microbial taxonomy were monitored in treatments amended with ferrihydrite or goethite. After three dilutions, Fe(III) reduction persisted only in bottles with ferrihydrite. Enhanced CH production was observed in the presence of goethite, highlighting the potential for reactive Fe(III) oxides to inhibit methanogenesis. Supplementing the media with hydrogen, nickel and selenium did not stimulate methanogenesis. There was limited evidence for Fe(III)-dependent CH oxidation, although some incubations displayed CH -stimulated Fe(III) reduction. 16S rRNA profiles continuously changed over the course of enrichment, with ultimate dominance of unclassified members of the order Desulfuromonadales in all treatments. Microbial diversity decreased markedly over the course of incubation, with subtle differences between ferrihydrite and goethite amendments. These results suggest that Fe(III) oxide mineralogy and availability of electron donors could have led to spatial separation of Fe(III)-reducing and methanogenic microbial communities in ferruginous marine sediments, potentially explaining the persistence of CH as a greenhouse gas throughout the first half of Earth history.

No abstract

19Reactive Fe(III) minerals can influence methane (CH 4 ) emissions by inhibiting microbial 20 methanogenesis or by stimulating anaerobic CH 4 oxidation. The balance between Fe(III) 21 reduction, methanogenesis, and methane oxidation in ferruginous Archean and Paleoproterozoic 22 oceans would have controlled CH 4 fluxes to the atmosphere, thereby regulating the capacity for 23 CH 4 to warm the early Earth under the Faint Young Sun. We studied CH 4 and Fe cycling in 24 anoxic incubations of ferruginous sediment from the ancient ocean analogue Lake Matano, 25 Indonesia over three successive transfers (500 days total). Iron reduction, methanogenesis, 26 methane oxidation, and microbial taxonomy were monitored in treatments amended with 27 ferrihydrite or goethite. After three dilutions, Fe(III) reduction persisted only in bottles with 28 ferrihydrite. Enhanced CH 4 production was observed in the presence of goethite, highlighting the 29 potential for reactive Fe(III)-oxides to inhibit methanogenesis. Supplementing the media with 30 hydrogen, nickel and selenium did not stimulate methanogenesis. There was limited evidence for 31 Fe(III)-dependent CH 4 oxidation, although some incubations displayed CH 4 -stimulated Fe(III)-32 reduction. 16S rRNA profiles continuously changed over the course of enrichment, with ultimate 33 dominance of unclassified members of the order Desulfuromonadales in all treatments. Microbial 34 diversity decreased markedly over the course of incubation, with subtle differences between 35 ferrihydrite and goethite amendments. These results suggest that Fe(III)-oxide mineralogy and 36

No abstract