Abstract
Abstract. Enhancing microbial U(VI) reduction with the addition of organic electron donors is a promising strategy for immobilizing uranium in contaminated groundwaters, but has yet to be optimized because of a poor understanding of the factors controlling the growth of various microbial communities during bioremediation. In previous field trials in which acetate was added to the subsurface, there were two distinct phases: an initial phase in which acetate-oxidizing, U(VI)-reducing Geobacter predominated and U(VI) was effectively reduced and a second phase in which acetate-oxidizing sulfate reducing bacteria (SRB) predominated and U(VI) reduction was poor. The interaction of Geobacter and SRB was investigated both in sediment incubations that mimicked in situ bioremediation and with in silico metabolic modeling. In sediment incubations, Geobacter grew quickly but then declined in numbers as the microbially reducible Fe(III) was depleted whereas the SRB grow more slowly and reached dominance after 30–40 days. Modeling predicted a similar outcome. Additional modeling in which the relative initial percentages of the Geobacter and SRB were varied indicated that there was little to no competitive interaction between Geobacter and SRB when acetate was abundant. Further simulations suggested that the addition of Fe(III) would revive the Geobacter, but have little to no effect on the SRB. This result was confirmed experimentally. The results demonstrate that it is possible to predict the impact of amendments on important components of the subsurface microbial community during groundwater bioremediation. The finding that Fe(III) availability, rather than competition with SRB, is the key factor limiting the activity of Geobacter during in situ uranium bioremediation will aid in the design of improved uranium bioremediation strategies.
Highlights
Stimulating microbial reduction of soluble U(VI) to less soluble U(IV) with the addition of organic electron donors has shown promise as a strategy for preventing the spread of uranium in contaminated groundwater (Lovley, 2003; Wall and Krumholz, 2006; Williams et al, 2011)
The interaction of Geobacter species and sulfate-reducing bacteria (SRB) was evaluated in sediment incubations that simulated conditions in the field experiments, but provided the opportunity to quantify the number of cells in each population over time
Addition of more acetate and sulfate as they became depleted stimulated additional growth of SRB, but not Geobacter species. This pattern of succession from Geobacter to SRB has previously been observed in field and column studies (Anderson et al, 2003; Komlos et al, 2008; Milleto et al, 2011) and the timing of the transition to SRB predominance in the batch studies reported here is similar to that seen in those previous studies
Summary
Stimulating microbial reduction of soluble U(VI) to less soluble U(IV) with the addition of organic electron donors has shown promise as a strategy for preventing the spread of uranium in contaminated groundwater (Lovley, 2003; Wall and Krumholz, 2006; Williams et al, 2011). The addition of acetate to sulfate-rich groundwater at a uranium bioremediation study site in Rifle, CO, produced an initial Fe(III) and U(VI) reducing phase, in which Geobacter species predominated, followed by a sulfate-reducing phase during which Fe(III) and U(VI) reduction ceased and acetate-oxidizing sulfate reducers related to Desulfobacter were more abundant (Anderson et al, 2003; Miletto et al, 2011). Previous studies on the interactions between Fe(III) reducers and sulfate reducers have focused on the competition for electron donors that takes place under steady-state conditions in sedimentary environments.
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