4). These findings are similar to previous work on Serratia sp. (Adams et al., 2007) where glycerol was found to be the most favourable electron donor tested and acetate and benzoate resulted in slow rates of Fe(III) reduction. The Serratia species isolated from Sellafield sediment was found to reduce Fe(III) optimally at the pH of 4.5–6.5 with a range of activity between pH 3.5 to 9.5 (Fig. 5). No Fe(III)
reduction was observed above pH 9.5, and rates of Fe(III) reduction were observed to slow above pH c. 6.5 and below pH c. 4.5 (Fig. 5a). In cultures where the pH was initially < 6.5, the microbial Fe(III) reduction was observed to shift the pH towards alkalinity Epigenetic inhibitor presumably because of the release of OH− during Fe(III) reduction (Fig. 5b) (Mortimer et al., 1997; Adams et al., 2007). However, the pH in Fe(III)-citrate cultures with an initial pH > 6.5 decreased during Fe(III) reduction presumably because of an increase in aqueous CO2 resulting from microbial
respiration and subsequent formation and dissociation of carbonic acid (Figs 1b,c and 5b). In addition, after Fe(III) reduction had developed, a white precipitate was observed above pH 7 and this PD0325901 chemical structure was identified via XRD analysis as containing both siderite and vivianite (data not shown). Siderite and vivianite production consumes and OH− acting to decrease the pH. It is interesting that the biogeochemical processes occurring in these microcosms act to buffer the pH towards the optimum growth pH for Serratia Amino acid sp. The bacterium isolated in this study appears to be a robust and highly adaptable species that is capable of surviving dramatic changes in sediment geochemistry. Serratia species are reported to reduce Fe(III) over a wide spectrum of pH values and utilize a diverse range of alternative electron acceptors and electron donors (This study and Adams et al., 2007). It appears that during microbial stimulation scenarios, changes in pH and available electron donors/acceptors can result in unusually resilient rather than
more commonly identified Fe(III)-reducing organisms becoming dominant. Here, an organism rarely reported as an Fe(III)-reducing bacterium with an optimum growth pH of < 7 was observed to dominate in a pH 9 system which had undergone extensive denitrification prior to metal reduction. Thus, it is possible that during remediation scenarios where sediment geochemistry is altered during bioremediation, the microbial community may shift to favour less typical, but more adaptable species. This work was funded by the Engineering and Physical Science Research Council (EPSRC) as part of the Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal (DIAMOND) consortium grant EP/F055412/1. We also acknowledge the support of NERC grants NE/H007768/1 for the molecular ecology work.