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Abstract

The impact of deep-subsurface pressure conditions on microbial activity is still poorly constrained. In particular it is unknown how pressure of deep environments affects microbial transformations of iron. We investigated the effects of high hydrostatic pressure (HHP) on the rate and the extent of bacterial dissimilatory iron reduction (DIR). We employed a novel experimental setup that enables in situ monitoring of Fe oxidation state and speciation in bacterial cultures in an optimized HHP incubation system using X-ray Absorption Near-Edge Structure (XANES) spectroscopy. The iron-reducing bacterium Shewanella oneidensis MR-1 was incubated at 30 °C with Fe(III) citrate and tryptone at pressures between 0.1 and 100 MPa. For pressures up to 70 MPa strain MR-1 (108 cells ml−1) was able to reduce all 5 mM Fe(III) provided. Above 70 MPa, the final amount of Fe(III) that MR-1 could reduce decreased linearly and DIR was estimated to stop at 109 ± 7 MPa. The decrease in the reduction yield was correlated with the dramatic decrease in survival (as determined by CFU counts) above 70 MPa. The initial rate of DIR increased with pressure up to 40 MPa, then decreased to reach zero at about 110 MPa. Increased rates of DIR activity and relatively high growth rates for pressures below 40 MPa would potentially ensure the maintenance of MR-1 in most of deep subsurface environments where moderate pressures occur, i.e. deep-sea environments. This study not only provides the first in situ quantitative results for microbial iron metabolism under HHP conditions but also sets the stage for future investigations of deep-sea pressure-adapted iron reducers. Moreover it demonstrates for the first time that XANES at the Fe K-edge is a powerful probe for in vivo monitoring of iron transformations in living microbial cultures.