The Impact of Ionizing Radiation on the Microbial Reduction of Fe(III)

Abstract:

Biogeochemical processes mediated by Fe(III)-reducing bacteria have the potential to impact on the post-closure evolution of a geological disposal facility (GDF) for radioactive waste. However, the organisms promoting these processes will likely be subject to significant radiation fluxes. Therefore, the impact of acute doses of ionizing radiation on the physiological status of the model Fe(III)-reducing bacterium Shewanella oneidensis was assessed. FT-IR spectroscopy and MALDI-TOF-MS suggested that the metabolic response to radiation is underpinned by alterations to lipids and proteins. Furthermore, the irradiated phenotype exhibits enhanced Fe(III)-reduction.

The impact of radiation on the extracellular environment was also assessed. Exposure to gamma radiation caused activation of ferrihydrite and hematite for enzymatic reduction by S. oneidensis. TEM, SAED and Mössbauer spectroscopy revealed that this effect was a result of radiation induced changes to crystallinity leading to an increase in bioavailability of Fe(III) for respiration.

To assess the impact of radiation on sediment microbial communities, a series of microcosm experiments were constructed and gamma irradiated over a two month period. Sediments irradiated at a dose rate of 0.5 Gy h^-1 exhibited enhanced Fe(III) reduction despite receiving doses potentially lethal to indigenous microorganisms, whilst biogeochemical processes in sediments irradiated with 30 Gy h^-1 were only partially restricted. Despite this, 16S rRNA gene pyrosequencing revealed significant dose-dependent shifts in the microbial communities in tandem with changes in microcosm biogeochemical profiles.

Collectively, these results indicate that, despite significant total absorbed doses, biogeochemical processes will likely not be restricted by dose rates expected in a deep geological repository. Indeed, electron accepting processes in such environments may even be stimulated by radiation.