Abstract
Biogeochemical processes mediated by Fe(III)-reducing bacteria such as Shewanella oneidensis have the potential to influence the post-closure evolution of a geological disposal facility for radioactive wastes and to affect the solubility of some radionuclides. Furthermore, their potential to reduce both Fe(III) and radionuclides can be harnessed for the bioremediation of radionuclide-contaminated land. As some such sites are likely to have significant radiation fluxes, there is a need to characterise the impact of radiation stress on such microorganisms. There have, however, been few global cell analyses on the impact of ionizing radiation on subsurface bacteria, so here we address the metabolic response of S. oneidensis MR-1 to acute doses of X-radiation. UV/Vis spectroscopy and CFU counts showed that although X-radiation decreased initial viability and extended the lag phase of batch cultures, final biomass yields remained unchanged. FT-IR spectroscopy of whole cells indicated an increase in lipid associated vibrations and decreases in vibrations tentatively assigned to nucleic acids, phosphate, saccharides and amines. MALDI-TOF-MS detected an increase in total protein expression in cultures exposed to 12 Gy. At 95 Gy, a decrease in total protein levels was generally observed, although an increase in a putative cold shock protein was observed, which may be related to the radiation stress response of this organism. Multivariate statistical analyses applied to these FT-IR and MALDI-TOF-MS spectral data suggested that an irradiated phenotype developed throughout subsequent generations. This study suggests that significant alteration to the metabolism of S. oneidensis MR-1 is incurred as a result of X-irradiation and that dose dependent changes to specific biomolecules characterise this response. Irradiated S. oneidensis also displayed enhanced levels of poorly crystalline Fe(III) oxide reduction, though the mechanism underpinning this phenomenon is unclear.
Highlights
Civil nuclear energy generation and nuclear weapon production since 1945 has generated significant volumes of legacy radioactive wastes and contaminated land [1]
As many of the sites contaminated by radionuclides are likely to have significant radiation fluxes [10,11,12], the utility of microorganisms in the remediation of highly radioactive wastes will largely be determined by the ability to survive radiation stress [13]
Future experiments may be expanded to determine whether this phenomenon is limited to insoluble Fe(III) or is applicable to chelated soluble Fe(III) and other inorganic anaerobic electron acceptors. These results suggest that ionizing radiation impacts upon the viability and growth of aerobic cultures of S. oneidensis over a range of doses
Summary
Civil nuclear energy generation and nuclear weapon production since 1945 has generated significant volumes of legacy radioactive wastes and contaminated land [1]. As physicochemical methods of remediation of contaminated land, e.g. soil washing and ‘pump and treat’, may incur great cost, the use of non-invasive in situ alternative technologies, such as bioremediation, may provide a more versatile and cost-effective substitute [2,3]. Many subsurface bacteria, such as Shewanella spp. have the ability to couple the oxidation of organic matter to the reduction of a range of metal cations, anions and radionuclides [4,5,6], providing the potential for use in the bioremediation of radionuclide contaminated land [2,7]. Radiation toxicity may govern the importance of microbially controlled processes in these environments and there is a requirement to deliver fundamental physiological information on the impact of ionizing radiation on Fe(III)-reducing bacteria such as Shewanella oneidensis
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