Microorganisms control many processes pertinent to the stability of radwaste inventories in nuclear storage and disposal facilities. Furthermore, numerous subsurface bacteria, such as Shewanella spp. have the ability to couple the oxidation of organic matter to the reduction of a range of metals, anions and radionuclides, thus providing the potential for the use of such versatile species in the bioremediation of radionuclide contaminated land. However, the organisms promoting these processes will likely be subject to significant radiation doses. Hence, the impact of acute doses of ionizing radiation on the physiological status of a key Fe(III)-reducing organism, Shewanella oneidensis, was assessed. FT-IR spectroscopy and MALDI-TOF-MS suggested that the metabolic response to radiation is underpinned by alterations to proteins and lipids. Multivariate statistical analysis indicated that the phenotypic response was somewhat predictable although dependent upon radiation dose and stage of recovery. In addition to the cellular environment, the impact of radiation on the extracellular environment was also assessed. Gamma radiation activated ferrihydrite and the usually recalcitrant hematite for reduction by S. oneidensis. TEM, SAED and Mössbauer spectroscopy revealed that this was a result of radiation induced changes to crystallinity. Despite these observations, environments exposed to radiation fluxes will be much more complex, with a range of electron acceptors, electron donors and a diverse microbial community. In addition, environmental dose rates will be much lower than those used in previous experiments. Sediment microcosms irradiated over a two month period at chronic dose rates exhibited enhanced Fe(III)-reduction despite receiving potentially lethal doses. The microbial ecology was probed throughout irradiations using pyrosequencing to reveal significant shifts in the microbial communities, dependent on dose and availability of organic electron donors. The radiation tolerance of an algal contaminant of a spent nuclear fuel pond was also assessed. FT-IR spectroscopy revealed a resistant phenotype of Haematococcus pluvialis, whose metabolism may be protected by the radiation induced production of an astaxanthin carotenoid. The experiments of this thesis provide evidence for a range of impacts of ionizing radiation on microorganisms, including the potential for radiation to provide the basis for novel ecosystems. These results have important implications to the long-term storage of nuclear waste and the geomicrobiology of nuclear environments.

Freshwater green algae within species complex Haematococcus pluvialis are best natural producers of secondary carotenoid astaxanthin famous for its strong antioxidant activity. Despite worldwide distribution, great biotechnological potential and extensive literature dealing with different approaches to the cultivation of these organisms, there still remains surprising gaps in knowledge of their complex life cycle and behaviour. First uncertainty concerns asexual cell division of zoospores, hypothetically limited to five cell doublings (Lee & Ding 1994). No such genetic limitation was observed in this thesis, number of cell divisions is thus influenced mainly by cultivation conditions. There is still considerable uncertainty regarding to the asexual reproduction, as well. Ambiguity might be caused by scarce observation of the whole process of gametogenesis and conjugation of gametes. Published studies differed in some particular points. Unfortunately, we did not succeed in induction of gametogenesis despite broad scale of tested cultivation conditions applied to two different species within the H. pluvialis complex, namely H. pluvialis and H. rubicundus. This failure might be caused by heterothalism of the studied strains. Haematococcus pluvialis inhabits mainly ephemeral pools. Its ability to...