Microorganisms are known to tolerate a variety of extreme environments, such as high and low pH, desiccation and a wide range of temperatures that would prove uninhabitable for most eukaryotic cells. However, extreme radiation exposure is a ubiquitous hazard to pro- and eukaryotic viability. Ionising and non-ionising radiation, and their associated high energies, cause damage to a cell in the form of DNA double-strand breaks, membrane deterioration, and lethal mutations. Radiation also induces secondary effects such as the production of reactive oxygen species, which attack and degrade organic compounds. It is therefore not surprising that radiation is considered by the scientific community to be one of the main influencing factors when regarding habitability on the early Earth, as well as other planets, such as present-day Mars. This thesis explores the response of select microbes that have been exposed to extreme radiation environments, i.e. both high and ultra-low radiation. Understanding how radiation affects the geochemical environment is key to the assessment of its potential to support life and to harbour molecules associated with life. The effect of radiation-induced photochemistry on the early terrestrial and present-day Martian surface is explored in conjunction with Fenton chemistry. Iron oxides are abundant on both Earth and Mars and act as catalysts in Photo-Fenton reactions, enabling the production of free radicals. The resulting consequences for habitability are shown to be antagonistic, with iron oxide enabling both the protection or destruction of cells, depending on the local geochemistry. In addition, the photo-reactivity of perchlorate is investigated. The UV-induced activation of the strong oxidant, and recently confirmed Martian surface constituent, is demonstrated, revealing its severe bacteriocidal effect on microbes. It is also shown to significantly reduce microbial viability when combined with further Martian soil constituents and components required for Photo-Fenton chemistry. In order to accurately analyse the effects of low earth orbit radiation on prokaryotic life, cyanobacterial samples were attached to the outside of the International Space Station as part of the EXPOSE-R2 mission for 1.5 years. The samples were subjected to various conditions, including exposure to a minimally filtered space radiation environment and simulated Mars conditions. The experiment is designed to test the protection that biogenic and non-biogenic substances may provide to cells. The results in this work present the post-flight analysis of the samples and demonstrate the ability of these substances to maintain cyanobacteria viability. They also show that the cyanobacterial cells themselves can effectively act as a shield for a secondary, co-cultured bacteria species. On the other end of the radiation dose scale, this work addresses the gaps in knowledge with regard to the little-understood effects of low, sub-background radiation on prokaryotes. Using the Boulby Underground Lab in the functioning Boulby Mine, Cleveland UK, microbes are cultivated under regulated, extremely low radiation environments to test multiple dose-response models. The results show no change in cell's growth rates or gradients in low radiation exposure when compared to surface-dose controls. They also fail to exhibit any enhanced susceptibility to stress factors, such as UV irradiation, as suggested by previous work in the field. These experiments mark the first extensive and tightly controlled research into microbial responses in the near-absence of radiation. This work illustrates the importance of understanding both primary and secondary effects of radiation on microbes and begins to bridge the knowledge gap from both ends of the dose axis. These approaches show the far-reaching influence radiation has on astrobiologically relevant topics, such as habitat geochemistry and life detection, and demonstrate the capacity of life to survive in extreme radiation environments.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:756517 |
| Date | January 2018 |
| Creators | Wadsworth, Jennifer Louise |
| Contributors | Cockell, Charles ; Pilizota, Teuta |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/31217 |
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