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dc.contributor.advisorCockell, Charles
dc.contributor.advisorAllen, Rosalind
dc.contributor.advisorRoyer, John
dc.contributor.advisorMcMahon, Malcolm
dc.contributor.authorDickinson, Andrew W.
dc.date.accessioned2021-12-23T15:39:25Z
dc.date.available2021-12-23T15:39:25Z
dc.date.issued2021-11-27
dc.identifier.urihttps://hdl.handle.net/1842/38381
dc.identifier.urihttp://dx.doi.org/10.7488/era/1646
dc.description.abstractThe habitable parameter space of a given environment is defined by the multiple constraints that restrict an organisms ability to propagate, and therefore by the maximum range of environmental conditions that life is able to tolerate. The habitability space within which biological processes occur is determined by the physicochemical conditions that restrict these processes. Natural habitats, however, often require organisms to tolerate multiple extreme conditions in combination, such as high or low temperatures, salinity, pH and pressure. Despite the existing laboratory and field data, and our understanding that natural environments can be best characterised by the net effect of multiple environmental parameters, basic studies on the interplay between concomitant environmental extremes on microorganisms is surprisingly limited. Subsequently the boundaries of the habitability space on Earth are yet to be confidently outlined and it is clear that these edges must be defined by the impacts of multiple environmental parameters. Understanding how multiple environmental extremes effect microbial life is crucial in the assessment of the limits of life on Earth. In order to accurately determine the true limits of life it is necessary to examine the effect of multiple stress parameters on microbial growth. In this thesis, we examine the effects of salinity (NaCl), temperature and pH on propagation of a deep-sea hydrothermal vent microbe, Halomonas hydrothermalis, both in isolation and in combination to appropriately determine the impact of these stresses and the potential synergistic or antagonistic relationship between them. We chose these three factors because they are known to establish limits to life in natural environments and have been the focus of a substantial number of studies individually. Here we ask the question of whether a combination of these extreme environmental parameters approaches the physicochemical boundary of habitability space on Earth. These data show that multiple extremes, when combined, act to restrict the limits of life compared to individual extremes. We see that changes in pH alters NaCl tolerance under optimal temperature conditions and under increased temperatures both acidic pH and temperature combine to further limit NaCl tolerance. In addition, we explore the effect of multiple stresses of salinity, pH and temperature on microbial propagation using three facultative anaerobic strains (Halomonas hydrothermalis, Escherichia coli and Carnobacterium pleistocenium) with an aim to provide a better understanding of the energetic limits to life by assessing the maximal growth values attained under different modes of respiration. It is known that energy yields differ between aerobic and anaerobic respiration and the presence of environmental factors such as pH, NaCl and temperature can significantly alter their efficacy, yet few laboratory studies have been done to systematically explore the interactions of three or more stresses on the limits of microbial growth under both aerobic and anaerobic conditions. Here we demonstrate these facultative anaerobic microbes display significantly different tolerances to a combination of stresses of pH, NaCl concentration and temperature when cultivated under aerobic and anaerobic conditions. These data show increased tolerance to higher saline concentrations under anaerobic culture conditions when compared with aerobic growth for two of the strains tested. Additionally, we demonstrate complex interactions between acidic pH and NaCl concentration under increased temperatures where we see anaerobic cultures yield higher growth values than aerobic cultures under a combination of these stresses. These data have significant implications when considering the effect a rise of atmospheric oxygen during The Great Oxidation Event may have had on Earths anaerobic microbial population. Here we demonstrate an essential need to assess the habitability of natural environments with a deeper understanding of the interplay between concomitant physicochemical parameters with a particular focus on cellular energetics. Furthermore, pressure is a fundamental parameter of Earths biosphere that has played a key role in the evolution and distribution of life on Earth, with higher than atmospheric pressure environments predicted to be major habitats for prokaryotic life that exceed numbers found in other components of the biosphere. The effect of high pressure imposed on organisms in addition to other environmental factors such as salinity and pH are yet to be fully understood. The necessary adaptations to deal with these extremes individually are well established, yet the potential synergistic or antagonistic nature of a combination of these stresses in high pressure environments requires a more robust understanding of the physical constraints imposed on life under high pressures conditions. Here we demonstrate that when cultivated under simulated hydrostatic pressures of 50-bar and 150-bar, the model organism H. hydrothermalis displays higher growth than when cultured under atmospheric pressure conditions over a range of salinities and pH values. These data show multiple extremes potentially restrict the boundaries of the biosphere more than single stresses imposed alone. Thus, the boundary space for life in natural environments may be smaller than research into the limits of life in individual extremes would suggest and habitable environments may in fact be less pervasive throughout the universe than previously thought. To determine the window of tolerance to environmental pressures imposed on microbial extremophiles is an essential tool in furthering our understanding of the evolution and diversification of life on Earth, and in making clearer the potential habitability of other planetary bodies, both within and beyond our Solar System.en
dc.language.isoenen
dc.publisherThe University of Edinburghen
dc.subjectn/aen
dc.titleLife under multiple extremes: exploring the boundaries of habitability on Earth:en
dc.typeThesis or Dissertationen
dc.type.qualificationlevelDoctoralen
dc.type.qualificationnamePhD Doctor of Philosophyen


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