|dc.description.abstract||Microorganisms are the most ancient, abundant and diverse form of life on Earth. Their ability to tolerate a variety of stresses has enabled microbial colonisation of extreme environments, including hot springs, soda lakes and deep-sea vents. For decades, habitability studies have focused on the environmental effects of singular extremes, such as temperature, pressure, pH and salinity. Environments posing more than one simultaneous physical or chemical stress, nonetheless, are abundant in the natural world. It is therefore notable that so few studies have investigated the relationship between different extremes. This thesis explores the response of select microbes to synchronously occurring stresses, with the use of a new quantitative method.
Understanding the relationship between stresses within multiple extreme environments is key to understanding the habitability of these settings. The effect of nutrient limitation and NaCl on bacterial growth was explored, allowing the expansion of kinetic growth models to produce the Mult-Min method, used throughout this thesis. This method investigates whether a multiplicative effect (a situation when all the extremes contribute to an effect on growth) or minimising effect (scenario where one stress dominates the effect on growth) is observed between extreme stresses. The resulting consequences for habitability are that these interactions (or indeed lack of) are not a simple process and cannot be assumed to be either multiplicative or minimising for all combinations of stresses; hence a method (such as the Mult-Min) is required.
In addition, the effects of salt, temperature and pH on growth are examined. Salt exhibits a dominant effect on growth, with small amounts of perchlorate shown to be beneficial to Mg2+ requiring microorganisms. It is shown to enhance microbial viability up until a concentration relevant to that on the Martian surface. Compatible solutes, organic molecules used for cellprotection under extreme conditions, were also studied. This work addresses gaps in knowledge by comparing three of the most prevalent natural extremes, finding that salt has the most dominant effect on growth.
Overall, use of the Mult-Min method shows the complexity of the relationship between extremes, illustrating the inadequacy of focusing on isolated extremes. This work demonstrates the importance of understanding not only how stresses interact in extreme environments, but also the extent to which particular stresses may affect growth.||en