Biophysical constraints on microbial systems subjected to solute and water induced stresses

Abstract

Water is indispensable for all known life and any reduction in its availability due to either loss of water, or increase in solute concentration, would result in water stress if the reduction falls below a cell’s tolerance threshold. Water activity (aw) better predicts the growth of microorganisms because microorganisms can only use "available" water, which is solute dependent. Water could be considered the most significant inhibitory stress to the biological system; even more so than temperature and pH, and therefore one of the most important, if not the most important parameter to consider when discussing the habitability of an environment, dictating the range of species able to survive. The overarching aim was to explore the fundamentals of water activity (i.e. how water activity affects the microbial cell and how the cell overcomes this effect), and illustrate the mechanisms and consequences with members of the Aspergillus and some other extremophilic fungi. The impacts on microbial diversity using different environmental extremes were studied- these were: 1) low- water activity, hypersaline acidic lakes; 2) a low-water activity, hypersaline Alkali Lake; and 3) a MgCl2saturated chaotropic underwater lake. An attempt was made to explore the limits, responses and some practical implications of water stress in the studied microorganisms. The thesis starts by discussing the importance of water as a lubricant, its role in metabolic activity, and water induced stresses. It also discusses the relationship of water activity and relative humidity and the methods of controlling this parameter in commercial and scientific contexts. The introduction also reviews our current understanding of the water activity limits of the three domains of microbial life (Archaea, Bacteria, and Fungi) and mechanisms each has to adapt to these stresses. It then moves on to address the effects of extreme hyper saline, low water activity natural environments (lakes) on resident microbes: highly acidic, and highly alkali Both studies empirically determined the water activities of their respective lakes and discussed the consequences for microbial habitability and offering suggestions as to how native microbes have adapted to such environmental extremes and what this could potentially mean for the new field of astrobiology and, in particular, the planetary protection of Mars. Water activity is also discussed in the context of high chaotropicity and how it limits the viability of the microbial cell beyond 2.03 M MgCl2 concentrations (0.790 aw) Here it is shown that magnesium chloride greatly reduces the water activity; e.g at 3.3 M MgCl2the water activity of the Lake Kryos brine was 0.399, far below the current established limit for life thus it is impossible for metabolically active cells to be found within the brine like the authors of Steinle et al. (2018) attempts to imply.

Date of Award	Jul 2019
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	John E. Hallsworth (Supervisor)