Response to osmotic stress by the haloalkaliphilic bacterium Halomonas campisalis

John Aston

This thesis is broken down into four parts. The first part (Chapter One) provides a background into the study of halophilic organisms. Diversity, survival mechanisms, ecological significance, and applications are all discussed. Chapter Two discusses the primary focus of my research. The response of the haloalkaliphilic microorganism, Halomonas campisalis, to changes in environmental salinity is investigated. Three phenotypic changes are examined in detail: (1) growth kinetics, (2) compatibles solute accumulation, and (3) phospholipid fatty acid (PLFA) analysis. The study was completed in parallel with aerobic and denitrifying samples. Growth kinetics were found to be highly dependent on media salinity. Under aerobic conditions optimal growth occurred at 20 g/L NaCl (0.5 h-1). Under denitrifying conditions optimum growth occurred at 30 g/L NaCl (0.3 h-1). The compatible solute ectoine was observed in the absence of salt as well as across the entire range of salinities examined, with optimum intracellular accumulation occurring at 90 g/L NaCl for both aerobic and denitrifying conditions. In much smaller amounts, glycine betaine was found at intermediate salinities, and hydroxyectoine was found at the highest salinities (175 g/L NaCl). PLFA analysis provided insights into cell stress at varying salinities. High ratios of trans monoenoic fatty acids indicated an increase in cell membrane permeability in the absence of salinity under both aerobic and denitrifying conditions. In contrast no such physiological marker was present at 175 g/L NaCl where growth kinetics were also depressed. This suggests that another cell stressor is responsible for decreased cell growth. It is possible that high salinity in the media begins to interfere with cation transfer between H. campisalis and the surroundings, a necessary exchange in order to balance the intracellular pH. Chapter Three briefly discusses the effects of salinity on the growth kinetics of the extremely halophilic archaeon Halobacterium salinarum strain NRC-1. Again parallel observations were made under both aerobic and denitrifying conditions. An optimum growth rate of 0.047 h-1 at 200 g/L NaCl was seen with aerobic samples. Under denitrifying conditions the growth optima occurred at 150 g/L with a maximum specific growth rate of 0.04. The fourth chapter provides suggestions for future work.

Response to osmotic stress by the haloalkaliphilic bacterium Halomonas campisalis

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