Extremophilic yeasts: plasma-membrane fluidity as determinant of stress tolerance.

Our aim was to investigate the response of selected yeasts and yeast-like fungi from extreme environments to various temperatures at the level of their plasma membranes, in order to elucidate the connections between their plasma-membrane fluidity (measured by electron paramagnetic resonance spectroscopy - EPR), growth temperature range, stress tolerance, and ecological distribution. Although all studied fungi can be considered mesophilic according to their growth temperature profiles, their plasma-membrane fluidity indicated otherwise. Arctic yeast Rhodosporidium diobovatum could be classified as psychrotolerant due to its higher average membrane fluidity. Extremely halotolerant black yeast-like fungus Hortaea werneckii isolated from solar salterns, on the other hand, is not adapted to low temperature, which is reflected in the higher average rigidity of its plasma membrane and as a consequence its inability to grow at temperatures lower than 10°C. The plasma membrane of Aureobasidium sp. isolated so far exclusively from an Arctic glacier with its intermediate fluidity and high fluidity variation at different temperatures may indicate the specialization of this yeast-like fungus to the specific glacial environment. Similar behaviour of plasma membrane was detected in the reference yeast, non-extremophilic Saccharomyces cerevisiae. Its membranes of intermediate fluidity and with high fluidity fluctuation at different temperatures may reflect the specialization of this yeast to mesophilic environments and prevent its colonization of extreme environments. Halotolerant Aureobasidium pullulans from salterns, and Arctic Cryptococcus liquefaciens and Rhodotorula mucilaginosa with moderately fluctuating plasma membranes of intermediate fluidity are representatives of globally distributed generalistic and stress-tolerant species that can thrive in a variety of environments. Keeping the membranes stable and flexible is one of the necessities for the microorganisms to survive changes in extreme habitats. Our data suggest that plasma-membrane fluidity can be used as an indicator of fitness for survival in the extreme environments. In addition to the average fluidity of plasma membrane, the fluctuation of fluidity is an important determinant of stress tolerance: high absolute fluidity fluctuation is tied to decreased survival. The fluidity and its variation therefore reflect survival strategy and fitness in extreme environments and are good indicators of the adaptability of microorganisms.