Adaptation of the psychrophilic Mucor psychrophilus (Mucorales, Mucoromycota) to lower temperatures and under conditions of heat and osmotic shocks

The definitions of psychrophilia and psychrotolerance are based on the optimal temperature of growth (≤15 and 20 °C, respectively), and it is not clear whether differences exist in adaptation mechanisms. We analyzed the composition of osmolytes and membrane lipids of a true psychrophile Mucor psychrophilus during submerged cultivation at 12.5 °C and 4 °C, as well as under heat and osmotic shocks. The main osmolyte at 12.5 °C is trehalose (70 % of the total), whereas at 4 °C, comparable proportions of glycerol, glucose, and trehalose are observed. Under heat shock, the amount of trehalose increases threefold, and osmotic shock leads to an increase of the glycerol level without a reduction in the amount of trehalose. The predominant membrane lipids at both temperatures are non-bilayer phosphatidic acids (about 65 % of the sum) and phosphatidylethanolamines (20–30 %). An increase in the degree of unsaturation and a decrease in the sterols proportion are observed during growth at 4 °C, whereas at 12.5 °C, as well as under heat and osmotic shocks, the changes are insignificant. Similarity of the adaptation mechanisms of the psychrophilic and psychrotolerant fungi indicates the ambiguity of psychrophilia and psychrotolerance definitions.