Energy Metabolism Response to Low-Temperature and Frozen Conditions in Psychrobacter cryohalolentis

Studies of cold-active enzymes have provided basic information on the molecular and biochemical properties of psychrophiles; however, the physiological strategies that compensate for low-temperature metabolism remain poorly understood. We investigated the cellular pools of ATP and ADP in Psychrobacter cryohalolentis K5 incubated at eight temperatures between 22 degrees C and -80 degrees C. Cellular ATP and ADP concentrations increased with decreasing temperature, and the most significant increases were observed in cells that were incubated as frozen suspensions (< -5 degrees C). Respiratory uncoupling significantly decreased this temperature-dependent response, indicating that the proton motive force was required for energy adaptation to frozen conditions. Since ATP and ADP are key substrates in metabolic and energy conservation reactions, increasing their concentrations may provide a strategy for offsetting the kinetic temperature effect, thereby maintaining reaction rates at low temperature. The adenylate levels increased significantly < 1 h after freezing and also when the cells were osmotically shocked to simulate the elevated solute concentrations encountered in the liquid fraction of the ice. Together, these data demonstrate that a substantial change in cellular energy metabolism is required for the cell to adapt to the low temperature and water activity conditions encountered during freezing. This physiological response may represent a critical biochemical compensation mechanism at low temperature, have relevance to cellular survival during freezing, and be important for the persistence of microorganisms in icy environments.