SUMMARY: Organisms of the phytoflagellate Prymnesium parvum Carter undergo swelling and lysis in the presence of ammonia or acetic acid. The lytic activity was pH-dependent and increased as the concentration of undis-sociated weak electrolyte in the suspension medium was increased. The kinetics of swelling were followed with the aid of an electronic particle coounter and by microscopic examination. In the presence of ammonia or acetic acid, the rate of swelling as well as the final volume of the Prym-nesium organisms were a function of external osmotic pressure and of temperature. It is suggested that the swelling and lysis are osmotic in nature and depend on the intracellular accumulation of weak electrolytes by a ‘pump’ driven by differences between environmental and intracellular pH values. The similarities between this phenomenon and concentration of weak electrolytes in mammalian erythrocytes and other cells is discussed. Differences in the morphology and lysis of Prymnesium by ammonia and acetic acid were found which suggest the existence of intracellular compartments maintained at different pH values.
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