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The amino acid metabolism of Cyclotella cryptica is strongly influenced by changes in salinity of the medium. A sudden increase in salinity (water stress) reduces the total uptake of amino acids by the cells and also their assimilation into proteins. Under water-stress conditions, proline is readily synthesized from glutamate, arginine, and ornithine. and proline synthesized under these conditions turns over very slowly and accumulates.Increases in osmolarity of the medium, whether caused by salts, mannitol, or sucrose (but not by glycerol), is followed immediately by rapid synthesis of proline. Proteolysis is not observed under water-stress conditions. The high level of proline accumulated under these conditions is rapidly reduced when the cells are transferred back to low osmolarity media. The loss of 14 C from proline is accompanied by rapid incorporation into proteins and also conversion of proline into other cell constituents.The proline level is relatively low in high-salinity-adapted cells, and proline turns over rapidly in such cells. This indicated that the accumulation of proline occurs mainly in response to abrupt changes of intracellular ionic strength during adaptation to increased salinities. Additions of salts and organic substances to cell suspensions in low-salinity media cause plasmolysis followed by rapid deplasmolysis. However, when cells are transferred to high-osmolarity solutions of single salts or organic substances, deplasmolysis is only observed in media containing KCl or glycerol. Increases of external osmolarity cause increases in concentrations of intracellular potassium and free amino acids.
Liu et al. (Sat,) studied this question.