Physiological Aspects of Adaptation of the Marine Microalga Tetraselmis (Platymonas) viridis to Various Medium Salinity

We studied the capability of the marine microalga Tetraselmis (Platymonas) viridis to adapt to low and high medium salinity. The normal NaCl concentration for growth of this alga is 0.5 M. It was shown that T. viridis cells could actively grow and maintain osmoregulation and cytoplasmic ion homeostasis in the wide range of external salt concentrations, from 0.01 to 1.2 M NaCl. Using the plasma membrane vesicles isolated from T. viridis cells grown at various NaCl concentrations (0.01, 0.05, 0.5, 0.9, and 1.2 M), we studied the formation of the phosphorylated intermediate of Na⁺-ATPase, the enzyme responsible for Na⁺ export from the cells with a mol wt of ca. 100 kD. Na⁺-ATPase was shown to function in the plasma membrane even in the cells growing at an extremely low NaCl concentration (0.01 M). When alga was grown in high-salt media, the synthesis of several proteins with molecular weights close to 100 kD was induced. The data obtained argue for the hypothesis, which was put forward earlier, that a novel Na⁺-ATPase isoform is induced by T. viridis growing at high NaCl concentrations.