Osmotic regulation in the marine alga, Codium decorticatum - II. Active chloride influx exerts negative feedback control on the turgor pressure

Codium decorticatum maintains its internal osmotic pressure 2.3 bar above the osmotic pressure of the external medium, which results in a hydrostatic pressure difference (turgor) of 2.3 bar. During hypoosmotic stress the primary mechanism for decreasing the internal osmotic pressure is a decrease in active Cl^- influx. The decreased Cl^- influx is accompanied by a 6.9 mV depolarization of the negative vacuole potential, while there is no consistent change in the protoplast resistance. The net Cl^- efflux which results is balanced primarily by a passive net loss of K⁺. During hyperosmotic stress the Cl^- influx increases, accompanied by a slight (2.6 mV) hyperpolarization and a decrease of 90 Ω cm² in the membrane resistance. A passive net uptake of Na⁺ accompanies the net active influx of Cl^-. Following the restoration of normal turgor, which requires about 2 hr, concentrations of Na⁺ and K⁺ are slowly readjusted over a period of about 30 hr until the normal K⁺/Na⁺ ratio of 1.0 is restored. Codium regulates its turgor as rapidly in the dark as in the light, which suggests that the Cl^- pump is not dependent directly on photosynthesis for its energy supply.