Regulation of intracellular Na⁺ content during NaCl upshock in the marine cyanobacterium spirulina subsalsa cells

Cells of the halotolerant cyanobacterium Spirulina subsalsa were found to exclude Na⁺ ions from the cells, after a salt upshock, following instant diffusion of sodium into the cells. Na⁺ exclusion activity was operating in the light only, and is suggested to involve sodium proton antiporter, coupled to respiration-driven proton translocation. This investigation provides evidence for salt-enhanced respiratory activity in spheroplasts as well as in enriched plasma membrane preparations. The cytochrome oxidase activity, enriched during growth in hypersaline medium, is suggested to conduct proton translocation, at the cell plasma membrane. Proton ATPase activity, enriched in cells grown in hypersaline medium, was also demonstrated, and its activity in plasma membranes was enhanced by NaCl in vitro. The enriched activities of these putative proton-carriers is suggested to maintain the proton gradient needed for sodium exclusion from the cells. The complementary accumulation of glycinebetaine as a compatible osmoregulant was also proven, and its biosynthesis upon salt upshock is slower than sodium exclusion in Spirulina subsalsa.