BIOLOGICAL ROLES OF SODIUM IONS, REQUISITE FOR THE PRIMARY PRODUCTION IN THE MARINE ENVIRONMENT

摘要

Sodium chloride is a major chaotropic effecter which limits primary productivity of land plants and freshwater photoautotrophs. Mechanisms of salt tolerance in freshwater plants have been well studied and it is well established that plants exclude excess Na+ from the cytosol to maintain proper internal Na+ concentrations. On the other hand, little information is available on relationships between photoautotrophs and salt in marine environment, which is now known to be a major fraction of global primary production. In the present study, effects of reduced salt concentration to cell physiology and intracellular localization of salt were investigated using the marine diatom Phaeodactylum tricornutum. Our results showed that cells of marine diatom require Na+ for normal growth and photosynthesis whereas Cl- acted inhibitory to photosynthesis. Intracellular behavior of these ions was investigated by labeling with specific indicators, Sodium Green and MEQ. It was shown that under salt concentrations below seawater level (<0.5 M), Na+ and Cl" were shown to be accumulated at the membrane system surrounding the chloroplast. Most probably, salt is reserved at the matrixes of four layered chloroplast membranes, which is a unique structure observed in secondary symbionts. These data strongly suggest that, in contrast to land plants, primary production by photoautotrophs in the ocean requires Na+.