Ion composition of unicellular marine and fresh-water algae, with special reference to Platymonas subcordiformis cultivated in media with different osmotic strengths

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Abstract
Ion compositions (K+, Na+ Mg2+, Ca2+, Cl-, phosphate) of the euryhaline algae, Platymonas subcordiformis, Chlorella salina, grown in media with a salinity range from 0.1 to 0.6 M NaCl and of the fresh-water algae, Ankistrodesmus braunii and Scenedesmus obliquus, were compared. Enhancement of ion concentrations with increasing salinity in Platymonas was attributed largely to decreasing cell volume. In both the euryhaline algae, Na+ and — partially — Cl- content per cell increased significantly with rising salinity. The contents per cell of the other ions were not affected. Considering the relevance of ions and mannitol (Platymonas) and proline (Chlorella) as osmotically active particles, it was found that the ions alone maintained osmotic balance with low external salinity. With increasing salinity the organic compounds contributed up to 20–30% of the cellular solute potential. The main cation, K+, was the main contributor to the osmotic balance; the accumulation of organic compounds as well as of Na+ and Cl- contributes further to the ability of the algae to adapt to high salinity. The results confirm the hypothesis of low Cl- concentrations in nonvacuolate cells in comparison to vacuolate cells.