Coordination of ionic relations and mannitol concentrations in the euryhaline unicellular alga, Platymonas subcordiformis (Hazen) after osmotic shocks.

Following small hypo-osmotic shocks, ion concentrations (Na(+), K(+), Cl(-)) in Platymonas subcordiformis decreased; this was due mainly to an increase of cell volume. With larger hypo-osmotic stresses, the decrease of ion concentration continued and, additionally, extrusion of mannitol was observed. The ion and mannitol concentrations were not regained after 240 min. In contrast, following hyperosmotic shocks, the ion concentrations increased transitorily during the first 20-40 min. The same was true for K(+) following small hyperosmotic stresses and for Na(+) and - partially - Cl(-) with larger shocks. Large hyperosmotic stresses caused permanent accumulation of mannitol, which levelled off after 60-80 min. Thus the transient increase of ions bridged the concentration gap until mannitol was accumulated to a high enough concentration to account for the osmotic adaptation of Platymonas, together with a basal level of the ions K(+), Na(+), Cl(-).