Osmotic adaptations of charophyte algae in the Coorong, South Australia and other Australian lakes

Lamprothamnium is found in salinities varying from nearly fresh to twice sea water in coastal saline lakes in South Australia, Victoria, and New South Wales. It maintains its turgor (ΔP≅ΔII) approximately constant over this range, varying internal osmotic pressure by altering KCl concentration (although at the highest salinity, Na⁺ concentration became more important). The ability to regulate turgor with alterations in salinity is wide-spread in marine algae, but has not been reported for the freshwater charophytes. Lamprothamnium is also unusual among algae and particularly among charophytes in having a high concentration of sucrose (up to 300 mM) in the vacuole. Vacuolar concentration is not correlated with salinity and its adaptive significance is not known, although in the cytoplasm sucrose may play a significant role in osmotic adaptation. In other respects, membrane transport has many of the same properties as the freshwater charophytes, and has less in common with marine algae. Membrane potential is more negative and appears due to an electrogenic proton pump, a feature of freshwater algae but unknown in marine algae. It is possible that ions are transported by H⁺ cotransport mechanisms, as may be the case in Chara australis. It seems likely, however, that sucrose is taken up by Na⁺ symport, a type of mechanism more common in the marine algae.