Lamprothamnium, a euryhaline charophyte: I. Osmotic relations and membrane potential at steady state

The charophyte Lamprothamnium papulosum (Wallr.) J. Gr. is found at salinities varying from nearly fresh water to twice that of sea water. It can maintain its turgor constant at 302 mosmol kg^-1 (0.73 MPa) when exposed to external osmotic pressures of 550 to 1350 mosmol kg^-1 (1.3-3.3 MPa). Turgor shows a tendency to rise slightly at lower osmotic pressure (388 mosmol kg^-1 of turgor at 150 mosmol kg^-1 external osmolality). K⁺ and Cl^- are the main solutes in the vacuole, and are most important in controlling internal osmotic pressure. Mg²⁺, Ca²⁺, and SO^2-₄ are present in significant amounts but their concentrations do not change with changes in external salinity. Na⁺ is present in lower concentration than K⁺, and plays a minor role in regulating turgor. Sucrose is present in significant concentrations, but changes little with changes in salinity. Two enzymes involved in sucrose metabolism, sucrose phosphate synthetase (EC 2.4.1.14), and sucrose synthetase (EC 2.4.1.13) are active in whole cell extracts of Lamprothamnium. As in the fresh water charophytes, Lamprothamnium membrane potential may be depolarized (close to E_K) or hyperpolarized, and presumably of electrogenic origin. Both types of potential are found at all salinities tested.