Osmotic Adaption in the Marine Alga Griffithsia monilis (Rhodophyceae): The Role of Ions and Organic Compounds

Abstract

The red alga G. monilis maintains its turgor pressure constant at 4.05 � 0.14 x 10*5 Pa (179 measurements), or 166 mosmol/kg, over a range of external osmotic pressures from 900 to 1300 mosmol/kg. It is capable of regulating turgor pressure in the dark or when sorbitol is used to increase the external osmotic pressure. Complete regulation of turgor requires 24-36 h, although much of the regulation is accomplished in the first 2 h. The change in II*i is achieved by controlling the concentrations of K+, Na+, and Cl-. In the vacuole, KCl concentration is higher than NaCl, and KCI is usually more important than NaCl in regulating turgor, although the importance of the individual cations varies with specific conditions. The steady-state concentration of the principal photosynthetic product, digeneaside, increases with increasing external osmotic pressure. Its concentration is too low to affect the internal osmotic pressure if it is distributed evenly throughout the cell but, if it is restricted to the cytoplasm, it can play a major role in regulating the volume of the cytoplasm.