Isofloridosid und die osmoregulation bei Ochromonas malhamensis

Abstract

Summary An increase or decrease in the osmotic value of the surrounding fluid seems to be balanced directly by the production or degradation of isofloridoside (O-α-D-galactopyranosyl-1 → 1-glycerol) inside the cells of Ochromonas malhamensis. Some more data on the physiology of this osmoregulation system will be reported here. 1. The amount of isofloridoside piled up in the cells does not depend on the chemical nature but on the osmolarity of the substances added (fig. 2). The observed increase in the concentration of some free amino acids contributes less than 8 percent towards the osmotic balance of the cells, whereas the concentrations of K+ and Na+ remain constant (fig. 3). This sustains the conclusion drawn from other experiments also [ref. 1, 2, 7] that the osmotic balance in Ochromonas is predominantly achieved by the isofloridoside mechanism. 2. The ecological significance of the mechanism is demonstrated by the fact that the level of isofloridoside in the algae changes parallel to the osmotic value of the nutrient solution during growth of the cultures (fig. 1). 3. After an increase in osmotic pressure a constant rate of isofloridoside formation is reached instantly or within a few minutes (fig. 4). This is not affected by preincubation of the algae with cycloheximide (fig. 5) or actinomycin D. Both findings suggest that the regulation of the isofloridoside pathway may occur by activation of already existing enzyme molecules rather than by the induction of enzyme new-formation. 4. The rate of isofloridoside formation and the termination of its production depend both on the extent of the osmotic stress (fig. 4). This suggests that the regulation of the isofloridoside production involves a mechanism which can sense gradual differences in the outside osmotic pressure as well as a feed-back type of mechanism which can measure the status of osmotic balance already reached. 5. Light scattering (fig. 6) as well as direct measurements (fig. 7 and 8) show that the cells shrink suddenly after an increase in osmotic pressure and regain the cell volume in the course of isofloridoside production. The extent of volume loss is the same with various osmotic pressure substances and is higher with higher concentrations (fig. 9). The shrinkage occurs roughly within the same time interval as the start of isofloridoside production. It seems, therefore, possible that on the various parameters altered by the shrinkage is involved in the translation of the physical-chemical stimulus into a first biochemical alteration which in turn leads to the regulation of the isofloridoside pathway.