Ionic relations of the alga Hydrodictyon reticulatum. The effects of light conditions and inhibitors

Abstract

Summary Flows of sodium, potassium and chloride ions across the plasmalemma of the alga Hydrodictyon reticulatum were studied and their light dependence examined. Both the inflows and the outflows of the ions were found to be light-dependent in the sense of being considerably reduced in the dark. In the case of potassium the two flows appear to be influenced approximately to the same degree, since the potassium content is not significantly changed in the dark. It seems that a special mechanism is responsible for changes of the content of sodium, the highly significant decrease of which during prolonged dark periods may be shown to be due to a release of sodium bound by intracellular structures. Most of the intracellular sodium is found to be in a non-exchangeable form under normal day-and-night pattern of illumination and part of this sodium fraction is released reversibly during 48 hours in the dark, irreversibly after still more prolonged dark periods. The outflow of chloride ions in the dark seems to be less reduced than the inflow, for a significantly lower content of chloride ions was found in the dark. Coupling of individual flows with photosynthetic processes was examined with specific inhibitors. Chloride flows were found to be practically abolished with DCMU, an inhibitor of the second photosynthetic system; the flows of sodium and potassium, on the other hand, were markedly reduced with CCCP, an inhibitor of photophosphorylation. The most important conclusion drawn from these experiments seems to be that not only the unidirectional flows proceeding against a gradient of electrochemical potential are inhibited by the inhibitors, the flows in the opposite direction and in the direction of simple physico-chemical forces being inhibited in the same sense and often to the same degree. Hence the passive permeability of the plasmalemma of Hydrodictyon reticulatum appears to be very low, the flows of the ions taking place almost exclusively through metabolically-coupled active pumps in both directions. When these mechanisms are impaired by inhibition, both net flows and tracer exchange are almost completely prevented, the plasmalemma becoming virtually impermeable. In addition to that, the sodium transport in Hydrodictyon reticulatum is once more found to be insensitive to ouabain like in some of the algal cells and unlike in those of other algae.