Uptake and Release of Ca²⁺ in the Green Algae Mougeotia and Mesotaenium

Summary

Mougeotia cells were immersed in unbuffered artificial pond water with low calcium concentration and irradiated with far-red light, followed by red light. During irradiations the calcium ion concentration and the pH of the medium were monitored with extracellular electrodes. The measurements show that in far-red light the calcium ion concentration of the medium remained constant or increased slowly. During red light irradiation, the pH of the medium increased by about two units, the cells took up calcium ions from the medium quite rapidly, and then released them slowly back into the medium. The effect of red light on the calcium ion uptake could be simulated partly by increasing the pH of the medium artificially with NaOH.

Protoplasts freshly prepared from Mougeotia cells by enzymatic digestion of the cell walls showed calcium ion uptake, but no release, during red light irradiation. Red light irradiation of protoplasts also caused the extracellular pH to increase and the chloroplasts to move. Thus, the red light- mediated uptake of calcium ions by intact Mougeotia cells is probably membrane controlled and not caused by binding of calcium to the cell walls. The results moreover indicate that a pH-gradient at the plasmalemma is involved in the uptake of calcium ions.

In Mesotaenium cells, surrounded by well buffered media, red light caused a decrease in the surface charge of the cells, as measured by cell electrophoresis. This decrease is dependent on the uptake of calcium ions. Addition of EDTA or Verapamil to the medium prevents the red light-induced decrease in surface charge. We assume that the decrease in surface charge is, in part, caused by an efflux of mucus and calcium from the cells.