Hyperosmotic stress induces the rapid formation of phosphatidic acid (PA) in Chlamydomonas moewusii via the activation of two signalling pathways: phospholipase D (PLD) and phospholipase C (PLC), the latter in combination with diacylglycerol kinase (DGK) (Munnik et al., 2000). A concomitant increase in cell Ca(2+) becomes manifest as deflagellation. When KCl was used as osmoticum we found that two concentration ranges activated deflagellation: one between 50 and 100 mm and another above 200 mm. Deflagellation in low KCl concentrations was complete within 30 sec whereas in high concentrations it took 5 min. PLC was not activated, as it was by high KCl concentrations that cause hyperosmotic stress. Moreover PLD was activated more strongly by low than by high KCl concentrations. Potassium was the most potent monovalent cation based on the induction of deflagellation and the formation of PA and PBut. During treatment, the external medium acidified, indicating an increase in H(+)-ATPase activity in order to re-establish the membrane potential. Activation of PLD and deflagellation at low KCl concentrations were abrogated by treatment with La(3+), Gd(3+) and EGTA, indicating the dependency on extracellular Ca(2+). This suggests that low concentrations of KCl depolarize the plasma membrane, resulting in the activation of H(+)-ATPases and opening voltage-dependent Ca(2+) +/- channels, observed as deflagellation and an increase in PLD activity.