The role of intracellular orthophosphate in triggering osmoregulation in the alga Dunaliella salina

A new hypothesis is presented for the mechanism of metabolic response during osmoregulation in the alga Dunaliella salina. We propose that the osmotic response is initiated by differential volume changes of the cytoplasm and the chloroplast (observed using the electron microscope) which alter the cytoplasmic orthophosphate concentration. This triggers a flow through the Pi/triose-phosphate shuttle, activating chloroplast enzymes in the direction of either starch or glycerol synthesis. The Pi-dependent response was investigated in vivo using NMR. The rates of glycerol synthesis or elimination following osmotic shocks were modulated by the intracellular Pi level as predicted by the hypothesis.