Regulation of proline content of Chlorella autotrophica in response to changes in salinity

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Abstract
In Chlorella autotrophica, proline, chlorophyll, and protein synthesis took place almost exclusively in the light. No evidence for proline degradation in the dark was obtained. An osmotic upshock from 50 to 150% artificial seawater caused a temporary inhibition of photosynthesis in this alga. An osmotic downshock from 150 to 50% artificial seawater had no effect on its photosynthetic rate. Synthesis of proline, the main osmoregulatory solute in C. autotrophica, showed no lag phase following an upshock and was not inhibited in the presence of cycloheximide. These results suggest that the enzymes of the proline pathway are not synthesized de novo in response to the upshock. In the dark, the rate of proline synthesis was lower, whereas the presence of acetate allowed a rate of proline synthesis similar to that of cells in the light. Synthesis of proline in the dark in the presence of acetate was dependent on the induction of isocitrate lyase. Thus the glyoxylate cycle plays a key role in furnishing carbon and reducing power for proline synthesis in the dark. Following a sudden downshock, more than 55% of the proline content leaked out of the cell in the first 5 min due to transient breakdown in membrne permeability. However, if the downshock is gradual, proline is oxidized by the cells rather than being leaking out.