Two marine microalgae (Haptophyceae), Hymenornonas elongata and Isochrysis galbana, were grown in turbidostats in order to investigate the influence of light intensity on cell volume and biochemical composition. At irradiance levels that saturated growth (400pE m-2 S-'), the algae contained a high level of carbon compounds, especially soluble sugars. At the irradiance that limited growth (25 pE m-2 S-') sugar pools were lower for the 2 algae and H. elongata decreased its volume by 25 %. In the

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... ransitional phase that followed a shift from 400 pE m-2 S-' to 25 pE m-2 S-' rates at which biochemical ratios reached stability differed for the 2 algae. Re-establishing high light intensity after a period under low light intensity influenced algal volume in H. elongata; t h s event followed a particular lunetic pattern that we attempt to explain on theoretical grounds. Results are discussed with regard to durations of irradiance perturbations that can occur in the sea.