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The growth rate of a diatom (Chaetoceros protubcram) was followed during transfers from moderate to higher or lower irradianccs (PAR) with differing spectral compositions ("white,"

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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 transitional 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.

Two nitrogen-limited continuous cultures of Dunaliella tertiolecta were grown on light/dark cycles. One was submitted to limiting photon flux density (PFD) and the other to nonlimiting PFD. The growth rate was identical in the two cultures despite the difference in the PFD conditions. Once equilibria were reached in both cultures, the PFD were reversed to simulate a decrease and an increase in light conditions. A large suite of variables was measured to characterize the response of the cells, mainly through the interactions of carbon and nitrogen assimilation pathways. A decrease in irradiance led to a rapid decrease in algal biovolume; the biovolume-based growth rate (p,) descended to levels lower than those estimated for cells of the light-limited culture. An increase in irradiance rapidly led to an increase in lo, which attained values greater than those observed in high-light cells of the other culture before the shift, For the two cultures before and after the shift, cell carbon and cell volume were strongly correlated, showing the same pattern of diel variations.The specific C fixation rates (~(2) of the two cultures before and after the light shift declined significantly during the light periods. Before the light shift, p,C was paradoxically higher in the low light culture and could not be predicted only from the light levels. This suggests that some compensatory phenomena may occur during light and nitrogen limitations, Time variations of cell Chl a due to photoacclimation in both cultures were correlated with their N status. Similarly, the regulation of carboxylase activities (Rubisco) by the light levels was sensitive to the degree of N limitation. We found that nitrogen limitation has an overriding effect compared to light for the regulation of cell volume, C fixation and respiration rates, Chl a synthesis, electron transport system, and Rubisco activities. In cultures subjected to low irradiance, NO,-uptake rate decreased several hours into the dark phase, suggesting a time lag between the end of photosynthate production and the exhaustion of compounds necessary for dissolved inorganic nitrogen assimilation. The implications of these results concerning phytoplankton growth modeling in a variable environment are important because there are no existing models that correctly integrate the simultaneous effects of light and nitrogen on primary production. It is shown that the effects of these limitations were not additive in the range of light and N limitations tested in this experiment.It is now evident that the understanding of physiological processes involved in the acclimation of phytoplanktonic organisms subjected to a variable light or nutrient environment AcknowledgmentsWe thank Gilbert Malara for technical help, Michele Etienne and Olivier Bernard for assistance in programming, Serge Dallot for the statistical analysis, Jean-Francois Berthon for the P vs. I curves, Valerie Martin for the electron transport system analysis, and Patrick Chang for improving the English. The manuscript was imp...