GROWTH-COMPENSATING PHENOMENA IN CONTINUOUS CULTURES OF DUNALIELLA-TERTIOLECTA LIMITED SIMULTANEOUSLY BY LIGHT AND NITRATE

Two nitrogen-limited continuous cultures of Dunaliella tertiolecta wer e grown on light/dark cycles. One was submitted to limiting photon flu x density (PFD) and the other to nonlimiting PFD. The growth rate was identical in the two cultures despite the difference in the PFD condit ions. Once equilibria were reached in both cultures, the PFD were reve rsed to simulate a decrease and an increase in light conditions. A lar ge suite of variables was measured to characterize the response of the cells, mainly through the interactions of carbon and nitrogen assimil ation pathways. A decrease in irradiance led to a rapid decrease in al gal biovolume; the biovolume-based growth rate (mu) descended to level s lower than those estimated for cells of the light-limited culture. A n increase in irradiance rapidly led to an increase in mu, which attai ned values greater than those observed in high-light cells of the othe r culture before the shift. For the two cultures before and after the shift, cell carbon and cell volume were strongly correlated, showing t he same pattern of diel variations. The specific C fixation rates (mu C) of the two cultures before and after the light shift declined signi ficantly during the light periods. Before the Light shift, mu C was pa radoxically higher in the low Light culture and could not be predicted only from the light levels. This suggests that some compensatory phen omena may occur during light and nitrogen limitations. Time variations of cell Chl a due to photoacclimation in both cultures were correlate d with their N status. Similarly, the regulation of carboxylase activi ties (Rubisco) by the Light levels was sensitive to the degree of N li mitation. We found that nitrogen limitation has an overriding effect c ompared to Light for the regulation of cell volume, C fixation and res piration rates, Chi a synthesis, electron transport system, and Rubisc o activities. In cultures subjected to low irradiance, NO3- uptake rat e decreased several hours into the dark phase, suggesting a time lag b etween the end of photosynthate production and the exhaustion of compo unds necessary for dissolved inorganic nitrogen assimilation. The impl ications of these results concerning phytoplankton growth modeling in a variable environment are important because there are no existing mod els that correctly integrate the simultaneous effects of light and nit rogen on primary production. It is shown that the effects of these lim itations were not additive in the range of light and N limitations tes ted in this experiment.