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Molecular and physiological responses of diatoms to variable levels of irradiance and nitrogen availability: Growth of Skeletonema costatum in simulated upwelling conditions

Molecular mechanisms that drive metabolic acclimation to environmental shifts have been poorly characterized in phytoplankton. In this laboratory study, the response of light- and N-limited Skeletonema costatum cells to an increase in light and NO, availability was examined. C assimilation was depressed relative to N assimilation early in enrichment, and the photosynthetic quotient (0, : CO,) increased, consistent with the shunting of reducing equivalents from CO, fixation to NO; reduction. The concomitant increase in dark respiration was consistent with the increased energetic demand associated with macromolecular synthesis. The accelerations of N-specific rates of NO,- uptake and nitrate rcductase activity (NRA) over the first 24 h were comparable to observations for coastal upwelling systems. Increases in cell-specific rates of these proccsscs, however, were confined to the first 8 h of enrichment. The abundance of 18s ribosomal ribonucleic acid (rRNA) increased immediately after the environmental shift, followed by increases in levels of NR-specific mRNA that coincided with the acceleration in NO,- assimilation. NRA, however, exhibited a diurnal rhythm that did not correspond to changes in NR protein abundance, suggesting that enzyme activity was also regulated by direct modulation of existing NR protein by light and NO, - availability.