The impact of fluctuating light on the dinoflagellate Prorocentrum micans depends on NO3− and CO2 availability

Increasing atmospheric pCO2 and its dissolution into oceans leads to ocean acidification and warming, which reduces the thickness of upper mixing layer (UML) and upward nutrient supply from deeper layers. These events may alter the nutritional conditions and the light regime to which primary producers are exposed in the UML. In order to better understand the physiology behind the responses to the concomitant climate changes factors, we examined the impact of light fluctuation on the dinoflagellate Prorocentrum micans grown at low (1 μmol L−1) or high (800 μmol L−1) [NO3−] and at high (1000 μatm) or low (390 μatm, ambient) pCO2. The light regimes to which the algal cells were subjected were (1) constant light at a photon flux density (PFD) of either 100 (C100) or 500 (C500) μmol m−2 s−1 or (2) fluctuating light between 100 or 500 μmol photons m−2 s−1 with a frequency of either 15 (F15) or 60 (F60) min. Under continuous light, the initial portion of the light phase required the concomitant presence of high CO2 and NO3− concentrations for maximum growth. After exposure to light for 3 h, high CO2 exerted a negative effect on growth and effective quantum yield of photosystem II (F′v/F′m ). Fluctuating light ameliorated growth in the first period of illumination. In the second 3 h of treatment, higher frequency (F15) of fluctuations afforded high growth rates, whereas the F60 treatment had detrimental consequences, especially when NO3− concentration was lower. F′v/F′m responded differently from growth to fluctuating light: the fluorescence yield was always lower than at continuous light at 100 μmol m−2 s−1, and always higher at 500 μmol m−2 s−1. Our data show that the impact of atmospheric pCO2 increase on primary production of dinoflagellate depends on the availability of nitrate and the irradiance (intensity and the frequency of irradiance fluctuations) to which the cells are exposed. The impact of global change on oceanic primary producers would therefore be different in waters with different chemical and physical (mixing) properties.