The modulating effect of light intensity on the response of the coccolithophore Gephyrocapsa oceanica to ocean acidification

Abstract

Global change leads to a multitude of simultaneous modifications in the marine realm among which shoaling of the upper mixed layer, leading to enhanced surface layer light intensities, as well as increased car- bon dioxide (CO2) concentration are some of the most critical environmental alterations for phytoplankton. In this study, we investigated the responses of growth, photosynthetic carbon fixation and calcification of the coccolithophore Gephyrocapsa oceanica to elevated pCO2 (51 Pa, 105 Pa, and 152 Pa) (1 Pa 10 μtm) at a variety of light intensities (50-800 µmol photons m-2 s-1). By fitting the light response curve, our results showed that rising PCO2 reduced the maximum rates for growth, photosynthetic carbon fixation and calcification. Increasing light intensity enhanced the sensitivity of these rate responses to PCO2, and shifted the PCO2 optima toward lower levels. Combining the results of this and a previous study (Sett et al. 2014) on the same strain indicates that both limiting low PCO2 and inhibiting high PCO2 levels (this study) induce similar responses, reducing growth, carbon fixation and calcification rates of G. oceanica. At limiting low light intensities the PCO2 optima for maximum growth, carbon fixation and calcification are shifted toward higher levels. Interacting effects of simultaneously occurring environmental changes, such as increasing light intensity and ocean acidification, need to be considered when trying to assess metabolic rates of marine phytoplankton under future ocean scenarios.