Combined effects of CO2 level, light intensity, and nutrient availability on the coccolithophore Emiliania huxleyi

Continuous accumulation of fossil CO2 in the atmosphere and increasingly dissolved CO2 in seawater leads to ocean acidification (OA), which is known to affect phytoplankton physiology directly and/or indirectly. Since increasing attention has been paid to the effects of OA under the influences of multiple drivers, in this study, we investigated effects of elevated CO2 concentration under different levels of light and nutrients on growth rate, particulate organic (POC) and inorganic (PIC) carbon quotas of the coccolithophorid Emiliania huxleyi. We found that OA treatment (pH 7.84, CO2 = 920 μatm) reduced the maximum growth rate at all levels of the nutrients tested, and exacerbated photo-inhibition of growth rate under reduced availability of phosphate (from 10.5 to 0.4 μmol l−1). Low nutrient levels, especially lower nitrate concentration (8.8 μmol l−1 compared with 101 μmol l−1), decreased maximum growth rates. Nevertheless, the reduced levels of nutrients increased the maximum PIC production rate. Decreased availability of nutrients influenced growth, POC and PIC quotas more than changes in CO2 concentrations. Our results suggest that reduced nutrient availability due to reduced upward advective supply because of ocean warming may partially counteract the negative effects of OA on calcification of the coccolithophorid.