Abstract

We examined the effects of increased temperature, pCO2, and irradiance on a calcifying strain of the marine coccolithophore Emiliania huxleyi in semi-continuous laboratory cultures. Emiliania huxleyi CCMP 371 was cultured in four temperature and pCO2 treatments at both low and high irradiance (50 and 400 µmol photons m−2 s−1): (i) 20°C and 375 ppm CO2 (ambient control); (ii) 20°C and 750 ppm CO2 (high pCO2); (iii) 24°C and 375 ppm CO2 (high temperature); and (iv) 24°C and 750 ppm CO2 ('greenhouse'). The growth of E. huxleyi was greatly accelerated by elevated temperature at low irradiance. Photosynthesis was significantly promoted by increases in both pCO2 and temperature at both irradiances. Higher cellular C/P ratios were found in the higher CO2 treatments at high irradiance, indicating a reduced requirement for P. The PIC/POC (particulate inorganic to organic carbon) ratio remained constant at low light, regardless of CO2 or temperature conditions. However, both the cellular PIC content and PIC/POC ratio were greatly decreased by elevated irradiance, and were further decreased by increased pCO2 only at high light, indicating a combined effect of CO2 and light on calcification. These results suggest that future trends of CO2 enrichment, sea-surface warming and exposure to higher mean irradiances from intensified stratification will have a large influence on the growth of Emiliania huxleyi, and potentially on the PIC/POC 'rain ratio'. Our study demonstrates that it is possible to obtain a more complete picture of global change impacts on marine phytoplankton by designing experiments that consider multiple global change variables and their mutual interactions.