Light-dependence of diatom growth responses to elevated CO2 (HC, 1000 μatm, pHT 7.68) compared to ambient CO2 level (LC, 390 μatm, pHT 8.02). Growth rates were stimulated by elevated CO2 under low to moderate PAR, but inhibited under higher PAR levels. The PAR thresholds for the transition from CO2 growth stimulation to inhibition were about 160, 125 and 178 μmol photons m−2 s−1 for Phaeodactylum tricornutum (A), Thalassiosira pseudonana (B) and Skeletonema costatum (C), respectively, which were correspondingly about 49%, 45% and 50% lower than those grown under the low CO2 level. These light levels correspond to 22- 36 % of the incident surface solar PAR levels and are equivalent to PAR levels at 26-39 m depth in the South China Sea. The semi-continuous cultures were maintained under the sun and diluted every 24 h to ensure stability of cell density ranges and the seawater carbonate system
