Photosynthetic affinity for DIC or CO2 was depressed in the HC cells, as reflected in the relationship of either photosynthesis or rETR (using PAM fluorometer) v DIC curves ( Fig. 1). The half-saturation concentration (K1/2) for both DIC and CO2 increased significantly (p Verteporfin ( Fig. 1C), reflecting a down-regulation of CCM under the ocean acidification condition. The efficiency of Ci acquisition, the ratio of Vmax to K1/2 for both DIC and CO2, decreased significantly (p = 0.03) in the HC cells, by about 39% and 29% for photosynthetic rates and rETR, respectively. Independent of the different growth CO2 levels, the maximum rates of photosynthesis and electron transport were unaffected (p = 0.62). The Vmax of photosynthetic carbon fixation and rETR in the HC cells were 82.47 ± 6.32 fmol cell−1 h−1 and 95.25 ± 4.18 (a.u.), compared to 94.15 ± 14.69 fmol cell−1 h−1 and 96.54 ± 2.40 (a.u.) in the LC cells. However, both the photosynthetic rates and rETR at limiting DIC levels were significantly lower (p = 0.03) in the HC than in the LC cells, by up to 57% and 15% (inset in Fig. 1), respectively, reflecting a higher extent of the rate-limitation by less CO2 availability under DIC limiting levels in the HC grown cells.