On the other hand, down-regulated CCMs might cause additional light stress (Wu et al., 2010). Previously, energy delivered from photochemical processes was suggested to fuel HKI-272 of CO2 and HCO3− (Li and Canvin, 1998; Sültemeyer et al., 1993; Sukenik et al., 1997). The major energy expended by the CCMs in diatoms is in active Ci transport (Hopkinson et al., 2011). Doubling of the atmospheric CO2 level would save about 20% of the energy demand for CCMs of diatoms such as T. weissflogii, T. pseudonana, Thalassiosira oceanica and P. tricornutum ( Hopkinson et al., 2011). Such saved light energy might lead to additional light stress and cause photoinhibition ( Wu et al., 2010). It is possible that the observed effects of ocean acidification on different diatom species depend on the balance between the down-regulated and up-regulated physiological processes due to increased CO2 availability and changes in seawater chemistry. The aim of this study was to examine the effect of ocean acidification on growth, photosynthesis, dark respiration, Ci acquisition, photosynthetic electron transport, photoinhibition and photoprotection of T. pseudonana when acclimated to elevated CO2/low pH conditions. We found that the enrichment of CO2 to 1000 μatm with the reduction of pH to 7.83 enhanced photosynthetic carbon fixation and mitochondrial respiration but result insignificant changes in growth rate or photoinhibition.