CO₂-induced seawater acidification affects physiological performance of the marine diatom Phaeodactylum tricornutum

Abstract. CO₂/pH perturbation experiments were carried out under two different pCO₂ levels (39.3 and 101.3 Pa) to evaluate effects of CO₂-induced ocean acidification on the marine diatom Phaeodactylum tricornutum. After acclimation (>20 generations) to ambient and elevated CO₂ conditions (with corresponding pH values of 8.15 and 7.80, respectively), growth and photosynthetic carbon fixation rates of high CO₂ grown cells were enhanced by 5% and 12%, respectively, and dark respiration stimulated by 34% compared to cells grown at ambient CO₂. The half saturation constant (K_m) for carbon fixation (dissolved inorganic carbon, DIC) increased by 20% under the low pH and high CO₂ condition, reflecting a decreased affinity for HCO₃^– or/and CO₂ and down-regulated carbon concentrating mechanism (CCM). In the high CO₂ grown cells, the electron transport rate from photosystem II (PSII) was photoinhibited to a greater extent at high levels of photosynthetically active radiation, while non-photochemical quenching was reduced compared to low CO₂ grown cells. This was probably due to the down-regulation of CCM, which serves as a sink for excessive energy. The balance between these positive and negative effects on diatom productivity will be a key factor in determining the net effect of rising atmospheric CO₂ on ocean primary production.