Physiological responses of the marine diatom Thalassiosira pseudonana to increased pCO2 and seawater acidity

We studied the effects of elevated CO2 concentration and seawater acidity on inorganic carbon acquisition, photoinhibition and photoprotection as well as growth and respiration in the marine diatom Thalassiosira pseudonana. After having grown under the elevated CO2 level (1000 μatm, pH 7.83) at sub-saturating photosynthetically active radiation (PAR, 75 μmol photons m−2 s−1) for 20 generations, photosynthesis and dark respiration of the alga increased by 25% (14.69 ± 2.55 fmol C cell−1 h−1) and by 35% (4.42 ± 0.98 fmol O2 cell−1 h−1), respectively, compared to that grown under the ambient CO2 level (390 μatm, pH 8.16), leading to insignificant effects on growth (1.09 ± 0.08 d−1 v 1.04 ± 0.07 d−1). The photosynthetic affinity for CO2 was lowered in the high-CO2 grown cells, reflecting a down-regulation of the CO2 concentrating mechanism (CCM). When exposed to an excessively high level of PAR, photochemical and non-photochemical quenching responded similarly in the low- and high-CO2 grown cells, reflecting that photoinhibition was not influenced by the enriched level of CO2. In T. pseudonana, it appeared that the energy saved due to the down-regulated CCM did not contribute to any additional light stress as previously found in another diatom Phaeodactylum tricornutum, indicating differential physiological responses to ocean acidification between these two diatom species.