Elevated CO2 and associated seawater chemistry do not benefit a model diatom grown with increased availability of light

Elevated CO2 is leading to a decrease in pH in marine environments (ocean acidification [OA]), altering marine carbonate chemistry. OA can influence the metabolism of many marine organisms; however, no consensus has been reached on its effects on algal photosynthetic carbon fixation and primary production. Here, we found that when the diatom Phaeodactylum tricornutum was grown under different pCO(2) levels, it showed different responses to elevated pCO(2) levels under growth-limiting (20 mu mol photons m(-2) s(-1), LL) compared with growth-saturating (200 mu mol photons m(-2) s(-1), HL) light levels. With pCO(2) increased up to 950 mu atm, growth rates and primary productivity increased, but in the HL cells, these parameters decreased significantly at higher concentrations up to 5000 mu atm, while no difference in growth was observed with pCO(2) for the LL cells. Elevated CO2 concentrations reduced the size of the intra cellular dissolved inorganic carbon (DIC) pool by 81% and 60% under the LL and HL levels, respectively, with the corresponding photosynthetic affinity for DIC decreasing by 48% and 55%. Little photoinhibition was observed across all treatments. These results suggest that the decreased growth rates under higher CO2 levels in the HL cells were most likely due to acid stress. Low energy demand of growth and energy saving from the down-regulation of the CO2 concentrating mechanisms (CCM) minimized the effects of acid stress on the growth of the LL cells. These findings imply that OA treatment, except for down-regulating CCM, caused stress on the diatom, reflected in diminished C assimilation and growth rates.