AME 79:137-147 (2017)  -  DOI: https://doi.org/10.3354/ame01820

ABSTRACT: 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 pCO₂ levels, it showed different responses to elevated pCO₂ levels under growth-limiting (20 µmol photons m^-2 s^-1, LL) compared with growth-saturating (200 µmol photons m^-2 s^-1, HL) light levels. With pCO₂ increased up to 950 µatm, growth rates and primary productivity increased, but in the HL cells, these parameters decreased significantly at higher concentrations up to 5000 µatm, while no difference in growth was observed with pCO₂ for the LL cells. Elevated CO₂ concentrations reduced the size of the intracellular 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.

KEY WORDS: Ocean acidification · Intracellular DIC · Photosynthesis