Influence of ocean acidification on thermal reaction norms of carbon metabolism in the marine diatom Phaeodactylum tricornutum

Abstract

Under the present CO₂ condition, the efficiency of biological pump mediating carbon sequestration is predicted to decline in the future because respiration tends to be more sensitive to rising temperature than is photosynthesis. However, it remains unknown whether the impacts of global warming on metabolic rates of phytoplankton can be modulated by elevated CO₂ induced ocean acidification. Here we show that in the model diatom species Phaeodactylum tricornutum, E_a (activation energy) of photosynthesis (~0.5 eV) was significantly lower than that of respiration (1.8 eV), while CO₂ concentration had no effect on the E_a value. E_h (deactivation energy) of respiration was increased to 2.5 eV, that was equivalent to E_h of photosynthesis in high CO₂-grown cells and 28.4% higher than that in low CO₂-grown ones. The respiration to photosynthesis ratio (R/P) was consistently higher in high CO₂ condition, which increased with temperature at the beginning and subsequently decreased in both CO₂ conditions. The ratio of R/P in high CO₂ to R/P in low CO₂ gradually increased with temperature above the optimal temperature. Our results imply that ocean acidification will aggravate the negative impacts or offset the alleviating effects of warming on the R/P ratio depending on the temperature range in Phaeodactylum tricornutum.

Keywords: Carbon fixation; Ocean acidification; Phaeodactylum tricornutum; Phytoplankton; Respiration; Temperature; Warming.