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

Under the present CO 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 CO induced ocean acidification. Here we show that in the model diatom species Phaeodactylum tricornutum, E (activation energy) of photosynthesis (~0.5 eV) was significantly lower than that of respiration (1.8 eV), while CO concentration had no effect on the E value. E (deactivation energy) of respiration was increased to 2.5 eV, that was equivalent to E of photosynthesis in high CO-grown cells and 28.4% higher than that in low CO-grown ones. The respiration to photosynthesis ratio (R/P) was consistently higher in high CO condition, which increased with temperature at the beginning and subsequently decreased in both CO conditions. The ratio of R/P in high CO to R/P in low CO 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.