Ocean acidification mediates photosynthetic response to UV radiation and temperature increase in the diatom Phaeodactylum tricornutum@en

Increasing atmospheric CO2 concentration is responsible for progressive ocean acidification, ocean warming as well as decreased thickness of upper mixing layer (UML), thus exposing phytoplankton cells not only to lower pH and higher temperatures but also to higher levels of solar UV radiation. In order to evaluate the combined effects of ocean acidification, UV radiation and temperature, we used the diatom Phaeodactylum tricornutum as a model organism and examined its physiological performance after grown under two CO2 concentrations (390 and 1000 µatm) for more than 20 generations. Compared to the ambient CO2 level (390 µatm), growth at the elevated CO2 concentration increased non-photochemical quenching (NPQ) of cells and partially counteracted the harm to PS II (photosystem II) caused by UV-A and UV-B. Such an effect was less pronounced under increased temperature levels. The ratio of repair to UV-B induced damage decreased with increased NPQ, reflecting induction of NPQ when repair dropped behind the damage, and it was higher under the ocean acidification condition, showing that the increased pCO2 and lowered pH counteracted UV-B induced harm. As for photosynthetic carbon fixation rate which increased with increasing temperature from 15 to 25 °C, the elevated CO2 and temperature levels synergistically interacted to reduce the inhibition caused by UV-B and thus increase the carbon fixation.大气二氧化碳浓度升高会引发渐进性海洋酸化、海洋变暖以及上层混合层（upper mixing layer, UML）厚度降低，从而使浮游植物细胞不仅面临更低pH值与更高温度的胁迫，还会受到更强的太阳紫外辐射照射。为评估海洋酸化、紫外辐射与温度的复合影响，本研究以硅藻三角褐指藻（Phaeodactylum tricornutum）作为模式生物，将其在两种二氧化碳浓度（390与1000 µatm）下培养超过20代后，检测其生理性能。与环境二氧化碳水平（390 µatm）相比，升高的二氧化碳浓度可提升细胞的非光化学淬灭（non-photochemical quenching, NPQ），并部分抵消紫外A（UV-A）与紫外B（UV-B）对光系统II（photosystem II, PS II）造成的损伤。在温度升高的条件下，该保护效应会显著减弱。修复与紫外B诱导损伤的比值随非光化学淬灭水平升高而降低，这反映了当修复速率滞后于损伤速率时非光化学淬灭的诱导过程；而该比值在海洋酸化条件下更高，表明升高的二氧化碳分压与降低的pH可抵消紫外B诱导的损伤。至于在15至25℃范围内随温度升高而提升的光合固碳速率，升高的二氧化碳浓度与温度可产生协同交互作用，降低紫外B造成的抑制效应，进而提升光合固碳速率。