Seawater carbonate chemistry, chlorophyll a and photosynthetic carbon fixation rate under different light and pCO2 conditions during experiments, 2010@en

We carried out short term pCO2/pH perturbation experiments in the coastal waters of the South China Sea to evaluate the combined effects of seawater acidification (low pH/high pCO2) and solar UV radiation (UVR, 280–400 nm) on photosynthetic carbon fixation of phytoplankton assemblages. Under photosynthetically active radiation (PAR) alone treatments, reduced pCO2 (190 ppmv) with increased pH resulted in a significant decrease in the photosynthetic carbon fixation rate (about 23%), while enriched pCO2 (700 ppmv) with lowered pH had no significant effect on the photosynthetic performance compared to the ambient level. The apparent photosynthetic efficiency decreased under the reduced pCO2 level, probably due to C-limitation as well as energy being diverged for up-regulation of carbon concentrating mechanisms (CCMs). In the presence of UVR, both UV-A and UV-B caused photosynthetic inhibition, though UV-A appeared to enhance the photosynthetic efficiency under lower PAR levels. UV-B caused less inhibition of photosynthesis under the reduced pCO2 level, probably because of its contribution to the inorganic carbon (Ci)-acquisition processes. Under the seawater acidification conditions (enriched pCO2), both UV-A and UV-B reduced the photosynthetic carbon fixation to higher extents compared to the ambient pCO2 conditions. We conclude that solar UV and seawater acidification could synergistically inhibit photosynthesis.本研究于南海近岸海域开展了短期二氧化碳分压（pCO2）/pH扰动实验，以评估海水酸化（低pH/高pCO2）与太阳紫外辐射（UVR，280–400 nm）对浮游植物群落光合碳固定的联合影响。仅在光合有效辐射（PAR）单独处理组中，低pCO2（190 ppmv）配合pH升高会使光合碳固定速率显著下降（约23%）；而与环境水平相比，高pCO2（700 ppmv）配合pH降低对光合性能无显著影响。在低pCO2水平下，表观光合效率有所下降，这可能源于碳限制以及能量被分流用于上调二氧化碳浓缩机制（CCMs）。在存在UVR的条件下，紫外线A（UV-A）与紫外线B（UV-B）均会造成光合抑制，不过在较低PAR水平下，UV-A似乎可提升表观光合效率。在低pCO2水平下，UV-B对光合的抑制作用更弱，这可能是因为其有助于无机碳（Ci）的获取过程。在海水酸化条件（即高pCO2）下，相较于环境pCO2条件，UV-A与UV-B对光合碳固定的抑制程度更高。本研究得出结论：太阳紫外辐射与海水酸化可协同抑制浮游植物的光合作用。