海洋浮游植物光合固碳量约占全球生态系统固碳总量的50%，太阳可见光驱动浮游植物固碳而紫外光通常被认为抑制固碳。我们近期的研究发现，光强较低时太阳紫外辐射促进近岸浮游植物群体的光合固碳，而对远洋群体的固碳仍有抑制作用。由近岸到远洋优势浮游植物从细胞粒径较大的转变为较小的，同时较大的细胞可以合成和储存紫外吸收物质而较小的则不能，这可能是导致紫外辐射对浮游植物固碳的促进(近岸海域)和抑制(远洋海域)效应空间变化的最关键因素之一。为此，本项目拟用室内机理探讨与现场实验验证相结合的方法(室内选择7-8种细胞直径<3-40μm间的单种浮游植物；近海和外海典型海域利用<3μm，3-20μm和>20μm的3个粒级浮游植物群体)，探讨紫外辐射对浮游植物光合固碳生理的影响与其细胞大小的关系，揭示低光条件下(阴天或较深水层)阳光紫外辐射对近岸浮游植物群体固碳的促进效应和对远洋群体固碳的抑制效应的生理学机制。

Marine phytoplankton has been estimated to be responsible for about 50% of global primary production. Solar photosynthetically active radiation drives photosynthetic carbon fixation of phytoplankton dwelling in the euphotic zone, while ultraviolet radiation (UVR, 280-400 nm) is often considered to reduce the photosynthesis. Recently, we carried out the field investigations in surface waters from coasts to pelagic oceans of the South China Sea and found that, on cloudy days (the sunlight level was lower), solar UVR positively enhanced the photosynthetic carbon fixation of phytoplankton assemblages in coastal waters, but negatively reduced the carbon fixation in oceanic waters. It's well known that the dominating groups usually change from phytoplankton with larger cells in coastal waters to the smaller ones in oceanic waters; meanwhile, the larger cells (e.g., >10 μm in diameter) have been identified to have ability to synthesize and accumulate UV-absorbing compounds whilst the smaller ones (e.g., <2 μm) do not have such a ability. We thus hypothesize that the UV-absorbing compounds that merely present in larger phytoplankton cells, can be used to absorb and transfer UV-energy to reaction center of chlorophyll a, and drive the photosynthetic processes and then promote the carbon fixation; thus, changes in cell-size of phytoplankton may be one of the key causes for the negative and positive impacts of solar UVR respectively in coastal and oceanic waters at lower sulight (e.g.,on cloudy days or at deep water layes). Therefore, this project was designed to testify this hypothesis through studying the physiological mechanisms of the positive and negative impacts of UV radiation on phytoplankton photosynthesis in laboratory, together with the testified-experiments on board. The laboratorial experiments will be conducted using 7 to 8 species of phytoplankton with different cell-size (<2-40 μm) to characterize the UV-impacts (based on carbon fixation) and analyze its relationship to cell-size; while the on-board experiments will be carried out with natural phytoplankton assemblages of differential cell-size (<3 μm，3-20 μm and >20 μm) from typical stations of the coastal and oceanic waters of the South China Sea or/and the Indian Ocean. The goals of this project are: i) to clarify the relationships of the UV-impacts upon phytoplankton physiological carbon fixation and its cell-size; ii) to better understand the physiological mechanisms of the positive and negative effects of solar UV radiation on photosynthetic carbon fixation by phytoplankton assemblages respectively in the coastal and oceanic waters at the low-light conditions i.e. on cloudy days or at deep water layers.