太阳紫外辐射对浮游植物光合固碳的促进和抑制效应与其细胞大小的关系

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.

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。

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(UV)通常被认为抑制固碳。近年来的研究结果显示，在光强较低时(如阴天情况下或在较深的水层内)阳光UV提高近岸浮游植物群体的光合固碳量，而降低远洋群体的固碳量。从近岸到远洋受营养盐等环境要素变化的影响，优势浮游植物类群由细胞粒径较大的转变为较小的，同时较大的浮游植物细胞内可以合成和储存紫外吸收物质而较小的细胞则不能，这可能是导致阳光UV对浮游植物群体固碳的促进(近岸海域)和抑制(远洋海域)效应空间变化的原因之一。因此，本项目选择在近岸现场将浮游植物按照粒径大小分为<5 µm，5-20 µm和>20 µm的 3个不同类群，研究阳光UV的效应，结果发现只有粒径较大的浮游植物(>5 µm)可以UV能量进行光合固碳，粒径较小的(<5 µm)则不能，而且粒径越大对UV利用能力越强，如粒径>20 µm浮游植物群体UV利用能力高于粒径为5-20 µm浮游植物群体的；同时，我们在外海海域利用原位浮游植物群体(粒径<5 µm浮游植物生物量占总生物量的90%以上)开展了进一步的实验，结果证实了外海浮游植物(细胞粒径较小)不能利用UV能量进行固碳这一结论。我们的研究结果从浮游植物种群结构组成的差异方面揭示了UV的正面效应仅发生在近岸海域的内在原因，也进一步阐析了前期关于阳光UV对浮游植物初级生产力的正面效应(近岸海域)和负面效应(外海海域)的报导不一致的内在原因。.我们室内单种培养实验结果显示，在营养盐氮素(N)的限制条件下阳光UV对浮游植物光合能力的抑制率提高，即UV负面效应增强，可能是由于N限制条件下细胞会优先把所获得的N素分配到其生存所必须的组份上，而不是合成用于防御UV损伤的物质，致使N限制条件下UV负面效应加强。同时，我们还发现较短的光照时间(光周期)更有利于粒径较大浮游植物的生长，而细胞较小的浮游植物对光周期变化不明显；考虑到受潮汐等因子影响近岸海域水体混合较外海更为剧烈，浮游植物在随水体混合而上下移动时接受光强变化较大，因此我们的结果也从光生物学角度揭示了近岸海域以粒径较大浮游植物占优势的内在原因。

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