Oxygenic photosynthesis converts light energy into chemical energy via electron transport and assimilates CO2 in the Calvin-Benson cycle with the chemical energy. Thus, high light and low CO2 conditions induce the accumulation of electrons in the photosynthetic electron transport system, resulting in the formation of reactive oxygen species. To prevent the accumulation of electrons, oxygenic photosynthetic organisms have developed photoprotection mechanisms, including non-photochemical quenching (NPQ) and alternative electron flow (AEF). There are diverse molecular mechanisms underlying NPQ and AEF, and the corresponding molecular actors have been identified and characterized using a model green alga Chlamydomonas reinhardtii. In contrast, detailed information about the photoprotection mechanisms is lacking for other green algal species. In the current study, we examined the photoprotection mechanisms responsive to CO2 in the green alga Chlorella variabilis by combining the analyses of pulse-amplitude-modulated fluorescence, O2 evolution, and the steady-state and time-resolved fluorescence spectra. Under the CO2-limited condition, ΔpH-dependent NPQ occurred in photosystems I and II. Moreover, O2-dependent AEF was also induced. Under the CO2-limited condition with carbon supplementation, NPQ was relaxed and light-harvesting chlorophyll-protein complex II was isolated from both photosystems. In C. variabilis, the O2-dependent AEF and the mechanisms that instantly convert the light-harvesting functions of both photosystems may be important for maintaining efficient photosynthetic activities under various CO2 conditions.

中文翻译：

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绿藻小球藻光合作用过程中不同CO2体制下的光保护机制。

氧合光合作用通过电子传输将光能转换为化学能，并在Calvin-Benson循环中利用化学能吸收二氧化碳。因此，高光和低CO 2条件引起光合电子传输系统中电子的积累，从而导致活性氧的形成。为了防止电子积累，含氧光合生物已开发出光保护机制，包括非光化学猝灭（NPQ）和替代电子流（AEF）。NPQ和AEF的分子机制多种多样，并且已经使用模型绿藻衣藻（Chlamydomonas reinhardtii）鉴定并表征了相应的分子演员。相反，其他绿色藻类物种缺乏有关光保护机制的详细信息。在当前的研究中，我们通过结合脉冲幅度调制荧光，O2演变以及稳态和时间分辨荧光光谱的分析，研究了绿藻小球藻中对CO2响应的光保护机制。在CO2限制条件下，光系统I和II中出现了依赖于pH的NPQ。此外，还诱导了O2依赖性AEF。在补充碳的CO2限制条件下，放松了NPQ，并从两个光系统中分离了捕光的叶绿素-蛋白质复合物II。在C. variabilis中，依赖O2的AEF和立即转换两个光系统的光收集功能的机制对于在各种CO2条件下维持有效的光合作用可能很重要。我们通过结合脉冲幅度调制荧光，O2演变以及稳态和时间分辨荧光光谱的分析，研究了绿藻小球藻对CO2响应的光保护机制。在CO2限制条件下，光系统I和II中出现了依赖于pH的NPQ。此外，还诱导了O2依赖性AEF。在碳限制的CO2限制条件下，放松了NPQ，并从两个光系统中分离了捕光的叶绿素-蛋白质复合物II。在C. variabilis中，依赖O2的AEF和即时转换两个光系统的光收集功能的机制对于在各种CO2条件下维持有效的光合作用可能很重要。我们通过结合脉冲幅度调制荧光，O2演变以及稳态和时间分辨荧光光谱的分析，研究了绿藻小球藻对CO2响应的光保护机制。在CO2限制条件下，光系统I和II中出现了依赖于pH的NPQ。此外，还诱导了O2依赖性AEF。在补充碳的CO2限制条件下，放松了NPQ，并从两个光系统中分离了捕光的叶绿素-蛋白质复合物II。在C. variabilis中，依赖O2的AEF和立即转换两个光系统的光收集功能的机制对于在各种CO2条件下维持有效的光合作用可能很重要。