Seawater carbonate chemistry and photoprotective strategies controlling electron flow through PSII and PSI in red macroalgae Pyropia yezoensis@en

While intertidal macroalgae are exposed to drastic changes in solar photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) during a diel cycle, and to ocean acidification (OA) associated with increasing CO2 levels, little is known about their photosynthetic performance under the combined influences of these drivers. In this work, we examined the photoprotective strategies controlling electron flow through photosystems II (PSII) and photosystem I (PSI) in response to solar radiation with or without UVR and an elevated CO2 concentration in the intertidal, commercially important, red macroalgae Pyropia (previously Porphyra) yezoensis. By using chlorophyll fluorescence techniques, we found that high levels of PAR alone induced photoinhibition of the inter-photosystem electron transport carriers, as evidenced by the increase of chlorophyll fluorescence in both the J- and I-steps of Kautsky curves. In the presence of UVR, photoinduced inhibition was mainly identified in the O2-evolving complex (OEC) and PSII, as evidenced by a significant increase in the variable fluorescence at the K-step (Fk) of Kautsky curves relative to the amplitude of FJ−Fo (Wk) and a decrease of the maximum quantum yield of PSII (Fv/Fm). Such inhibition appeared to ameliorate the function of downstream electron acceptors, protecting PSI from over-reduction. In turn, the stable PSI activity increased the efficiency of cyclic electron transport (CET) around PSI, dissipating excess energy and supplying ATP for CO2 assimilation. When the algal thalli were grown under increased CO2 and OA conditions, the CET activity became further enhanced, which maintained the OEC stability and thus markedly alleviating the UVR-induced photoinhibition. In conclusion, the well-established coordination between PSII and PSI endows P. yezoensis with a highly efficient photochemical performance in response to UVR, especially under the scenario of future increased CO2 levels and OA.潮间带大型藻类在昼夜周期中会经历光合有效辐射（photosynthetically active radiation, PAR）与紫外辐射（ultraviolet radiation, UVR）的剧烈变化，同时还会面临伴随CO₂浓度升高而来的海洋酸化（ocean acidification, OA），但目前学界对这些胁迫因子共同作用下其光合性能的认知仍较为匮乏。本研究以具有商业开发价值的潮间带红藻类——条斑紫菜（Pyropia yezoensis，原分类学名为Porphyra yezoensis）为研究对象，探究了其在有无UVR以及升高CO₂浓度条件下，调控光系统II（photosystem II, PSII）与光系统I（photosystem I, PSI）间电子流的光保护策略。 研究采用叶绿素荧光技术开展实验，结果显示：仅高剂量PAR即可引发光系统间电子传递载体的光抑制，具体表现为考茨基曲线的J相与I相的叶绿素荧光水平均出现升高。当体系中存在UVR时，光诱导的抑制主要发生在放氧复合体（O₂-evolving complex, OEC）与PSII中，该现象可通过考茨基曲线K相的可变荧光（Fk）相较于FJ-Fo振幅（Wk）的显著升高，以及PSII最大量子产额（Fv/Fm）的下降得到佐证。此类抑制似乎能够改善下游电子受体的功能，从而使PSI免于过度还原。反过来，稳定的PSI活性可提升PSI周围的循环电子传递（cyclic electron transport, CET）效率，以耗散过剩能量并为CO₂同化提供ATP。 当藻体在升高CO₂浓度与OA条件下培养时，循环电子传递活性会进一步增强，这维持了OEC的稳定性，进而显著缓解了UVR诱导的光抑制。综上，PSII与PSI之间良好的协同调控能力，赋予了条斑紫菜在UVR胁迫下高效的光化学性能，尤其是在未来CO₂浓度升高与海洋酸化的情景中。