Diatoms are a group of silicified algae that play a major role in marine and freshwater ecosystems. Diatom chloroplasts were acquired by secondary endosymbiosis and exhibit important structural and functional differences from the primary plastids of land plants and green algae. Many functions of primary plastids, including photoacclimation and inorganic carbon acquisition, are regulated by calcium-dependent signalling processes. Calcium signalling has also been implicated in the photoprotective responses of diatoms; however, the nature of calcium elevations in diatom chloroplasts and their wider role in cell signalling remains unknown. Using genetically encoded calcium indicators, we find that the diatom Phaeodactylum tricornutum exhibits dynamic calcium elevations within the chloroplast stroma. Stromal calcium ([Ca2+]str) acts independently from the cytosol and is not elevated by stimuli that induce large cytosolic calcium ([Ca2+]cyt) elevations. In contrast, high light and exogenous hydrogen peroxide (H2O2) induce large, sustained [Ca2+]str elevations that are not replicated in the cytosol. Measurements using the fluorescent H2O2 sensor roGFP2-Oxidant Receptor Peroxidase 1 (Orp1) indicate that [Ca2+]str elevations induced by these stimuli correspond to the accumulation of H2O2 in the chloroplast. [Ca2+]str elevations were also induced by adding methyl viologen, which generates superoxide within the chloroplast, and by treatments that disrupt non-photochemical quenching (NPQ). The findings indicate that diatoms generate specific [Ca2+]str elevations in response to high light and oxidative stress that likely modulate the activity of calcium-sensitive components in photoprotection and other regulatory pathways.

中文翻译：

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硅藻在强光和氧化应激下表现出动态的叶绿体钙信号


硅藻是一组硅化藻类，在海洋和淡水生态系统中发挥着重要作用。硅藻叶绿体是通过次生内共生获得的，与陆地植物和绿藻的初级质体表现出重要的结构和功能差异。初级质体的许多功能，包括光驯化和无机碳采集，都受钙依赖性信号传导过程的调节。钙信号传导也与硅藻的光保护反应有关;然而，硅藻叶绿体中钙升高的性质及其在细胞信号传导中的更广泛作用仍然未知。使用遗传编码的钙指示剂，我们发现硅藻 Phaeodactylum tricornutum 在叶绿体基质内表现出动态钙升高。基质钙 （[Ca2+]str） 独立于胞质溶胶起作用，不会因诱导细胞溶质钙 （[Ca2+]cyt） 大幅升高的刺激而升高。相比之下，高光和外源性过氧化氢 （H2O2） 会诱导大的、持续的 [Ca2+] str 升高，这些升高不会在胞质溶胶中复制。使用荧光 H 2 O 2 传感器 roGFP2 氧化剂受体过氧化物酶 1 （Orp1） 的测量表明，这些刺激诱导的 [Ca2 + ] str 升高对应于 H 2 O 2 在叶绿体中的积累。[Ca2+]str 升高也通过添加甲基紫精（在叶绿体内产生超氧化物）和破坏非光化学淬灭 （NPQ） 的处理来诱导。研究结果表明，硅藻在强光和氧化应激下产生特异性的 [Ca2+] str 升高，这可能调节光保护和其他调节途径中钙敏感成分的活性。