Nitrous oxide (N2O), a potent greenhouse gas in the atmosphere, is produced mostly from aquatic ecosystems, to which algae substantially contribute. However, mechanisms of N2O production by photosynthetic organisms are poorly described. Here we show that the green microalga Chlamydomonas reinhardtii reduces NO into N2O using the photosynthetic electron transport. Through the study of C. reinhardtii mutants deficient in flavodiiron proteins (FLVs) or in a cytochrome p450 (CYP55), we show that FLVs contribute to NO reduction in the light, while CYP55 operates in the dark. Both pathways are active when NO is produced in vivo during the reduction of nitrites and participate in NO homeostasis. Furthermore, NO reduction by both pathways is restricted to chlorophytes, organisms particularly abundant in ocean N2O-producing hot spots. Our results provide a mechanistic understanding of N2O production in eukaryotic phototrophs and represent an important step toward a comprehensive assessment of greenhouse gas emission by aquatic ecosystems.

中文翻译：

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藻类的光合作用将一氧化氮转化为一氧化二氮。

一氧化二氮（N2O）是大气中一种有力的温室气体，主要来自水生生态系统，藻类对此做出了重要贡献。但是，光合生物生产N2O的机制描述得很少。在这里，我们显示了绿色微藻莱茵衣藻利用光合电子传输将NO还原为N2O。通过研究缺乏黄素二铁蛋白（FLV）或细胞色素p450（CYP55）的莱茵衣藻突变体，我们发现FLV有助于在光下减少NO，而CYP55在黑暗中运行。当在亚硝酸盐的还原过程中在体内产生NO时，这两种途径都是活跃的，并参与NO体内稳态。此外，通过这两种途径减少的NO仅限于绿藻类，而绿藻类是在产生海洋N2O的热点中特别丰富的生物。