Skidaway Institute of Oceanography, Department of Marine Sciences, University of Georgia, Savannah, GA 31411;
Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.
Proc Natl Acad Sci U S A. 2019 Aug 13;116(33):16448-16453. doi: 10.1073/pnas.1821233116. Epub 2019 Jul 25.
Reactive oxygen species (ROS) like superoxide drive rapid transformations of carbon and metals in aquatic systems and play dynamic roles in biological health, signaling, and defense across a diversity of cell types. In phytoplankton, however, the ecophysiological role(s) of extracellular superoxide production has remained elusive. Here, the mechanism and function of extracellular superoxide production by the marine diatom are described. Extracellular superoxide production in exudates was coupled to the oxidation of NADPH. A putative NADPH-oxidizing flavoenzyme with predicted transmembrane domains and high sequence similarity to glutathione reductase (GR) was implicated in this process. GR was also linked to extracellular superoxide production by whole cells via quenching by the flavoenzyme inhibitor diphenylene iodonium (DPI) and oxidized glutathione, the preferred electron acceptor of GR. Extracellular superoxide production followed a typical photosynthesis-irradiance curve and increased by 30% above the saturation irradiance of photosynthesis, while DPI significantly impaired the efficiency of photosystem II under a wide range of light levels. Together, these results suggest that extracellular superoxide production is a byproduct of a transplasma membrane electron transport system that serves to balance the cellular redox state through the recycling of photosynthetic NADPH. This photoprotective function may be widespread, consistent with the presence of putative homologs to GR in other representative marine phytoplankton and ocean metagenomes. Given predicted climate-driven shifts in global surface ocean light regimes and phytoplankton community-level photoacclimation, these results provide implications for future ocean redox balance, ecological functioning, and coupled biogeochemical transformations of carbon and metals.
活性氧（ROS）如超氧自由基驱动水生系统中碳和金属的快速转化，并在多种细胞类型中发挥生物健康、信号传递和防御的动态作用。然而，在浮游植物中，细胞外超氧自由基产生的生态生理学作用仍然难以捉摸。本文描述了海洋硅藻细胞外超氧自由基产生的机制和功能。胞外超氧自由基的产生与 NADPH 的氧化偶联。一种推测的 NADPH 氧化黄素酶具有跨膜结构域和与谷胱甘肽还原酶（GR）高度相似的序列，与该过程有关。GR 还通过黄素酶抑制剂二苯基碘（DPI）和氧化型谷胱甘肽（GR 的首选电子受体）与整个细胞的细胞外超氧自由基产生相关。细胞外超氧自由基的产生遵循典型的光合作用-光照曲线，并在光合作用的饱和光照之上增加了 30%，而 DPI 在广泛的光照水平下显著降低了光系统 II 的效率。这些结果表明，细胞外超氧自由基的产生是跨质膜电子传递系统的副产物，通过光合作用 NADPH 的循环利用来平衡细胞的氧化还原状态。这种光保护功能可能很普遍，与其他代表性海洋浮游植物和海洋宏基因组中存在的 GR 假定同源物一致。鉴于全球表层海洋光照条件和浮游植物群落水平光适应的预测气候驱动变化，这些结果对未来海洋氧化还原平衡、生态功能以及碳和金属的耦合生物地球化学转化具有重要意义。
