Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA, USA.
Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, Cambridge, MA, USA.
Nat Microbiol. 2022 Feb;7(2):300-311. doi: 10.1038/s41564-021-01028-1. Epub 2022 Jan 10.
Cyanobacteria of the genus Trichodesmium provide about 80 Tg of fixed nitrogen to the surface ocean per year and contribute to marine biogeochemistry, including the sequestration of carbon dioxide. Trichodesmium fixes nitrogen in the daylight, despite the incompatibility of the nitrogenase enzyme with oxygen produced during photosynthesis. While the mechanisms protecting nitrogenase remain unclear, all proposed strategies require considerable resource investment. Here we identify a crucial benefit of daytime nitrogen fixation in Trichodesmium spp. that may counteract these costs. We analysed diel proteomes of cultured and field populations of Trichodesmium in comparison with the marine diazotroph Crocosphaera watsonii WH8501, which fixes nitrogen at night. Trichodesmium's proteome is extraordinarily dynamic and demonstrates simultaneous photosynthesis and nitrogen fixation, resulting in balanced particulate organic carbon and particulate organic nitrogen production. Unlike Crocosphaera, which produces large quantities of glycogen as an energy store for nitrogenase, proteomic evidence is consistent with the idea that Trichodesmium reduces the need to produce glycogen by supplying energy directly to nitrogenase via soluble ferredoxin charged by the photosynthesis protein PsaC. This minimizes ballast associated with glycogen, reducing cell density and decreasing sinking velocity, thus supporting Trichodesmium's niche as a buoyant, high-light-adapted colony forming cyanobacterium. To occupy its niche of simultaneous nitrogen fixation and photosynthesis, Trichodesmium appears to be a conspicuous consumer of iron, and has therefore developed unique iron-acquisition strategies, including the use of iron-rich dust. Particle capture by buoyant Trichodesmium colonies may increase the residence time and degradation of mineral iron in the euphotic zone. These findings describe how cellular biochemistry defines and reinforces the ecological and biogeochemical function of these keystone marine diazotrophs.
束毛藻属的蓝藻每年向海洋表面固定约 80 太克氮，并促进海洋生物地球化学，包括二氧化碳的固存。束毛藻在白天固定氮，尽管固氮酶与光合作用产生的氧气不相容。虽然保护固氮酶的机制尚不清楚，但所有提出的策略都需要大量的资源投入。在这里，我们确定了束毛藻属白天固氮的一个关键好处，这可能会抵消这些成本。我们分析了培养和野外种群的束毛藻的昼夜蛋白质组，与夜间固定氮的海洋固氮生物 Crocosphaera watsonii WH8501 进行了比较。束毛藻的蛋白质组非常动态，同时展示了光合作用和固氮作用，导致平衡的颗粒有机碳和颗粒有机氮的产生。与大量产生糖原作为固氮酶储能的 Crocosphaera 不同，蛋白质组学证据与这样的观点一致，即束毛藻通过光合作用蛋白 PsaC 充电的可溶性铁氧还蛋白直接为固氮酶提供能量，从而减少了对糖原的需求。这最大限度地减少了与糖原相关的负荷，降低了细胞密度并降低了沉降速度，从而支持束毛藻作为一种有浮力、高光适应的群体形成蓝藻的生态位。为了占据其同时进行固氮和光合作用的生态位，束毛藻似乎是铁的明显消费者，因此开发了独特的铁获取策略，包括使用富含铁的灰尘。浮质束毛藻群落对颗粒物质的捕获可能会增加营养盐铁在透光带的停留时间和降解。这些发现描述了细胞生物化学如何定义和加强这些海洋关键固氮生物的生态和生物地球化学功能。
