Recent studies have described active nitrogen fixation in high-latitude waters, but the ecological controls on the occurrence or activity of nitrogen-fixing organisms (diazotrophs) in such systems remain unknown. Turnover rates and top-down controls are also general knowledge gaps for marine diazotrophs. We detected abundant UCYN-A (endosymbiotic nitrogen-fixing cyanobacteria) in the Gulf of Anadyr, western Bering Sea, which correlated with high dissolved iron to dissolved inorganic nitrogen ratios (Fe : DIN) due to riverine input. Growth and grazing mortality of UCYN-A sublineages were almost balanced with higher biomass-turnover rates compared to the whole phytoplankton community, indicating selective grazing of UCYN-A in nitrogen-depleted waters. Grazing rates on UCYN-A1 (small cells) were higher than for UCYN-A2/3/4 (large cells), consistent with the general size dependence of phytoplankton growth and grazing mortality. We found that Fe : DIN is a major determinant of UCYN-A abundances in high-latitude waters, where UCYN-A could make substantial contributions to plankton food-web cycling.

中文翻译：

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白令海西部内共生固氮蓝藻的高生物量周转率

最近的研究描述了高纬度水域的活性固氮作用，但对此类系统中固氮生物（固氮生物）的发生或活动的生态控制仍然未知。周转率和自上而下的控制也是海洋固氮菌的一般知识空白。我们在白令海西部的阿纳德尔湾检测到丰富的 UCYN-A（内共生固氮蓝藻），这与河流输入导致的高溶解铁与溶解无机氮比率（Fe：DIN）相关。与整个浮游植物群落相比，UCYN-A 亚系的生长和放牧死亡率几乎与更高的生物量周转率平衡，表明 UCYN-A 在氮耗尽的水域中选择性放牧。UCYN-A1（小细胞）的放牧率高于 UCYN-A2/3/4（大细胞），与浮游植物生长和放牧死亡率的一般大小依赖性一致。我们发现 Fe : DIN 是高纬度水域中 UCYN-A 丰度的主要决定因素，其中 UCYN-A 可以对浮游生物食物网循环做出重大贡献。