High biomass turnover rates of endosymbiotic nitrogen‐fixing cyanobacteria in the western Bering Sea

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.