In this study, we have for the first time analyzed diel microzooplankton grazing selectivity on unicellular cyanobacterial diazotroph (i.e., Crocosphaera watsonii WH8501) and nondiazotrophic unicellular microalga (i.e., Chlorella autotrophica). A mixed diet consisting of these two phytoplankton was supplied to four species of protistal grazers during daytime and nighttime, respectively. C. watsonii fixes nitrogen during nighttime and showed a stronger diel pattern of cellular C:N ratio than C. autotrophica. All four grazers ingested more nighttime C. watsonii than daytime C. watsonii, suggesting the diazotroph became more nutritious (inferred by C:N ratio) and thus a preferred prey for grazers when it fixes nitrogen. In particular, Oxyrrhis marina changed from preferring C. autotrophica during daytime to preferring C. watsonii during nighttime. The rest grazers showed speciesspecific grazing preferences, which could be explained by extracellular polysaccharide production of C. watsonii, feeding mode, cingulum size and cell size of grazers.

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.

The warming ocean is expected to be more phosphorus (P) limited due to increasing stratification. P is a major limiting nutrient of marine diazotrophs, while the interactive effect of temperature elevation and P limitation on marine unicellular diazotrophs is unknown. Here, we examined the physiology of a major unicellular diazotroph, Crocosphaera watsonii, grown under P‐limited and P‐replete conditions at 25°C, 28°C, and 31°C. Growth, N2, and CO2 fixation rates of C. watsonii increased with temperature under P limitation, and growth rates were similar between P‐limited and P‐replete treatments at 31°C. At high temperature, the P use efficiencies for N2 and CO2 fixation under P limitation were more than twice higher than under P‐replete conditions. Expression of genes involved in P acquisition, intracellular recycling, and substitution in C. watsonii was upregulated at higher temperature under P limitation. These results suggest that P limitation in C. watsonii was relieved with elevated temperature through various temperature‐dependent economic strategies on P metabolism. Through meta‐analysis of a field data set using general additive model, we found that C. watsonii abundance was correlated mainly with temperature and phosphate, and predicted to increase significantly with further warming.