Seasonal Patterns of Picocyanobacterial Community Structure in the Kuroshio Current

SIMPLE SUMMARY: The cell enumeration, 16S rRNA phylogenetic analysis, and hydrography determination were conducted to reveal the ecology of prokaryotic picoplankton in the subtropical Kuroshio current. The picocyanobacteria (i.e., Synechococcus and Prochlorococcus), contributing more than 50% of chlorophyll a, were important primary producers in the subtropical Kuroshio current. The notable seasonal distributions of picocyanobacteria and hydrography were also well described. We suggested the ambient nutrient contents should be the crucial parameter to determine the seasonal patterns of Synechococccus and Prochlorococcus in the study area. Because of the ability to compete for nutrients in an oligotrophic environment, picocyanobacteria would become dominant primary producers in marine ecosystems under the scenario of global warming. ABSTRACT: The nutrient-scarce, warm, and high-salinity Kuroshio current has a profound impact on both the marine ecology of the northwestern Pacific Ocean and the global climate. This study aims to reveal the seasonal dynamics of picoplankton in the subtropical Kuroshio current. Our results showed that one of the picocyanobacteria, Synechococcus, mainly distributed in the surface water layer regardless of seasonal changes, and the cell abundance ranged from 10(4) to 10(5) cells mL(−1). In contrast, the maximum concentration of the other picocyanobacteria, Prochlorococcus, was maintained at more than 10(5) cells mL(−1) throughout the year. In the summer and the autumn, Prochlorococcus were mainly concentrated at the water layer near the bottom of the euphotic zone. They were evenly distributed in the euphotic zone in the spring and winter. The stirring effect caused by the monsoon determined their distribution in the water column. In addition, the results of 16S rRNA gene diversity analysis showed that the seasonal changes in the relative abundance of Synechococcus and Prochlorococcus in the surface water of each station accounted for 20 to 40% of the total reads. The clade II of Synechococcus and the High-light II of Prochlorococcus were the dominant strains in the waters all year round. Regarding other picoplankton, Proteobacteria and Actinobacteria occupied 45% and 10% of the total picoplankton in the four seasons. These data should be helpful for elucidating the impacts of global climate changes on marine ecology and biogeochemical cycles in the Western Boundary Currents in the future.