Seasonal Assemblages of picocyanobacteria in the subtropical segment of the Kuroshio Current

Abstract A long-term observation has been conducted on the community composition diversity of picoplankton in the subtropical segment of Kuroshio Current from 2009 to 2015. The results showed that Synechococcus mainly distributed in the surface water layer regardless of seasonal changes, and the number of cells ranged from 104 to 105 cells mL− 1. In contrast, the maximum value of Prochlorococcus was maintained at more than 105 cells mL− 1 throughout the year, and the stirring effect caused by the monsoon determined their distribution in the water layer. In the summer and the autumn, Prochlorococcus were mainly concentrated at the water layer near the bottom of the euphotic zone. In the spring and the winter, they were evenly distributed in the euphotic zone. 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. Proteobacteria and Actinobacteria approximately accounted for 45% and 10% of the total picoplankton in the four seasons. The clade-II of Synechococcus and the High-light II of Prochlorococcus were the dominant strains in the waters all year round. These data will be important basic parameters for us to study the impact of global environmental changes on marine biogeochemical cycles in the western boundary currents in the future.