Composition and genetic diversity of picoeukaryotes in the northeastern South China Sea during the Luzon winter bloom

We analyzed picoeukaryotes during the Luzon winter bloom, in combination with remote sensing, flow cytometry and high throughput sequencing. Satellite images showed a seasonal increase in phytoplankton in the study area during winter, and the in situ campaign observed an extremely high abundance of photosynthetic picoeukaryotes, which were the main contributors to the total phytoplankton biomass. Auto- and mixotrophic picoalgae (Chlorophyta, Ochrophyta, and Dinophyta), heterotrophic flagellates (Radiolaria) and parasites (Syndiniale) were found to be potentially numerous components of picoeukaryote communities. Overall, auto- and mixotrophic photosynthetic picoeukaryotes were mainly distributed in the upper layers (0–100 m) while heterotrophic flagellates and parasites dominated the deep layers (200–300 m). The Kuroshio-induced upwelling stimulated the growth of photosynthetic picoeukaryotes, which mainly belonged to Chlorophyta, especially the class Mamiellophyceae.