Resolving phytoplankton taxa based on high-throughput sequencing during brown tides in the Bohai Sea, China.

Large-scale blooms formed by pico-sized phytoplankton, which strongly inhibited feeding activity and growth of cultured scallops, have been recorded along the coast of Qinhuangdao in the Bohai Sea since 2009. Based on pigment profiles and clone library analysis of phytoplankton samples during the blooms, the major bloom-forming species was primarily identified as pelagophyte Aureococcus anophagefferens Hargraves et Sieburth, the causative species of intensive brown tides in the United States and South Africa. Due to the indistinct morphological features of the bloom-forming microalgae and limited knowledge on the composition of phytoplankton communities, there were still disputes concerning the causative species of the blooms. In this study, the method of high-throughput sequencing targeted 18S rDNA V4 region was used to study the composition of pico- and nano-sized phytoplankton communities in 2013 and 2014. A total of 18 groups of eukaryotic microalgae at the class level and more than 2000 operational taxonomic units (OTUs) were identified in phytoplankton samples collected from the brown-tide zone in the Qinhuangdao coastal waters. For both years, A. anophagefferens was the most dominant species during the bloom period and its maximum relative abundance exceeded 60 percent. Along with other evidence, the results further confirm that A. anophagefferens is the major causative species of the pico-sized phytoplankton blooms in the Bohai Sea. The outbreak of brown tide exhibited a strong inter-annual variation between 2013 and 2014, and an increasing dominance of dinoflagellates could be observed in the Qinhuangdao coastal waters.