Picoeukaryotic diversity and distribution in the subtropical-tropical South China Sea

Little is known regarding the diversity and distribution of picoeukaryotes in the northwestern Pacific Ocean, although these organisms are vital components of their environment. Here, we used a culture-independent approach to assess the 18S rDNA diversity of picoeukaryotes at six sampling sites along a transcontinental section of the South China Sea. The Alveolata group comprised 58.6% of the clones and was mainly represented by the novel marine alveolates (MALV)-I (18.8%) and MALV-II (30.6%), corresponding to 66.5% of all operational taxonomic units. Sequences affiliated with seven clades of the novel marine stramenopiles (MAST) were widely distributed in different clone libraries. We report an entirely new group representing the deepest evolutionary branch of the Hacrobia; this finding suggests the existence of novel picoeukaryotes at a high taxonomic level. Many phylotypes could not be taxonomically assigned, indicating the presence of numerous previously unknown groups. Horizontally, picoeukaryotic assemblages in the coastal water characterized with the rare occurrence of MALV-I were distinct from offshore communities. Vertically, MAST-4 were mainly retrieved in surface waters; however, the Radiolaria (Rhizaria) were mainly detected in clone libraries from depths of 60 m. Our findings further emphasize the immense diversity of picoeukaryotes, especially in the subtropical-tropical northwestern Pacific Ocean.