Protist diversity and community structure around the Xianbei Seamount in the South China Sea

Seamounts are topographic rises of the seabed that represent one of the most common underwater ecosystems worldwide. Marine protists comprise various species with complex ecological functions. However, protist communities in seamount-associated ecosystems have not been adequately studied. In this study, we investigated planktonic protist communities around the Xianbei Seamount in the South China Sea. The entire protist community was profiled using universal primers for the 18S rRNA gene, whereas diplonemids (i.e., partial protist communities) were specifically explored using group-specific primers, and the cell numbers of the diplonemids were quantified based on copy numbers. We found that the alpha diversity (richness and Shannon indices) of the entire and partial protist communities significantly decreased and increased, respectively, across the water columns. The water depth-regulated patterns of beta diversity were remarkable, and a depth–decay relationship of community similarity was observed for both categories. Moreover, all four families of diplonemids (Eupelagonemidae, Hemistasiidae, DSPD II, and Diplonemidae) were recovered from water masses around the seamount, and the diplonemid assemblages showed horizontal and vertical changes in cell numbers (ranging from 1.8 × 10 3 to 5.4 × 10 4 cells/L). In summary, this study sheds light on the diversity and compositional structures of both entire and partial (i.e., diplonemid-related) protist communities in pelagic ecosystems, with an emphasis on seamount-associated areas. IMPORTANCE This study provides a reference for protist communities around the Xianbei Seamount in the South China Sea. Protists are characterized by their vast diversity and fundamentally distinct functions, whereas studies on subclades performed to obtain a better understanding remain scarce. In particular, we explored the diversity and distribution of diplonemids, which have previously been characterized as the most species-rich lineage. Our results, based on environmental DNA metabarcoding using the newly designed primers, support the great diversity of diplonemids. We further quantified the cell number of diplonemids based on the copy number of the 18S rRNA gene and determined the horizontal and vertical variations around the seamount. In summary, in this study, we not only describe protist communities in rarely investigated habitats (i.e., seamount-associated ecosystems) but also provide new information on diplonemids that apparently act as the dark taxa of marine protists.