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