Disentangling protist communities identified from DNA and RNA surveys in the Pearl River–South China Sea Continuum during the wet and dry seasons

Little is known about the mechanisms that regulate the assembly of microbial communities in the river–sea continuum. Here, we performed HiSeq paired-end sequencing of the V4 region of 18S rRNA gene, using both DNA and RNA extracts, to identify protist communities in the surface and bottom water layers along a transect of the Pearl River–South China Sea Continuum (PSC) during the wet (summer) and dry (winter) seasons. We found that during the summer but not during the winter, protist communities, identified from their DNA or RNA signatures, could be better explained by mass effects and species sorting, respectively. Moreover, protist diversity in the DNA and RNA surveys exhibited similar trends along the transect, that is, a linear upstream-to-downstream decrease during the summer and a weakly U-shaped curve during the winter. In contrast, the taxonomic compositions in the DNA- and RNA-derived communities were remarkably different during either the summer or the winter. In summary, the results of our DNA and RNA surveys revealed the temporal assembly of protist communities in the PSC, which suggests that protist diversity and composition are highly responsive to the hydrographic conditions of the river–sea continuum.