Abundance and community composition of bacterioplankton in the Northern South China Sea during winter: geographic position and water layer influences

The abundance and composition of bacterioplankton of the Northern South China Sea (NSCS) were investigated using flow cytometry and high-throughput sequencing. The results showed that the absolute abundance of bacterioplankton retained high values in surface waters at both continental shelf and oceanic sites and Proteobacteria, Cyanobacteria, and Bacteroidetes represented the three typical dominant phyla in NSCS. The average bacterioplankton abundances at 5 m, 75 m, and 200 m were 9.55, 5.04, and 1.32 × 10 cells mL, respectively, and there was a significantly positive correlation between bacterioplankton abundance and Chl a content (r = 0.84, p < 0.01). Drastic changes of the bacterioplankton community occurred in different water layers. Three operational taxonomic units (OTUs), whose distribution were significantly different between 5-m and 75-m water layers, all belonged to Flavobacteriales of the Bacteroidetes (p < 0.05). In addition, bacterioplankton community richness and diversity at the continental shelf (CS) was generally higher than at oceanic stations (SB and KI). Five OTUs, which favored the habitat of continental shelf, belonged to Alphaproteobacteria including the orders of the SAR11 cluster, Rhodospirillales, Rhodobacterales and other unclassified orders (p < 0.05). Two OTUs, which favored the habitat of oceanic stations, were assigned to the orders of Flavobacteriales and Alteromonadales. Furthermore, the abundances of two OTUs belonging to the Cyanobacteria phylum and Verrucomicrobiales order were significantly different between the sea basin (SB) and Kuroshio influenced area (KI) stations (p < 0.05).