Diversity, distribution and nitrogen use strategies of bacteria in the South China Sea basin

Abstract. The diversity and abundance of bacteria and diazotrophs in the euphotic and aphotic layers of the South China Sea (SCS) basin were investigated based on high-throughput sequencing of the 16S rRNA and nifH genes. Bacterial communities in the aphotic layers significantly differed from those at the euphotic layers, and were characterized by geographical specificities. Prochlorococcus and Alphaproteobacteria were abundant in the surface layer, whereas Gammaproteobacteria was more common in the aphotic layers. Moraxellaceae was the most abundant group in the aphotic layer in the northern basin of the SCS (nSCS), while SAR324, SAR202 and SAR406 occurred mainly in the southern basin of the SCS (sSCS). Diazotrophic Alphaproteobacteria was the predominant group in the SCS basin, whereas Marine Group II Euryarchaeota emerged in the euphotic bottom of both nSCS and sSCS. Abundances of genes encoding amino acid transporters and ammonium assimilating enzymes were relatively high in the SCS surface and the entire water column of the sSCS, while expression levels of urea and ammonium transporter-encoding genes were the highest at the surface of the SEATS site. Iron deficiency-induced gene IdiA and urease were highly expressed at the A2 site. Our results indicated that bacterial communities in the SCS were depth-stratified and exhibited geographic divergency in the aphotic layers between nSCS and sSCS. Amino acids and ammonium were the major nitrogen sources for bacteria while urea, ammonia and nitrite played important roles in regulating cell growth of Prochlorococcus in different regions of the SCS.