Microbial Diversity, Selective Isolation and Bioactivity Characterization of Bacterial Populations in Nearshore Eutrophic Seawater

Marine microorganisms process significant potential for varied application in the environment, medicine, agriculture and various industries. The nearshore coastal area usually exhibit unique characteristics due to the combined effects of terrestrial runoff, tidal exchange, and human activities, and is a high-incidence area of eutrophication of the seawater. In this study, 16S rRNA amplicon sequencing and functional analysis were used to characterize the bacterial populations in the eutrophic seawater samples from the nearshore S1 site, in which the red-tide frequently occurred in the East China Sea (ECS). Based on the obtained results, the Proteobacteria, Actinobacteria, and Bacteroidetes were identified as the predominant phyla of bacterial populations in station S1. The composition of the bacterial populations of this station showed significantly difference compared with the other five nearshore sites in addition to the taxonomic associations. Functional prediction of bacterial populations via both KEGG and MetaCyc analysis indicated the L-tryptophan biosynthesis pathway related to natural auxin, indole-3-acetic acid (IAA) biosynthesis appeared in the bacterial populations in eutrophic seawater from station S1. By using targeted isolation of the cultivable bacteria, a new red bacterium named S1-TA-50 was recovered, and it was identified as a potential novel species of the genera Sulfitobacter in the family Rhodobacteraceae which is the predominant taxa group for the phycobacterial populations. The new isolate exhibited attracting antibacterial activity against the four model testing pathogenic strains. Additionally, it proved to be a new microalgae growth-promoting bacterium with a substantial IAA production. This study is believed to contribute to the accumulation of scientific information of dynamic of marine bacterial ecosystems in the eutrophic nearshore areas. It is also beneficial for deeper understanding the bacteria roles in marine ecological balance and comprehensive utilization of microbial resources.