Microbial Diversity, Selective Isolation and Bioactivity Characterization of Bacterial Populations in Eutrophic Seawater of Coastal East China Sea

Marine bacteria possess significant potential for numerous applications including environmental remediation, creation of natural products and medicines, agriculture, and various industrial sectors. In this study, the diversity of bacterial populations in the seawater at the nearshore S1 station which is a frequent red-tide occurrence area in the East China Sea, was characterized using 16S rRNA gene amplicon sequencing analysis. The three predominant phyla in the bacterial communities were identified as Proteobacteria, Actinobacteria, and Bacteroidetes, with the families Rhodobacteraceae, Mycobacteriaceae, and Flavobacteriaceae as the dominant groups, respectively. The bacterial community composition at the S1 station significantly differed from those of the other five investigated coastal sites, and demonstrated its own unique taxonomic associations with the Rhodobacteraceae as the keystone species. Functional prediction through KEGG and MetaCyc analyses revealed the presence of an L-tryptophan biosynthesis pathway responsible for indole-3-acetic acid (IAA) production. By using the targeted isolation of cultivable bacterial strains, a novel red-pigmented bacterium, designated S1-TA-50, which produced IAA metabolites, was recovered from the S1 station. It was identified as a potential novel species within the genus Sulfitobacter in the family Rhodobacteraceae. This bacterium demonstrated notable antibacterial activity against four model pathogenic strains and also acted as a new microalgae growth-promoting bacterium with substantial IAA production after bacterial culture optimization. This study contributes to the accumulation of scientific knowledge regarding the dynamics of marine bacterial ecosystems in nearshore eutrophic environments and facilitates a better understanding of phycosphere bacterial roles in coastal ecosystems, as well as the comprehensive utilization of microbial resources.