Abstract
Macroalgae growth depends on biologically available nitrogen, such as ammonium and nitrate. Therefore, nitrogen is the most common growth limiting factor of macroalgae. However, it is unclear how the surface microorganisms of marine macroalgae promote nitrogen transformation and improve the utilization of nitrogen by macroalgae. In this study, 228 strains of bacteria were isolated from the surface of U. fasciata. The composition and structure of U. fasciata surface epiphytes under different nitrogen concentrations were determined by high throughput sequencing. Network analysis was used to identify highly correlated strains and predict possible functions. The bacterial strains of microbiota were identified based on the presence of nitrogen-fixing genes involved in the production of growth hormones and the appearance of dissolving P genes. The characteristics of promoting plant growth and increasing plant biomass accumulation were further determined. The ability of beneficial microbial combinations to assist in the acquisition of N/P nutrients was tested using synthetic communities (SynCom). Finally, the expression levels of several key genes were used to elucidate the effect of SynCom on the nutrient acquisition of U. fasciata.