Effects of Salinity on the Growth, Biochemical Components, and Epiphytic Bacterial Community of Desmodesmus intermedius

Salinity is a key determinant governing microalgal growth, biochemical composition, and the structure of associated epiphytic bacterial communities. To investigate the effects of salinity on the structure and function of the epiphytic bacterial community in Desmodesmus intermedius, this study utilized 16S rRNA gene high-throughput sequencing to analyze the communities across the control (S0) and experimental groups (S5, S10, S15). The results demonstrated that salinity is a key environmental driver governing the structural and functional succession of the bacterial community. Alpha diversity analysis revealed that the control group exhibited the highest bacterial diversity and greater evenness. In contrast, the experimental groups showed a significant increase in the relative abundance of Thauera and a concurrent decrease in Roseococcus with increasing salinity. Beta diversity analysis revealed clear segregation of the epiphytic bacterial communities across the salinity groups. FAPROTAX functional prediction revealed that increasing salinity led to a reduction in chemoheterotrophy, photoheterotrophy, and aerobic chemoheterotrophy, while enhancing nitrogen respiration, nitrate reduction, and other denitrification processes. This shift indicates a substantial reconfiguration of carbon and nitrogen metabolic pathways. BugBase phenotype analysis further revealed that the experimental groups exhibited a higher proportion of Gram-positive bacteria and enhanced biofilm-forming capacity. Canonical correspondence analysis identified salinity as the predominant factor shaping bacterial community structure. This study comprehensively investigates the response mechanisms of the D. intermedius epiphytic bacterial community to salt stress, laying a foundation for understanding microbial functions within the phycosphere.