Ocean currents and environmental gradients shape prokaryotic community structure and function in the South China Sea.

The South China Sea (SCS) is characterized by complex hydrodynamic conditions that influence the structure and function of prokaryotic microbial communities. This study conducted a comprehensive analysis of prokaryotic diversity, community assembly, and functional potential across various water masses within the SCS. Using 16S rRNA gene sequencing and co-occurrence network analyses, we found that geographic distance and environmental gradients, particularly temperature and nutrient levels, significantly impacted community composition. Our findings indicate that ecological drift is the primary mechanism governing community assembly, with spatial turnover primarily driven by the dispersal of microorganisms facilitated by ocean currents. Distinct modules in co-occurrence networks were associated with specific environmental factors, reflecting potential environmental selection processes along the SCS current. Keystone species and biomarkers identified through network analysis and random forest modeling exhibited varying associations with environmental variables, highlighting their adaptability to changing conditions. This work underscores the importance of ocean currents and environmental factors in shaping prokaryotic community dynamics and provides insights into microbial biogeography and ecosystem function in the SCS. IMPORTANCE: Microorganisms, especially prokaryotes, are fundamental in sustaining marine ecosystems through nutrient cycling and organic matter decomposition. However, understanding what shapes their diversity and distribution remains challenging. Our study highlights the significant role ocean currents and environmental conditions play in influencing prokaryotic communities in the South China Sea-a critical marine environment due to its dynamic currents and ecological complexity. We found that currents facilitate microbial dispersal, shaping community composition over vast areas, while temperature gradients act as key selective pressures, determining which species thrive. Additionally, we reveal that both predictable environmental selection and random ecological drift significantly contribute to community structuring. By identifying keystone microbes and biomarkers sensitive to environmental change, our work offers essential insights into marine microbial ecology. These findings are crucial for predicting how microbial communities, and thus ocean health and productivity, respond to ongoing environmental changes.