Contrasting spatial and temporal structuring of seawater and sediment bacterial communities in coastal environments.

Bacterial communities in seawater and sediment are essential to biogeochemical processes in coastal systems, yet their spatial and temporal dynamics remain poorly understood, and few studies have examined how bacterial communities in these habitats co-vary. We characterized bacterial communities along the northern Gulf of Mexico coast using 16 S rRNA gene sequencing of > 800 seawater and sediment samples collected over 15 months from five beaches spanning 40 km. Seawater and sediment harbored distinct bacterial assemblages that exhibited contrasting patterns of variation. Seawater communities showed seasonal turnover and a temporal distance–decay relationship driven by temperature and salinity, with the most abundant populations showing species-specific relationships to environmental conditions. Sediment bacterial communities were more diverse and were more stable over time with greater spatial differentiation. These results show that seawater and sediment bacterial communities in nearshore systems are structured by distinct spatiotemporal processes and provide new insights into the dynamics of coastal microbiomes. Understanding how bacterial communities in nearshore ecosystems respond to spatiotemporal and environmental variation is essential for predicting ecosystem resilience and informing monitoring and management strategies in an understudied coastal system.