Bacterial Communities in Sand and Seawater of Northern Gulf Coast Beaches: Temporal, Spatial, and Environmental Influences.

Coastal microbial communities play critical roles in marine food webs and biogeochemical cycling, yet their diversity and function remain poorly characterised in many regions. This is especially evident along the northern Gulf coast, a dynamic system with substantial freshwater influences. We used high throughput 16S rRNA sequencing to characterise bacterial communities in sand and seawater collected every 3 months (March 2024 through March 2025) from 10 beaches along a 53 km stretch of the Mississippi coast. The diversity and composition of these communities were related to environmental variation and to biogeochemical function as determined from the activity of enzymes related to carbon, nitrogen, and phosphorus mineralisation. Our findings revealed distinct bacterial communities in sand and seawater, with the microbiome of each habitat showing greater temporal variation over the course of the study than spatial variation between beaches. Patterns in bacterial community structure and proportions of abundant taxa were strongly linked to physicochemical variables, while enzyme activities suggested how microbial communities may contribute to biogeochemical processes in these habitats. Collectively, these findings provide critical information for understanding microbial ecology in this system and highlight the central role of bacteria in mediating ecosystem function along a dynamic and understudied coastline.