Fine-Scale Patterns in Bacterial Communities on a Gulf Coast Beach

Despite being low-resource environments, sandy beaches can contain diverse bacterial assemblages. In this study we examined the spatial heterogeneity of bacterial communities in sand on a beach on the Mississippi Gulf Coast, USA. 16S ribosomal RNA gene sequencing was used to characterize bacterial communities in surface sand along 10 m transects from dry sand towards the upper beach to fully submerged sand, as well as up to 0.4 m deep into the sand. There were clear gradients in bacterial community structure based on position on the beach and depth, and community richness and diversity was greater in moist sand subject to tidal influence than drier sand. Bacterial communities in sand higher up the beach were characterized by members of the phyla Bacillota and Actinomycetota, whereas there was an increased presence of picocyanobacteria (phylum Cyanobacteriota) in sand closer to the water and greater diversity overall. Along with gradients in community structure, microbial activity also showed spatial patterns, with microbial extracellular enzyme activity being greatest in surface sand at intermediate positions along the beach transects that were subject to tidal influences but not fully submerged. This research supports the idea of beaches containing diverse bacterial communities and demonstrates that the existence of gradients in beach environments means that these communities show clear patterns in their spatial distribution.