Seasonal variation in microbial diversity and community composition of coastal sediments in Saint Martin Island, Bay of Bengal

Coastal sediments harbor diverse microbial communities that regulate key biogeochemical processes, yet seasonally resolved data from the Bay of Bengal are limited. This study investigated seasonal variation in sediment-associated microbial communities from Saint Martin Island using 16S rRNA gene metabarcoding. Sediment samples were collected from five coastal sites during summer and winter seasons and sequenced targeting multiple hypervariable regions of the bacterial 16S rRNA gene. Across ten samples, over 7.6 million high-quality reads generated 1,192 operational taxonomic units (OTUs). Alpha diversity analyses revealed significantly higher microbial richness in winter sediments, while evenness indices showed no significant seasonal differences. Beta diversity analyses demonstrated clear seasonal separation (p < 0.05) of microbial communities. Proteobacteria dominated both seasons, with significantly higher relative abundance in summer, whereas Actinobacteria and Firmicutes were enriched in winter. Summer communities were characterized by marine-associated genera such as Vibrio, Neptuniibacter, and Alcanivorax, while winter sediments were enriched with Bacillus, Flavobacterium, and Gramella. Predictive functional profiling indicated seasonal shifts in metabolic potential, with winter communities enriched in sulfur metabolism and complex organic matter degradation, and summer communities associated with enhanced nitrogen and sulfur cycling. These findings highlight strong seasonal structuring of sediment microbiomes and provide baseline insight into coastal microbial dynamics in the Bay of Bengal.