Seasonal shifts in microbial communities drive differential carbon accumulation and remineralization in the eastern Arabian Sea

Marine sediments are crucial to the carbon cycle, yet the balance between carbon burial and microbial remineralization remains unclear, particularly in regions experiencing seasonal variations. In the eastern Arabian Sea (EAS), monsoon-driven seasonal changes significantly influence the primary productivity and the associated microbial activities. This study investigated the bacterial community composition and its functions in coastal and offshore sediments along the EAS during the non-monsoon and monsoon seasons, using metabarcoding analysis to understand their role in sediment carbon dynamics. The results indicate distinct seasonal shifts in bacterial communities and predictive metabolic pathways. A significant difference in carbon dynamics was detected between coastal and offshore stations (p < 0.05), with slightly higher carbon values recorded during the non-monsoon season. During the non-monsoon season, the anaerobic bacterial taxa affiliated with the phylum Firmicutes, including Bacillus, Clostridiales and Planctomycetes class Pirellulaceae, were predominant, particularly in offshore stations, which were predicted to contribute to carbon retention through fermentation and nitrate reduction pathways. In contrast, during the monsoon season, aerobic genera from the Proteobacteria, such as Idiomarina, Halomonas, Marinobacter and Salegentibacter, were dominant and associated with functions including chemoheterotrophy, aerobic chemoheterotrophy and aromatic compound degradation, indicating enhanced microbial carbon processing that likely limited sedimentary carbon accumulation despite elevated primary productivity. This study emphasizes the dynamic nature of microbial communities in regulating carbon processing in response to seasonal environmental changes. The findings have significant implications for understanding the impact of seasonal changes on carbon storage and remineralization, particularly in coastal upwelling ecosystems.