Insights into microbial carbon sequestration mechanisms in the Eastern Arabian Sea using metagenomic analysis

Abstract

This investigation elucidated how depth- and season-dependent environmental gradients shape microbial community composition, metabolic potential, and carbon sequestration pathways in the Eastern Arabian Sea (EAS). The study encompassed six stations (L1-L6) spanning coastal to offshore regimes, three depth zones (surface, 200 m, and 1000 m), and three monsoonal phases: Spring Inter-Monsoon (SIM), Summer Monsoon (SM), and Winter Monsoon (WM). A total of 10,500 taxa were identified across all samples. Alpha-diversity indices showed peak diversity during the SM and SIM periods. Across all depths, Pseudomonadota (53.2 ± 16.2%) remained the dominant phylum, underscoring its broad ecological adaptability. Cyanobacteria (31.3 ± 19%) were abundant in surface waters during SIM and WM, but declined sharply with depth (<2%), where Actinomycetota dominated (25 ± 16%), highlighting strong vertical niche portioning. Distinct seasonal restructuring was evident, particularly during the SM, when upwelling-driven nutrient enrichment resulted in a marked decline in Cyanobacteria and a concomitant increase in copiotrophic taxa such as Rhodobacterales, Flavobacteriales, Pseudomonadales, and Oceanospirillales, indicative of intensified heterotrophic processing of organic matter. In contrast, oligotrophic taxa (Pelagibacterales, Prochlorococcus, Synechococcus) prevailed during SIM and WM, suggesting nutrient-limited and microbially driven carbon cycling. Remarkably, even deep-water communities (200-1000 m) exhibited significant seasonal restructuring (p < 0.05), with Alteromonadales and Oceanospirillales enriched during SM and Sphingomonadales and Rhodobacterales dominating during WM, indicating active coupling between surface productivity and deep microbial assemblages. Functional analyses revealed pronounced depth-dependent stratification of metabolic potential (p < 0.05) reflecting shifts from growth-oriented processes in surface waters to adaptive and recycling strategies at depth. Collectively, these findings reveal robust monsoon-driven and depth-stratified microbial dynamics in the EAS and provide novel evidence inferred based on microbial community structure and functional potential that both the Biological Carbon Pump and the Microbial Carbon Pump operate concurrently across this climatically sensitive and highly productive region.

Keywords: BCP; Bacterial diversity; MCP; Spring-inter monsoon; Summer monsoon; Winter monsoon.