Spatiotemporal dynamics of the gut microbiota in a temperate intertidal bivalve (Mactra veneriformis): seasonal and tidal zone shifts drive compositional and functional restructuring

Host-associated gut microbiota are hypothesized to facilitate acclimation to fluctuating environments; yet, how predictable seasonal changes and smaller-scale habitat variation interact to shape the microbiome of intertidal bivalves remains poorly understood. Here, we investigated the spatiotemporal dynamics of the gut bacterial community of the commercially important clam Mactra veneriformis across four seasons and two tidal zones (middle vs. low) in the northern Yellow River Delta using 16 S rRNA gene amplicon sequencing. Although gut microbial alpha diversity showed no consistent directional change across seasons and tidal zones, we observed marked shifts in community composition (beta diversity) and co-occurrence network structure. Seasonal variation was the dominant driver, with autumn samples forming a distinct compositional cluster, irrespective of tidal zone. The winter community exhibited the highest network complexity (more nodes, edges, and a greater average clustering coefficient), indicative of intensified microbial interactions under cold stress. Functional predictions revealed season-specific pathway enrichment: energy and glycan metabolism pathways were enriched in autumn and winter, whereas lipid and carbohydrate metabolism pathways were more prominent in spring and summer. Furthermore, Procrustes analysis demonstrated a strong, significant concordance between stomach content (diet) composition and gut microbiota structure in both tidal zones. Our findings reveal that seasonal environmental shifts and tidal position restructure the gut bacterial community and its functional potential in M. veneriformis . This study provides critical baseline insights into the eco-evolutionary dynamics of the bivalve gut microbiome in a rapidly changing temperate intertidal ecosystem.