Impact of spatio-temporal variations and altitudinal gradient on bacterial dynamics along the Eastern Himalayan river Teesta

Teesta, an Eastern Himalayan River is an important lifeline of the state of Sikkim and parts of West Bengal. However, the river is under the influence of human activities and hydrological disturbances. Microbes are sensitive to environmental changes which make them indicators of pollution, nutrient load, heavy metal and ecological disturbances. This study investigated spatial and temporal variations in bacterial communities along the altitudinal gradient of the Teesta River using 16S metabarcoding of water and sediment samples. Multivariate analyses were used to assess seasonal shifts and the influence of physicochemical factors on microbial community structure. Our study revealed that bacterial species richness significantly varied across the sites along the river gradient but bacterial composition was less influenced by the sites, seasons and sample type. While dominant phyla remain consistent across samples, season-driven shifts and niche preferences such as winter enrichment of Actinobacteriota and LEfSe-identified genus-level biomarkers highlight the ecological heterogeneity and functional diversification of riverine microbial communities. Altitude and heavy metal lead (Pb) was found to strongly influence on occurrence of bacterial phyla like Proteobacteria, Chloroflexota, Acidobacteriota, and Actinobacteriota. Whereas, environmental parameters like pH, temperature, alkalinity and dissolved oxygen exhibited the most significant association with major bacterial phyla. The Teesta River displayed distinct taxonomic signatures in water and sediment, particularly at the genus level that is strongly shaped by both seasonal dynamics and habitat-specific conditions. Our findings offer valuable insights on how seasonal shifts and habitat-specific conditions within the Teesta River shape bacterial community structure and composition with significant implications in predicting ecosystem responses to environmental change in an Eastern Himalayan river system.