Bacterial diversity along the geothermal gradients: insights from the high-altitude Himalayan hot spring habitats of Sikkim.

Geothermal habitats present a unique opportunity to study microbial adaptation to varying temperature conditions. In such environments, distinct temperature gradients foster diverse microbial communities, each adapted to its optimal niche. However, the complex dynamics of bacterial populations in across these gradients high-altitude hot springs remain largely unexplored. We hypothesize that temperature is a primary driver of microbial diversity, and bacterial richness peaks at intermediate temperatures. To investigate this, we analysed bacterial diversity using 16S rRNA amplicon sequencing across three temperature regions: hot region of 56-65 °C (hot spring), warm region of 35-37 °C (path carrying hot spring water to the river), and cold region of 4-7 °C (river basin). Our findings showed that Bacillota was the most abundant phylum (45.51 %), followed by Pseudomonadota (32.81 %) and Actinomycetota (7.2 %). Bacillota and Chloroflexota flourished in the hot and warm regions, while Pseudomonadota thrived in cooler areas. Core microbiome analysis indicated that species richness was highest in the warm region, declining in both cold and hot regions. Interestingly, an anomaly was observed with , which was more abundant in cases where ponds were used for bathing and recreation. In contrast, was mostly found in cold regions, likely due to its viability in soil and ability to remain dormant as a spore-forming bacterium. The warm region showed the highest bacterial diversity, while richness decreased in both cold and hot regions. This highlights the temperature-dependent nature of microbial communities, with optimal diversity in moderate thermal conditions. The study offers new insights into microbial dynamics in high-altitude geothermal systems.