Coupled Thermal-pH and Sulfur Gradients Shape Chloroflexota-Dominated Bacterial Communities at a Residual Xinjiang Coal-Fire Vent

Residual coal-fire gas vents form steep thermal and geochemical gradients that create sharply distinct microbial habitats over short distances. We compared the microbial communities of two microhabitats at a residual vent near Kuitun City, Xinjiang: BS1, a gas-emitting fracture at 64 °C, and BS2, adjacent cooler sediment at 43 °C. BS1 had pH 6.8 and total sulfur 5.86 g/kg, whereas BS2 had pH 8.4 and total sulfur 1.90 g/kg. Illumina sequencing of the 16S rRNA gene V3–V4 region yielded 52,327 and 64,212 merged read pairs retained for downstream analyses for BS1 and BS2, respectively. BS1 contained fewer observed amplicon sequence variants (ASVs; 524 versus 1366) and lower Shannon diversity (5.626 versus 6.588). Chloroflexota dominated both communities but was more abundant in BS1 (43.83% versus 36.33%), whereas BS2 contained larger Proteobacteria and Bacteroidota fractions. BS1 was enriched in Pyrinomonas, unclassified Chloroflexota, unclassified Anaerolineae, and Thermorudis, while BS2 was enriched in Thermogemmatispora, Massilia, and Arthrobacter. At this vent, the hot fracture therefore supported a lower-diversity, Chloroflexota-enriched community, whereas cooler sediment supported a richer and more compositionally diverse community. Together with the mineralogical observations, these results reveal sharp site-scale partitioning of bacterial assemblages across a coupled thermal-pH-sulfur gradient.