Archaeal communities as indicators of hydrothermal influence in the Tianxiu vent field, Northwest Indian Ocean

Deep-sea hydrothermal sediments represent critical zones for archaea-driven biogeochemical cycling, yet the ecological differentiation of archaeal communities across hydrothermal gradients remains poorly understood. Here, we used 16S rRNA gene amplicon sequencing of sediment cores from two contrasting sites in the Tianxiu hydrothermal field of the Northwest Indian Ocean, and performed metagenomic analysis on the near-vent BC12 sediments, to investigate archaeal community composition, co-occurrence patterns, and metabolic potential in response to the hydrothermal activity. Comparative analysis revealed marked divergence between near-vent site BC12 and far-vent site JL218P. The site BC12, under stronger hydrothermal influence, was enriched in Hydrothermarchaeia , along with Nanoarchaeia and Thermoplasmata , and exhibited a more complex, highly connected co-occurrence network. Correlation analyses further showed that Hydrothermarchaeia abundance was significantly associated with hydrothermal-related geochemical gradients, supporting this lineage as a potential indicator of hydrothermal influence. Metagenomic analysis of BC12 further revealed Hydrothermarchaeia genomes encoding the Wood-Ljungdahl carbon fixation pathway, while genome-centric functional inference suggested enhanced potential for methanogenesis and hydrogen oxidation. In contrast, JL218P was dominated by Nitrososphaeria , showed limited vertical variation, and formed a simpler network structure, with predicted functional profiles more closely associated with nitrification and aerobic ammonia oxidation. Together, these findings identify hydrothermal-related geochemical heterogeneity as a major driver of archaeal community composition, ecological organization, and metabolic differentiation in deep-sea sediments, and advance our understanding of the ecological drivers structuring deep-sea hydrothermal ecosystems.