Multilayered Human Activities Shape the Microbial Communities of Groundwater-Dependent Ecosystems on an Arid Oceanic Island

: Island coastal aquifers, though spatially limited, sustain key ecosystem functions linked to locally critical provisioning, maintenance and cultural ecosystem services. These functions are largely dependent on the presence of highly adapted biological communities, whose microbial components remain poorly understood. Here, we describe bacterial communities across groundwater-dependent ecosystems on Lanzarote (Canary Islands, Spain), spanning habitats with contrasting environmental conditions and degrees of human influence, using 16 S rRNA gene amplicon sequencing. We then infer the processes shaping community variation by integrating diversity partitioning, indicator species analysis, and machine-learning classification. Bacterial taxonomic diversity varied significantly among habitats, with community composition primarily structured by turnover, consistent with environmental filtering. In contrast, predicted human-associated and potentially pathogenic taxa showed patterns dominated by nestedness, indicating localized enrichment linked to anthropogenic inputs. Caves, enclosed marine bays, and hypersaline systems hosted the most compositionally distinct microbial communities, whereas wells and anchialine pools showed greater overlap in community composition. Together, our results suggest that groundwater microbial communities are influenced by the interplay between environmental filtering and anthropogenic inputs, and that coastal aquifers can act simultaneously as reservoirs of natural biodiversity and sinks of human-associated bacteria. These findings highlight the need for integrative monitoring and conservation strategies that incorporate both hydrological and biological components to safeguard groundwater-dependent ecosystems on oceanic islands.