Ancient Roman saltworks drive the present-day microbial community profiles in a coastal aquifer

Study region: This study was conducted in the coastal aquifer of the Tiber River delta (Fiumicino, Italy), where groundwater salinization reflects the long-lasting legacy of ancient Roman saltworks. The area combines complex coastal hydrogeology with historical salt production, making it an ideal setting to examine how past human activities continue to influence present-day groundwater systems. Study focus: We investigated non-salinized and salinized groundwater units from the same aquifer using integrated hydrogeochemical characterization, flow cytometry, 16S rRNA gene amplicon sequencing, and Biolog-based functional assays. Salinized samples showed higher chloride, bromide, sodium, and sulfate concentrations, along with ion ratios consistent with dissolution of salt deposits. Salinization reduced microbial diversity and shifted communities from freshwateradapted families toward a halotolerant assemblage dominated by Campylobacterota, especially Sulfurimonadaceae and Sulfurovaceae. Functional annotation suggested broadly conserved potentials for carbon, nitrogen, and sulfur cycling. However, the Biolog assays revealed higher heterotrophic respiration and carbon substrate use but lower functional diversity in salinized samples. New hydrological insights for the region: Ordination analyses showed a clear separation of aquifer units along the salinization gradient, with coordinated chemical and microbial vectors indicating alternative ecosystem states sustained by millennia‑old anthropogenic salt inputs. Our findings showed that ancient saltworks can drive persistent hydrogeochemical alteration, select specialized halotolerant microbiomes, and reconfigure carbon and nutrient processing while maintaining core biogeochemical functions, with critical implications for coastal groundwater management strategies.