Microbial communities traits and hydrogeochemistry in a coastal karst aquifer subject to salinization: a closer look at interaction

Aquifers of coastal areas are impaired by saline water intrusion owing to either natural or anthropogenic causes. Different inputs of surface and marine salt waters can consistently affect the local hydrological, geochemical, and biological settings, with direct consequences on groundwater quality, the aquifer ecological status, and related ecosystem services. The aquatic microbial community represents a fundamental component of the groundwater resident biota by playing a major role in carbon and nutrient cycling through rapid structural and functional adaptations to changing environmental conditions. However, the structural and functional traits of the aquatic microbial community in coastal aquifers were largely disregarded in the current literature. The present work aimed to explore the groundwater quality in a coastal karst aquifer located in southern Italy (Region Apulia) and the microbial community responses to groundwater salinization. The target area (1227 km2) is located in a semi-arid climate region with agricultural vocation. Water withdrawal for irrigation is among the major factors promoting saline intrusion. A total of 200 samples, collected during four sampling campaigns, were analyzed in situ for the main physical-chemical parameters (oxidation reduction potential, pH, electrical conductivity, T, dissolved oxygen) together with UV-VIS determinations of ammonia, nitrites, and cyanides. Chemical analysis in the laboratory included major anions/cations, trace elements, and dissolved organic carbon (DOC). The groundwater microbial community was characterized by assessing the total microbial load (flow cytometry), the microbiological contamination of fecal origin (Colilert- 18 assay), the microbial metabolic potential (Biolog EcoPlates), the heterotrophic respiration (Biolog MT2 MicroPlates), the extracellular enzyme expression (API ZYM test), and the phylogenetic community composition (16rRNA gene amplicon sequencing). Preliminary results from 47 sampling sites allowed identifying four major groups of waters with different salinity levels according to Cl concentration patterns. The aquatic microbial load and fecal contamination levels were generally low and not significantly affected by salinization. However, the microbial functional activities were likely stimulated at increasing salt concentration and the phylogenetic community profiles changed consistently during the water withdrawal period. The observed changes can have potential repercussions on biogeochemical cycling and ecosystem services. The approach here utilized can represent a fast, reliable and sensitive tool for tracking saline water impact in the groundwater environment in view of better management of the water resource.