Small-Scale Variability of Microbial Communities and Their Functional Potential in a Mediterranean Beach Aquifer
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Coastal aquifers are dynamic interfaces where groundwater interacts with seawater environments through submarine groundwater discharge (SGD). This process transports dissolved compounds and essential elements that influence the productivity of coastal areas and biogeochemical cycles occurring in them. Microbial communities residing in these underground habitats play an essential role in modifying the chemical characteristics of groundwater, ultimately influencing the composition of SGD and its effects into the ocean. Despite this, the contribution of microorganisms in controlling groundwater composition and nutrient fluxes has been largely overlooked, as research on SGD has been driven by other disciplines, often disregarding the microbiological aspects. Recent studies have shown that microbial communities from coastal aquifers can be very heterogeneous at small spatial scales, but few investigations have considered this small-scale variability in porewater microbiota located next to groundwater discharge sites. In this study, we aimed at exploring the small-scale spatial patterns of microbial communities inhabiting a karst beach aquifer discharging in the Mediterranean Sea. We investigated changes in prokaryotic abundance, activity and the abundance of different functional genes involved in nitrogen transformations along two transects perpendicular to the coast, and linked them to variations in nutrient concentrations and other physicochemical characteristics. Our initial findings reveal highly heterogeneous porewater microbial communities along the studied transects, which vary depending on salinity and other factors. Among them, we identify abundant small-sized populations of ultramicrobacteria, which could alter important biogeochemical processes and are the focus of ongoing research. Additionally, next generation sequencing and functional gene tracking are being used to understand how their metabolic activity modulates nutrient fluxes associated with SGD. Gaining a deeper understanding of coastal groundwater microbial populations will provide valuable insights into the drivers of SGD to the ocean.