Digging into the Role of Microorganisms in Mediterranean Coastal Aquifers
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Submarine groundwater discharge (SGD) is a key global process that delivers significant amounts of nutrients and other solutes to coastal oceans, profoundly impacting marine ecosystems. Both the quality of groundwater and the effects of SGD are shaped by biogeochemical processes occurring at the land-sea interface, many of which are mediated by microorganisms inhabiting the transition zones where freshwater mixes with seawater. However, studies on SGD and subterranean estuaries have largely overlooked the microbial component, leading to an incomplete understanding of the functioning of coastal aquifers and their relevance for coastal ecosystems. Integrating microbial information into SGD research is challenging, as it requires interdisciplinary approaches (ecology, hydrology, geochemistry) and cross-ecosystem perspectives, given that subterranean estuaries act as connectors between terrestrial, freshwater, and marine systems. In recent years, we have characterized microbial communities from various coastal aquifers along the Spanish coast to gain insight into their poorly known microbial inhabitants, and to better understand their role in regulating SGD quality and its consequences in the marine environment. Our results reveal that coastal groundwater communities are extremely diverse and heterogeneous, exhibiting a unique composition as they share very few species with connected freshwater, sediment, and seawater microbial communities. They harbor many potentially unknown species and metabolisms, and are dominated by some poorly known groups such as Patescibacteria and Nanoarchaeota, which might be mobilized to the sea via SGD. Ongoing research targeting microbial functional genes will allow to characterize the functional potential of these communities to elucidate their role in modulating nutrient fluxes to the sea.