Subterranean estuaries (STEs) modulate the chemical composition of continental groundwater before it reaches the coast, but their microbial community is poorly known. Here, we explored the microbial ecology of two neighbouring, yet contrasting STEs (Panx on and Ladeira STEs; Ría de Vigo, NW Iberian Peninsula). We investigated microbial composition (16S rRNA gene sequencing), abundance, heterotrophic production and their geochemical drivers. A total of 10,150 OTUs and 59 phyla were retrieved from porewater sampled during four surveys covering each STE seepage face. In both STEs, we find a very diverse microbial community composed by abundant cosmopolitans and locally restricted rare taxa. Porewater oxygen and dissolved organic matter are the main environmental predictors of microbial community composition. More importantly, the high variety of benthic microbiota links to biogeochemical processes of different elements in STEs. The oxygen-rich Panx on beach showed strong associations of the ammonium oxidizing archaea Nitrosopumilales with the heterotrophic community, thus acting as a net source of nitrogen to the coast. On the other hand, the prevailing anoxic conditions of Ladeira beach promoted the dominance of anaerobic heterotrophs related to the degradation of complex and aromatic compounds, such as Dehalococcoidia and Desulfatiglans, and the co-occurrence of methane oxidizers and methanogens.

Subterranean estuaries (STEs), where continental groundwaters and saltwaters meet, are zones of intense biogeochemical reactivity. As such, STEs significantly modify groundwater-borne nutrient fluxes to the coastal zone. Thus, evaluating their reactive role is crucial to anticipate impacts of submarine groundwater discharge (SGD) over coastal ecosystems. Here, we studied the nitrogen biogeochemistry of two STEs with contrasting wave-exposure and redox conditions in Panxón and Ladeira beaches (Ría de Vigo, NW Iberian Peninsula). Seasonal surveys were performed at the permanently saturated zone of both beaches during low tide in February, May, July, and October 2019. Sediment was sampled and porewater samples collected using push-pull piezometers. Salinity, 222Rn and 226Ra activities were used to trace water circulation inside each beach. Porewater nitrate, ammonium, nitrite and dissolved oxygen were used to evaluate the role of these STEs as reactive sinks or sources of inorganic nitrogen. Our results showed a marked seasonal variability of water circulation inside both beaches, with strong salinity gradients in February and May and weakened circulation in July and October. The presence of a gravel layer in Panxón beach completely altered the typical structure of STEs by increasing porewater transport and mixing through the beach interior. As a result, Panxón beach profiles were highly enriched in nitrate and oxygen. Conversely, suboxic, and anoxic conditions were prevalent in Ladeira beach during the study period, with ammonium being the prevailing inorganic nitrogen form. High nitrate concentrations occurred associated to the tidal circulation cell during February and May, being the only effective mechanism of sediment oxygenation in Ladeira beach. Although nitrate reduction and production were observed in both STEs, comparison with averaged conservative mixing porewater profiles showed that Ladeira beach acted as a net nitrogen sink whereas Panxón beach acted as a net nitrogen source. The presence of a gravel layer oxygenates the interior of Panxón beach, thus limiting nitrate reduction and promoting the amplification of groundwater-borne nitrogen fluxes to the coast.