The three domains of life within the discharge area of the shallow STE at a high energy beach

The seaward side of the German North Sea Island of Spiekeroog is characterized by sandy high energy beaches. Those are discharge areas of a subterranean estuary (STE), influenced by the island´s freshwater lens. While bacterial communities of STEs have received increasing scientific attention, archaeal and meiofaunal diversities were mostly neglected. Previous studies at the Spiekeroog beach STE focussed on spatial and seasonal patterns of geochemical and bacterial diversity. By additionally investigating the archaeal and meiofaunal diversity and distribution, we now aimed to fill this gap of knowledge to understand the microbial response to subterranean groundwater discharge (SGD). Due to an altered beach topography, discharge areas of the STE were geochemically defined much clearer than observed in our preceding study. This led to a higher evenness of prokaryotic communities in areas impacted by SGD compared to unimpacted areas. Archaea contributed 1-4% to the 16S rRNA gene sequence dataset. Those were dominated by Nitrosopumilaceae, corresponding well to higher concentrations of NH4 in the discharge area of the STE. The unimpacted sites had elevated abundances of Wosearchaeia, which were also detected in another STE. While a large proportion of prokaryotes were present in the entire intertidal area, meiofaunal community compositions were site specific and dominated by nematodes. Nematode communities of the high-water line differed distinctively from the other stations and showed no adaptation to the STE. Correlation analysis revealed that variations of the nematode community significantly influenced bacterial community composition and vice-versa, while Archaea were unaffected. Overall, our findings give a unique insight into the microbial community composition within the discharge area of an STE, spanning all three domains of life.