Diversity and relationship between dissolved organic matter and porewater communities in the subterranean estuary of a sandy high-energy beach
Subterranean estuaries (STEs) are dynamic interfaces where terrestrial groundwater mixes with seawater, creating steep gradients in salinity, redox potential, and dissolved organic matter (DOM) composition. Yet, it remains unclear how variations along these gradients shape microbial community structures within these environments. In this study, we combined 16S rRNA gene sequencing with ultrahigh-resolution DOM characterization to investigate temporal variabilities of microbial and molecular patterns along a coastal groundwater transect on Spiekeroog Island, Germany. Both DOM composition and microbial communities varied markedly across space and time, reflecting the interplay between freshwater and marine inputs. Prokaryotic communities across the beach transect were clearly dominated by ultra-small bacteria and archaea related to the Nanoarchaeota and Patescibacteria. Along the land-sea beach transect, DOM composition transitioned from a terrestrial to a marine signature, reflecting inputs from dune vegetation and soil organic matter near the dunes and increasing marine influence seaward. Redox conditions, salinity, and DOM composition together defined discrete ecological niches that structured microbial assemblages along the STE. Oxic, freshwater-influenced zones hosted taxa such as Nitrosarchaeum, SAR202 (Chloroflexota), and Ralstonia, adapted to the degradation of aromatic, terrestrially derived DOM. In contrast, anoxic, saline and discharge zones were enriched in sulfate-reducing Desulfobacterota, fermentative Bacillota, and nitrifying Nitrosopumilaceae, linked to nitrogen-rich, microbially reworked DOM. These results reveal that interactions between DOM chemistry, redox zonation, and salinity gradients shape microbial niches and drive the pronounced spatial and temporal variability observed in high-energy STEs.