11 pages, 5 figures, Subterranean estuaries (i.e., seawater-fresh groundwater mixing zones at coastal aquifers) are highly reactive boundaries between continental groundwater and coastal surface seawater. Because particulate organic matter is retained in shallow sediments, internal microbial transformations rely on dissolved organic matter (DOM) supply and bioavailability. Here, we investigated DOM carbon content and optical characteristics in two nearby subterranean estuaries with contrasting oxygenation. Coastal organic carbon processing in the anoxic subterranean estuary resulted in the export of DOM enriched in recalcitrant compounds compared to the oxygenated one, which was a net sink of DOM. This contrasting behavior was not driven by opposite redox conditions but from the fast transfer of labile DOM and oxygen to the beach interior of the oxygenated subterranean estuary. There, heterotrophic processes, which rely almost exclusively on DOM, are enhanced, resulting in net DOM consumption prior discharge to surface waters, This research was financed by the SUBACID project (SUBmarine groundwater discharge [submarine groundwater discharge] impact on coastal ACIDification processes in contrasting Atlantic Shores: toward securing ecosystem services and food production), funded by the Irish Research Council and the European Union's Horizon 2020 Research and Innovation Program under the Marie Skłodowska-Curie grant agreement No. 713279 through the CAROLINE program (CLNE/2017/210). Elisa Calvo-Martin received funding from the Spanish government through the FPU PhD grant number FPU20/04707, Peer reviewed