Different dissolved organic matter (DOM) sources sustain microbial life in beach subterranean estuary

Dissolved organic matter (DOM) is diverse in composition and serves as substrate for microbial metabolism. Within subterranean estuaries (STEs), DOM is introduced from different sources along the groundwater flow paths. These different DOM sources make it challenging to disentangle degradation pathways, especially in high-energy beaches with dynamic porewater advection and changing redox conditions. We performed sediment incubations in flow-through reactors (FTRs) to investigate how DOM from different sources is transformed by STE microbial communities. We used sediment and groundwater from the STE of a high-energy beach on Spiekeroog Island (Germany). Intertidal beach sediments were incubated for 13 days in FTRs with groundwater of low (~1.6) and high salinity (~29.1) as marine and terrestrial endmember, respectively, in triplicate setups, and additional control FTRs with artificial seawater of respective salinities. The FTRs ran under oxic conditions with recirculating advective flow. Porewater samples were taken daily for quantification of dissolved organic carbon (DOC) and nutrient concentrations, and samples from the start and the end of the incubation were taken for the analysis of microbial community composition, microbial cell numbers, and DOM composition. DOM samples were isolated through solid phase extraction and molecularly characterized via ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry. Over the course of the incubation, DOC concentrations increased, presumably from sediment leaching and potentially also by primary production in light-exposed parts of the setup, as oxygen concentrations also increased. The DOM composition of the porewater samples at start and end of the incubation was highly diverse, with a total of up to 2900 different molecular formulae detected in each sample. As expected, the low salinity porewater had a more terrestrial DOM signature with a higher proportion of aromatic compounds compared to the DOM in the high salinity porewater. In all setups, the DOM composition changed significantly from start to end. We observed an increase in DOM lability in both endmember setups indicating the mobilization of fresh DOM from sediments and/or microbial activity, including primary production. Interestingly, the changes observed were similar for both DOM endmembers. Our results indicate that the microbial communities of the high-energy beach STE thrive on a similar fraction of DOM, independent of its source.