<p>Subterranean estuaries (STEs) are subsurface environments where oxygen-poor inland groundwater mixes with oxygen-rich seawater, driving biogeochemical transformations along redox and salinity gradients. These environments regulate the land–ocean exchange of dissolved constituents in the form of submarine groundwater discharge (SGD). For example, marine and terrestrial dissolved organic matter (DOM) undergoes degradation and can be transformed, retained, or released through interactions with the solid phase. Concurrently, electron acceptors are sequentially consumed, driving redox reactions that influence carbon and nutrient cycling. We investigated a microtidal STE connected to an organic-rich peatland on the Baltic coast. First, we hypothesized that the mixing of peat-derived terrestrial DOM and dissolved Fe²⁺ with oxygenated seawater promotes Fe³⁺-DOM coprecipitation. Additionally, we proposed that sulfate reduction contributes to terrestrial DOM sulfurization in the coastal aquifer. Fe and Mn as well as sulfur species were analyzed to characterize redox conditions, while ion and isotopic analyses, as well as ultra-high resolution mass spectrometry, were employed to identify the sources and processing of DOM. Our data revealed a strong seawater-groundwater zonation in the shallow (&lt; 20&#xa0;cm) STE, and an accumulation of sulfide and iron in deeper layers. DOM in seawater and surficial STE porewater was dominated by hydrogen-rich aliphatic compounds, and in the deeper anoxic STE it was comprised of aromatic, highly unsaturated compounds characteristic of peat. Dissolved organic sulfur (DOS) in STE porewaters was high and rich in aromatic compounds, suggesting significant contributions from peat-derived sulfurized DOM in the STE. Comparatively, the DOS/DOC ratios in the STE porewaters and surficial seawater samples were higher than those in the highly sulfidic Black Sea but comparable to those in the North Sea and porewaters from the Wadden Sea tidal flat. Experimental post-sampling aeration of Fe²⁺- bearing DOM samples and subsequent Fe³⁺-DOM coprecipitation selectively removed high-molecular-weight, oxygen-rich, and sulfurized DOM fractions at low Fe²⁺/DOC ratios and near-neutral pH conditions. However, DOC removal remained below values reported previously from terrestrial peat environments, indicating that under the neutral to alkaline conditions commonly found in STEs, DOM-Fe³⁺ coprecipitation may not contribute substantially to organic carbon sequestration. Our results suggest that temperate microtidal STEs could be direct conduits for sulfurized, peat-derived DOM to the sunlit, oxic coastal ocean, with yet unknown consequences to its subsequent reactivity and fate.</p>