Sediments from a seasonally euxinic coastal ecosystem show high nitrogen cycling potential

Coastal ecosystems are susceptible to eutrophication and deoxygenation, which may alter their nitrogen cycle dynamics. Here, we investigated the microbial nitrogen cycling potential in the sediment of a seasonally euxinic coastal ecosystem (Lake Grevelingen, NL), in winter and summer. Porewater profiles showed ammonium (NH ₄ ⁺ ) concentrations exceeding 10 mM and rapid depletion of electron acceptors with depth. Activity tests revealed NH ₄ ⁺ oxidation potential up to 53 µmol g ⁻¹ day ⁻¹ , even in anoxic sediment layers. A nitrifying microbial community was present in both oxic and anoxic sediment sections (up to 1.4% relative abundance). NO₃ ^⁻ , nitrite (NO ₂ ⁻ ) and nitrous oxide (N ₂ O) reduction potential were prominent across all sediment sections, with the highest rates (167 µmol NO ₃ ⁻ g ⁻¹ day ⁻¹ ) in the surface sediment in summer. Denitrification (79.3-98.4%) and dissimilatory nitrate reduction to ammonium (DNRA; 1.6-20.7%) were the major NO ₃ ⁻ removal pathways, as supported by the detection of the narG/napA, nirK/nirS, norB, nosZ and nrfA/otr genes in all sediment sections. The DNRA contribution increased with depth and with the addition of electron donors, such as monomethylamine. Anaerobic ammonium oxidation (anammox) was not detected in these eutrophic sediments. Combined, our results show that there is high potential for nitrogen removal in eutrophic coastal ecosystems which may help further restoration measures.