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. Activity tests revealed ammonium (NH₄ ⁺) oxidation potential with maximum potential rates 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). Nitrate (NO₃ ^-), nitrite (NO₂ ^-), and nitrous oxide (N₂O) reduction potential were prominent across all sediment sections, with the highest potential rates (167 μmol NO₃ ^-∙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 NO₃ ^- 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.