Oxygen and light determine the pathways of nitrate reduction in a highly saline lake

Abstract. Nitrate (NO₃⁻) removal from aquatic ecosystems involves several microbially mediated processes including denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and anaerobic ammonium oxidation (anammox) regulated by slight changes in environmental gradients. Saline lakes are prone to the accumulation of anthropogenic contaminants, making them highly vulnerable environments to NO₃⁻ pollution. We investigated nitrate removal pathways in mesocosm experiments using lacustrine, undisturbed, organic-rich sediments from Pétrola Lake (Spain), a highly saline waterbody subject to anthropogenic NO₃⁻ pollution. We used the revised ¹⁵N-isotope pairing technique (¹⁵N-IPT) to determine NO₃⁻ sink processes. Our results demonstrate the coexistence of denitrification, DNRA, and anammox processes, and their contribution was determined by environmental conditions (oxygen and light). DNRA and N₂O-denitrification were the dominant nitrogen (N) removal pathways when oxygen and/or light were present (up to 82 %). In contrast, anoxia and darkness promoted NO₃⁻ reduction by DNRA (52 %) and N loss by anammox (28 %). Our results highlight the role of coupled DNRA-anammox, as yet has never been investigated in hypersaline lake ecosystems. We conclude that anoxia and darkness favored DNRA and anammox processes over denitrification and therefore reduce N₂O emissions to the atmosphere.