Anaerobic Ammonium Oxidation Measured in Sediments along the Thames Estuary, United Kingdom

Abstract

Until recently, denitrification was thought to be the only significant pathway for N ₂ formation and, in turn, the removal of nitrogen in aquatic sediments. The discovery of anaerobic ammonium oxidation in the laboratory suggested that alternative metabolisms might be present in the environment. By using a combination of ¹⁵ N-labeled NH ₄ ⁺ , NO ₃ ⁻ , and NO ₂ ⁻ (and ¹⁴ N analogues), production of ²⁹ N ₂ and ³⁰ N ₂ was measured in anaerobic sediment slurries from six sites along the Thames estuary. The production of ²⁹ N ₂ in the presence of ¹⁵ NH ₄ ⁺ and either ¹⁴ NO ₃ ⁻ or ¹⁴ NO ₂ ⁻ confirmed the presence of anaerobic ammonium oxidation, with the stoichiometry of the reaction indicating that the oxidation was coupled to the reduction of NO ₂ ⁻ . Anaerobic ammonium oxidation proceeded at equal rates via either the direct reduction of NO ₂ ⁻ or indirect reduction, following the initial reduction of NO ₃ ⁻ . Whether NO ₂ ⁻ was directly present at 800 μM or it accumulated at 3 to 20 μM (from the reduction of NO ₃ ⁻ ), the rate of ²⁹ N ₂ formation was not affected, which suggested that anaerobic ammonium oxidation was saturated at low concentrations of NO ₂ ⁻ . We observed a shift in the significance of anaerobic ammonium oxidation to N ₂ formation relative to denitrification, from 8% near the head of the estuary to less than 1% at the coast. The relative importance of anaerobic ammonium oxidation was positively correlated ( P < 0.05) with sediment organic content. This report of anaerobic ammonium oxidation in organically enriched estuarine sediments, though in contrast to a recent report on continental shelf sediments, confirms the presence of this novel metabolism in another aquatic sediment system.