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

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Abstract
Until recently, denitrification was thought to be the only significant pathway for N2 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 15N-labeled NH4+, NO3−, and NO2− (and 14N analogues), production of 29N2 and 30N2 was measured in anaerobic sediment slurries from six sites along the Thames estuary. The production of 29N2 in the presence of 15NH4+ and either 14NO3− or 14NO2− confirmed the presence of anaerobic ammonium oxidation, with the stoichiometry of the reaction indicating that the oxidation was coupled to the reduction of NO2−. Anaerobic ammonium oxidation proceeded at equal rates via either the direct reduction of NO2− or indirect reduction, following the initial reduction of NO3−. Whether NO2− was directly present at 800 μM or it accumulated at 3 to 20 μM (from the reduction of NO3−), the rate of 29N2 formation was not affected, which suggested that anaerobic ammonium oxidation was saturated at low concentrations of NO2−. We observed a shift in the significance of anaerobic ammonium oxidation to N2 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.