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

摘要

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.