Response of anaerobic ammonium oxidation to inorganic nitrogen fluctuations in temperate estuarine sediments

The discovery of anaerobic ammonium oxidation (anammox) highlighted the importance of alternative metabolic pathways to inorganic nitrogen removal in natural environments, particularly in those subjected to increased nitrate inputs, such as estuaries. Laboratory enrichment experiments were used to test the effect of increasing loads of nitrate (NO₃ ⁻), nitrite (NO₂ ⁻), and ammonium (NH₄ ⁺) on the anammox process. Three Atlantic temperate estuaries (NW Portugal) were investigated along a salinity gradient, and anammox activity was measured under different NO₃ ⁻, NO₂ ⁻, and NH₄ ⁺ treatments, using the isotope pairing technique. Obtained results showed that NO₃ ⁻ stimulated denitrification but not anammox, whereas NO₂ ⁻ additions had a positive effect on anammox activity, confirming its role as a key environmental control. On the other hand, increasing NH₄ ⁺ concentrations seemed to inhibit anammox for low salinity sites. Our findings suggested an important role of the natural availability of nitrogen compounds in regulating anammox and the magnitude of anammox versus denitrification in estuarine environments.