Potential rates and environmental controls of anaerobic ammonium oxidation in estuarine sediments

The importance of anaerobic ammonium oxidation (anammox) - a metabolic pathway that can generate dinitrogen - remains poorly described in temperate estuarine systems. We evaluated the relative importance of anammox and denitrification along the salinity gradient of the Cávado River estuary (NW Portugal) during a seasonal survey. Potential rates of anammox and denitrification were measured in anaerobic sediment slurries using ¹⁵N-labeled NO ₃ ^- and NH ₄ ⁺ amendments. Production of ²⁹N ₂ and ³⁰N ₂ in the slurries was quantified using membrane inlet mass spectrometry (MIMS). Environmental parameters such as salinity, temperature and inorganic nitrogen were also monitored. Anammox and denitrification potentials in Cávado estuarine sediments varied from 0 to 3.3 and from 1.1 to 10.8 nmol N cm ^-3 wet sediment h ^-1, respectively. During 1 sampling occasion, anammox activity accounted for as much as 72% of the measured production of dinitrogen, while annual averages varied from 17 to 33% depending on location. Nitrate availability and temperature appeared to be the primary environmental controls of anammox. Higher concentrations of NO ₃ ^- and intermediate temperatures in the estuarine water (14 to 16°C) supported higher anammox activities. Using 16S rRNA gene-specific primers, anammox-like bacterial sequences were recovered from Cávado sediments - corroborating the measured anammox activities. Our results support the environmental importance of anammox for removal of nitrogen in estuarine sediments, and suggest that this process is apparently regulated by few variables.