Granule Deterioration Triggers Increased Dissimilatory Nitrate Reduction to Ammonium in Anammox Granular Sludge

Deterioration of anammox (anaerobic ammonium oxidation) granular sludge (anammox granular sludge (AGS)) is common in bioreactor studies experiencing reactor perturbations, but the resulting metabolic stress responses have not been related to changes in the community structure and function. This research provides evidence that an AGS-based sequencing batch reactor began performing dissimilatory nitrate reduction to ammonium (DNRA) during a major disintegration event. Granule disintegration was accompanied by the release of 150 mg/L dissolved organic carbon (DOC), which provided the electron donor for DNRA. Although the anammox bacterium Candidatus Brocadia persisted, the release of an organic substrate promoted the activity of DNRA genera─Thauera, Anaerolineae, and Denitratisoma. The release of DOC led to a high COD/N ratio (approximately 8 g of COD/g of N), which promoted heterotrophic bacteria, as well as Candidatus Brocadia, to perform DNRA and consumed nitrite, the electron acceptor for anammox. This work documents that granule disintegration and high COD/N ratios are promoting DNRA in anammox-based systems.