Mixotrophic anammox bacteria outcompete dissimilatory nitrate reduction and denitrifying bacteria in propionate-containing wastewater.

Organic carbon can influence nitrogen removal during the anaerobic ammonia oxidation (anammox) process. Propionate, a common organic compound in pretreated wastewater, its impacts on mixotrophic anammox bacteria and the underlying mechanisms have not been fully elucidated. This study investigated the core metabolism and shift in behavior patterns of mixotrophic Candidatus Brocadia sapporoensis (AMXB) under long-term propionate exposure. Genome-resolved metagenomic analysis revealed that AMXB could convert nitrate generated by anammox bacteria to ammonium via the DNRA pathway, leveraging propionate as an electron donor. This recycled ammonium was then used to sustain the anammox process, thereby enhancing nitrogen removal efficiency. Notably, AMXB grew more efficiently than DNRA and denitrifying bacteria due to its more energy-efficient propionate metabolic pathway. This finding suggests that AMXB, as a mixotrophic anammox bacterium, has a competitive advantage in nitrogen metabolism in low C/N wastewater, contributing to efficient nitrogen removal.