Deciphering the partial denitrification function of companion bacteria in mixotrophic anammox systems under different carbon/nitrogen ratios

Although partial denitrification coupled with the anammox process has received considerable attention, the partial denitrification function of companion bacteria in mixotrophic anammox systems remains experimentally investigated. Herein, a partial denitrification process was realized in mixotrophic anammox systems with a COD/TN ratio of 0.75. Furthermore, the experimental verification results revealed that nitrate and ammonia were eliminated concurrently, showing that partial denitrifying bacteria were present in the mixotrophic anammox systems. Denitratisoma and Norank Xanthobacteraceae were the dominant genera of companion bacteria containing functional genes related to partial denitrification. Especially, Denitratisoma was a companion bifunctional bacterium for Candidatus Kuenenia shifted between complete and partial denitrification at different COD/TN ratios. Moreover, nir gene abundance in the mixotrophic anammox system increased by 135.4% compared to that in the autotrophic anammox system. In contrast, nor gene abundance decreased by 28.00%, indicating that NO might be used as a medium for cross-feeding anammox and partial denitrification. This study provides a more thorough understanding of companion bacteria in mixotrophic anammox systems.