Re-discussing the synergistic mechanisms in dual metabolism of microalgae and N-cycling bacteria on joint N removal

Nitrogen (N) removal in microalgal-bacterial system (MBS) depends on the dual metabolism of microalgae and N-cycling bacteria. However, the effect of specific synergistic mechanisms involved in the dual metabolism on N removal remains unclear. Here, N removal performance, community compositions, N-cycling related metabolism, and biosynthesis and transport of cofactors were explored to clarify the synergistic mechanism between microalgae and N-cycling bacteria. Results indicated that microalgae hindered the N removal capability of dissimilatory nitrate reducing (DNR) bacteria, denitrifying bacteria, nitrifying bacteria, and assimilating bacteria by suppressing the electron transport chain of N-cycling bacteria. Fortunately, although the N removal ability of N-cycling bacteria decreased, the N-cycling bacterial community collaborative mode among multiple N-cycling pathways could drive microalgal growth and assimilation via providing the essential vitamins B1, B2, and B7, thus enhancing the synergistic effects of microalgae and N-cycling bacteria on N removal. Compared with the control, the removal efficiencies of TN and NH₄⁺-N in MBS increased by 6.64-fold and 7.46-fold. Meanwhile, microalgal growth and chlorophyll a increased by 1.53-fold and 1.52-fold in MBS. But, the NO₃^–-N removal efficiency decreased by 0.94-fold in MBS, the abundance of denitrifying bacteria was higher than DNR bacteria in MBS, suggesting that NO₃^–-N removal capacity in MBS was predominantly associated with denitrifying bacteria, while microalgae exhibit a tendency to prefer absorbing NH₄⁺-N rather than NO₃^–-N. Together, this work improves our understanding of N removal mechanisms in MBS and provides valuable insights for the effective design and operation of MBS in real wastewater.