The role of anthraquinone-2-sulfonate on intra/extracellular electron transfer of anaerobic nitrate reduction.

To improve the electron (e-) transfer efficiency, exogenous redox mediators (RMs) were usually employed to enhance the denitrification efficiency due to the electron shuttling. Previous studies were mainly focused on how to improve the extracellular electron transfer (EET) by exogenous RMs. However, the intracellular electron transfer (IET), another crucial e- transfer pathway, of biological denitrification was scarcely reported, especially for the relationship between the denitrification and IET. In this study, Coenzyme Q, Complexes I, II and III were determined as the core components in the IET chain of denitrification by using four specific respiration chain inhibitors (RCIs). Anthraquinone-2-sulfonate (AQS) partially recovered the IET of denitrification from NO3--N to N2 gas when the RCIs were added. Specifically, the generations of N2 gas were improved by 9.68%-18.25% in the experiments with RCIs and AQS, comparing to that with RCIs. nrfA gene was not detected by reverse transcription-polymerase chain reaction, suggesting that Klebsiella oxytoca strain could not conduct dissimilatory nitrate reduction to ammonium. Nitrate assimilation was considered as the main NH4+-N formation way of K. oxytoca strain. The two e- transfer pathways of denitrification were constructed and the roles of AQS on the IET and EET of denitrification were specifically discussed. The results of this study provided a better understanding of the e- transfer pathways of denitrification, and suggested a potential practical use of exogenous RM on bio-treatment of nitrate-containing wastewater.