Various electron transports involved in nitrate reduction within electrochemically active biofilm with periodic...

Periodic polarity reversal (PPR) presents an efficient strategy to improve nitrate (NO₃^--N) reduction in electrochemically active biofilm (EAB), but the involved extracellular electron transfer (EET) processes are still unclear. Here, the contributions of unidirectional/bidirectional EET and electron storage for NO₃^--N reduction in EABs were quantified with different polarity reversal operations. The NO₃^--N reduction efficiencies reach 6.5 % for single polarity reversal, 14.7 % for non-polarity reversal, and 28.8 % for PPR, respectively. The percentages were 50.2 % and 21.6 % for NO₃^--N reduction driven by electron storage and unidirectional EET, which is mediated by outer membrane cytochromes and redox substances. Bidirectional EET contributed 28.2 % to NO₃^--N reduction, with the ammonia selectivity of 77.3 % under PPR operation. The genus Petrimonas dominated in the EAB, and genes related to nitrogen metabolism were overexpressed under PPR operation. These findings advance our understanding of EET driven NO₃^--N reduction in EAB and inspire the optimization of the bioelectrochemical systems.