Autotrophic nitrate reduction to ammonium via reverse electron transfer in Geobacter dominated biofilm

Geobacter dominated electroactive biofilms (EABs) have been demonstrated to perform bidirectional extracel-lular electron transfer (EET) in bioelectrochemical systems, but it is largely unknown when nitrate is the electron acceptor at the cathode. If reverse EET occurs on biocathode, this EAB has to perform dissimilatory nitrate reduction to ammonia (DNRA) rather than denitrification according to genomes. Here, we have proven the feasibility of reverse bioelectron transfer in EAB, achieving a DNRA efficiency up to 93 +/- 3% and high Faraday efficiency of 74 +/- 1%. Constant current was found to be more effective than constant potential to maintain Geobacter on the cathode, which highly determines this electrotrophic respiration. The prevalent DNRA at constant current surpassed denitrification, demonstrated by the reverse tendencies of DNRA (nrfA) and deni-trification (nirS and nirK) gene transcription. Metatranscriptomics further revealed the possible electron uptake mechanisms by which the outer membrane (OmcZ and OmcB) and periplasmic cytochromes (PpcB and PpcD) may be involved. These findings extend our understanding of the bidirectional electron transfer and advance the applications of EABs.

Automated summary

This study demonstrates the possibility of reverse bioelectron transfer in Geobacter-dominated electroactive biofilms, achieving a high efficiency of dissimilatory nitrate reduction to ammonia (DNRA). The use of constant current is found to be more effective in maintaining Geobacter on the cathode, and the prevalence of DNRA at constant current is shown through gene transcription analysis. Metatranscriptomics reveals potential electron uptake mechanisms in the biofilms.