Microbial nitrate reduction electro-assisted by exo-electrogenic reduction of dioxygen with a Pseudomonas dominated cathodic biofilm

A nitrate reducing microbial biocathode was developed through constant polarization at -0.5 V vs SCE using two types of inoculums: a pure culture of Thiobacillus denitrificans and water collected from the artificial wetland of Rampillon (France). The results show a clear increase of the nitrate removal efficiency for the Pilots with the natural water although no catalytic nitrate reduction can be evidenced by cyclic voltammetry. Further studies show a catalytic oxygen reduction through exo-electrogenic metabolism and a correlation between the cathode polarization at -0.5 V vs SCE and the nitrate remediation. 16S rRNA gene amplicon sequencing of the biofilm bacterial DNA shows a very large predominance of Pseudomonas, a genus that includes many species able to reduce nitrate and/or reduce dioxygen (O2) using electrons from a cathode. The increased nitrate reduction performance is hypothesized to arise from an indirect bioelectro-assistance that allows the emergence of local anoxic conditions caused by microbial endo-electrogenic pathway for oxygen reduction.