Denitrifying nirK-containing alphaproteobacteria exhibit different electrode driven nitrite reduction capacities

Biocathodic electrode-driven denitrification has been proved experimentally in complex biofilms. However, experimentation with isolated bacteria in pure culture is still limited. In this paper, six Alphaproteobacteria (Rhizobiales), found to be dominant in a denitrifying biocathode, have been characterized bioelectrochemically. Bacteria were isolated using strict autotrophic conditions in the presence of nitrate or nitrite. Six representative isolates were selected and proven able to denitrify under autotrophic and heterotrophic conditions in liquid media. Bioelectrochemical reduction of nitrate, nitrite and nitrous oxide was tested using cyclic voltammetry. Electrode-driven nitrite reduction was only detected in four of the six isolates. However, no electrode-driven nitrate or nitrous oxide reduction could be detected for any of them. In the presence of nitrite, estimated midpoint potentials for bioelectrocatalyzed reactions ranged from −500 to −534 mV vs. SHE. Two of the isolates exhibited midpoint potentials at −450 and −486 mV vs. SHE when incubated in the absence of any external nitrogenous electron acceptor. These redox peaks were attributed to electrode-driven hydrogen production in the biofilm. We have proven that electrode-driven nitrite reduction is feasible in monospecific biofilms. However, significant variability in relation to electrode-driven nitrogen reduction processes was observed in closely related species, confirming a strain-specific behavior.