Electrochemical enrichment of marine denitrifying bacteria to enhance nitrate metabolization in seawater

High concentrations of nitrate from industrial discharges to coastal marine environments are a matter of concern owing to their ecological consequences. In the last years, Bioelectrochemical Denitrification Systems (BEDS) have emerged as a promising nitrate removal technology. However, they still have limitations, such as the enrichment strategy for specific microbial communities in the electrodes under natural conditions. In this study, threeelectrode electrochemical cells were used to test microbial enrichment from natural seawater by applying three reported potentials associated with the dissimilatory denitrification process (-130, -260, and -570 mV vs. Ag/AgCl). The microbial community analysis showed that by applying -260 mV (vs. Ag/AgCl) to the working electrode, it was possible to significantly enrich denitrifying microorganisms, specifically Marinobacter, in comparison with the control. Furthermore, -260 mV (vs. Ag/AgCl) led to a significantly higher nitrate removal than other conditions, which, combined with cyclic voltammetry analysis, suggested that the polarized electrodes worked as external electron donors for nitrate reduction. Hence, this work demonstrates for the first time that it is possible to enrich marine denitrifying microorganisms by applying an overpotential of -260 mV (vs. Ag/AgCl) without the need for a culture medium, the addition of an exogenous electron donor (i.e., organic matter) or a previously enriched inoculum.