Marine aerobic biofilm as biocathode catalyst

Stainless steel electrodes were immersed in open seawater and polarized for some days at − 200 mV vs. Ag/AgCl. The current increase indicated the formation of biofilms that catalysed the electrochemical reduction of oxygen. These wild, electrochemically active (EA) biofilms were scraped, resuspended in seawater and used as the inoculum in closed 0.5 L electrochemical reactors. This procedure allowed marine biofilms that are able to catalyse oxygen reduction to be formed in small, closed small vessels for the first time. Potential polarisation during biofilm formation was required to obtain EA biofilms and the roughness of the surface favoured high current values. The low availability of nutrients was shown to be a main limitation. Using an open reactor continuously fed with filtered seawater multiplied the current density by a factor of around 20, up to 60 µA/cm2, which was higher than the current density provided in open seawater by the initial wild biofilm. These high values were attributed to continuous feeding with the nutrients contained in seawater and to suppression of the indigenous microbial species that compete with EA strains in natural open environments. Pure isolates were extracted from the wild biofilms and checked for EA properties. Of more than thirty different species tested, only Winogradskyella poriferorum and Acinetobacter johsonii gave current densities of respectively 7% and 3% of the current obtained with the wild biofilm used as inoculum. Current densities obtained with pure cultures were lower than those obtained with wild biofilms. It is suspected that synergetic effects occur in whole biofilms or/and that wild strains may be more efficient than the cultured isolates.