Sampling Natural Biofilms: A New Route to Build Efficient Microbial Anodes

Electrochemically active biofilms were constructed on graphite anodes under constant polarization at -0.1V vs saturated calomel reference (SCE)with 10 mM acetate as substrate. The reactors were inoculated with three different microbial samples that were drawn from exactly the same place in a French Atlantic coastal port (i)by scraping the biofilm that had formed naturally on the surface of a floating bridge, (ii)by taking marine sediments just under the floating bridge, and (iii)by taking nearby beach sand. Current densities of 2.0 A/m2 were reached using the biofilm sample as inoculum while only 0.4 A/m2 and 0.8 A/m2 were obtained using the underlying sediments and the beach sand, respectively. The structure of bacterial communities forming biofilms was characterized by denaturing gradient gel electrophoresis (DGGE)analysis, and revealed differences between samples with the increase in relative intensities of some bands and the appearance of others. Bacteria close related to Bacteroidetes, Halomonas, and Marinobacterium were retrieved only from the efficient EAbiofilms formed from natural biofilms, whereas, bacteria close related to Mesoflavibacter were predominant on biofilm formed from sediments. The marine biofilm was selected as the inoculum to further optimize the microbial anode. Epifluorescence microscopy and SEM confirmed that maintaining the electrode under constant polarization promoted rapid settlement of the electrode surface by a bacterial monolayer film. The microbial anode was progressively adapted to the consumption of acetate by three serial additions of substrate, thus improving the Coulombic efficiency of acetate consumption from 31 to 89