Using planktonic microorganisms to supply the unpurified multi-copper oxidases laccase and copper efflux oxidases at a biofuel cell cathode

The feasibility to apply crude culture supernatants that contain the multicopper oxidases laccase or copper efflux oxidase (CueO) as oxygen reducing catalysts in a biofuel cell cathode is shown. As enzyme-secreting recombinant planktonic microorganisms, the yeast Yarrowia lipolytica and the bacterium Escherichia coli were investigated. The cultivation and operation conditions (choice of medium, pH) had distinct effects on the electro-catalytic performance. The highest current density of 119 ± 23 μA cm−2 at 0.400 V vs. NHE was obtained with the crude culture supernatant of E. coli cells overexpressing CueO and tested at pH 5.0. In comparison, at pH 7.4 the electrode potential at 100 μA cm−2 is 0.25 V lower. Laccase-containing supernatants of Y. lipolytica yielded a maximum current density of 6.7 ± 0.4 μA cm−2 at 0.644 V vs. NHE. These results open future possibilities to circumvent elaborate enzyme purification procedures and realize cost effective and easy-to-operate enzymatic biofuel cells.