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uni'wissen 01-2015

shower, and waste water even contains residue from pesticides. The bacteria used to clean up waste water at conventional treatment plants are often unable to remove these pollutants: “They are persistent against bacteria and are still present in more or less the same amounts when the water leaves the plant.” The plants thus try to remove the pol- lutants from the water using activated carbon, which has the disadvantage of being difficult to dispose of, or they destroy them with the help of ozone, although the danger posed by the degra- dation products created with this method is not yet clear. “These two methods are not very environ- mentally friendly, have not yet been perfected, and are not particularly cost-effective either.” Helpful Parasite This is where Trametes versicolor comes in. The fungus is very common in European forests and grows primarily on European beech trees. It produces a constant supply of laccase. “This enzyme actually degrades wood,” says Sané – especially the wood component lignin. In this way, the fungus sustains itself as a tree parasite. The enzyme is adapted perfectly to the com- plex structure of wood: The fungus can oxidize many substances and break them down into their constituent parts. It can also break down micropollutants. Sané came up with the idea of exploiting this ability to purify waste water in treatment plants while working on her doctoral dissertation under Prof. Dr. Roland Zengerle at the Laboratory for MEMS Applications. In the “Bioelectrochemical Systems” research group, led by Dr. Sven Kerzen- macher, she initially set her sights on an entirely different potential area of application for Trametes versicolor and its enzyme: improving biofuel cells that can be powered in waste water and produce energy for water treatment plants. In order for electricity to be produced, electrons need to travel from one pole, the anode, to the other, the cathode (see illustration below). In the case of a biofuel cell in waste water, bacteria already living there build up at the anode: “They form a really thick layer.” While breaking down organic matter found in the water, the bacteria use the anode instead of oxygen to “breathe” – meaning that there is no need to expend any en- ergy oxygenating them and that they can transfer their electrons to the anode. The electrons then migrate to the cathode, where the laccase works as a catalyst, conveying the electrons to the oxygen as they arrive. “The enzymes are so catalytic that there is no need to use precious metals like platinum.” “At one point it struck me that this was an opportunity to kill two birds with one stone.” How a biofuel cell works Bacteria build up at the negative pole, the anode. They break down organic substances and form carbon dioxide (CO2 ). In the process, electrons (e- ) and protons (H+ ) are emitted. The positively charged protons pass through a membrane directly to the cathode, the positive pole. The negatively charged electrons migrate there too over an external circuit, thus producing electricity. At the cathode they reduce oxygen, which then fuses with the proteins to create water. The enzyme laccase, which is produced by Trametes versicolor, supports this reaction as a catalyst. Grafik: Sabine Sané Biofuel Cell uni wissen 01 201530 uni wissen 01201530