1. Department of Environmental Microbiology, Eawag, Swiss Federal Institute of Aquatic Science and Technology , 8600 Dübendorf, Switzerland
2. Department of Environmental Systems Science, Institute of Biogeochemistry and Pollutant Dynamics , ETH Zurich, 8092 Zurich , Switzerland
3. Department of Plant and Microbial Biology, University of Zurich , 8008 Zurich , Switzerland