1. Department of Environmental Microbiology, Eawag, Swiss Federal Institute of Aquatic Science and Technology, CH-8600, Dübendorf, Switzerland, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, CH-8092 Zürich, Switzerland, and Universität Basel, Zentrum für Mikroskopie, Pharmazentrum, CH-4056 Basel, Switzerland