1. Department of Environmental Microbiology, Swiss Federal Institute of Aquatic Science and Technology (Eawag), P.O.Box 611, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland, Ghent University, Faculty of Bioscience Engineering, Laboratory of Microbial Ecology and Technology (LabMET), Coupure Links 653, B-9000 Gent, Belgium, and Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, CH-8092 Zürich, Switzerland