1 Department of Environmental Microbiology, Swiss Federal Institute of Aquatic Science and Technology (Eawag), 8600 Dübendorf, Switzerland. Electronic address: David.Johnson@eawag.ch.
2 Institute of Bio- and Geosciences, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany. Electronic address: s.noack@fz-juelich.de.