1. Department of Environmental Sciences, The Man-Society-Environment Program, University of Basel, Vesalgasse 1, 4051 Basel, Switzerland
2. Department of Microbial Ecology, Biologische Anstalt Helgoland, Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, Kurpromenade, 27498 Helgoland, Germany