1. Department of Marine Biogeochemistry and Toxicology, Royal Netherlands Institute for Sea Research, 1790 AB Den Burgh
2. Department of Microbiology, Faculty of Science, University of Nijmegen, 6500 GL Nijmegen, The Netherlands
3. Max Planck Institute for Marine Microbiology, D-28359 Bremen, Germany
4. Limnological Research Center, Swiss Federal Institute for Environmental Science and Technology, CH 6047 Kastanienbaum, Switzerland