1. Molecular Ecology Group, Max-Planck-Institute for Marine Microbiology, Celsiusstraβe 1, D-28359 Bremen, Germany
2. Netherlands Institute for Sea Research (NIOZ), PO Box 59, NL-1790 AB Den Burg (Texel), The Netherlands
3. Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
4. Department of Microbiology, Max-Planck-Institute for Marine Microbiology, Celsiusstraβe 1, D-28359 Bremen, Germany