1. Department of Oceanography, Darling Marine Center, University of Maine, Walpole, Maine 04573, Department of Microbiology, Miami University, Oxford, Ohio 45056, School of Oceanography, University of Washington, Seattle, Washington 98195, and Laboratoire de Photochemie et Photophysique Moleculaire, URA CNRS 348, Universite de Bordeaux 1, Talence, France