** To be submitted soon to PNAS, please do not quote or cite ** BIOLOGICAL SCIENCES Ecology, Microbiology A latitudinal diversity gradient in planktonic marine bacteria Jed A. Fuhrman, Joshua A. Steele, Ian Hewson, Michael S. Schwalbach, Mark V. Brown, Jessica L. Green 1 , James H. Brown 2 USC Wrigley Institute and Department of Biological Sciences Los Angeles, CA 90089-0371, USA fuhrman@usc.edu (213) 740-5757, fax 740-8123 1. Center for Ecology and Evolutionary Biology, University or Oregon, Eugene, Oregon, 97403-5289, USA jlgreen@uoregon.edu 2. Department of Biology, University of New Mexico, Albuquerque, NM 87131 , USA, jhbrown@unm.edu. CORRESPONDING AUTHOR 22 pages, 3 figures,1 table (Supplement has 1 table and 1 figure) 209 words in abstract 32347 Characters including spaces in paper Abbreviations: ARISA, Amplified Ribosomal Intergenic Spacer Analysis 1