Biogeographic analysis of ribosomal RNA clusters from marine bacterioplankton

Bacterial cosmopolitanism, which has for a long time been accepted by most microbiologists, confronts recent reports of endemic species and restricted distributions. From a marine perspective, our current knowledge is lacking global pictures of bacterioplankton distribution. Public databases storing nucleotide sequences grow daily from recurrent environmental analyses of the microbial community using the small subunit ribosomal RNA gene (SSU rDNA) sequence as a marker of bacterial occurrence. These studies significantly contribute to our understanding of the global microbial processes. To address the question of whether marine bacterioplankton are cosmopolitan, 7070 SSU rDNA sequences submitted to GenBank were analyzed for sampling information and clustered into distinct ribotypes, based on sequence similarity levels. The resulting distribution maps displayed remarkable patterns of distributions of some members of the bacterioplankton community. Despite a strong bias toward sampling sites in the northern temperate regions of the globe, specific ribotypes showed ubiquitous dispersal whereas other ribotypes showed a distribution restricted to polar regions. These results suggest that marine bacterioplankton components present unexpected global patterns of distributions, where cosmopolitanism appears as an occasional trait.