Global patterns of diversity and community structure in marine bacterioplankton

Because of their small size, great abundance and easy dispersal, it is often assumed that marine planktonic microorganisms have a ubiquitous distribution that prevents any structured assembly into local communities. To challenge this view, marine bacterioplankton communities from coastal waters at nine locations distributed world-wide were examined through the use of comprehensive clone libraries of 16S ribosomal RNA genes, used as operational taxonomic units (OTU). Our survey and analyses show that there were marked differences in the composition and richness of OTUs between locations. Remarkably, the global marine bacterioplankton community showed a high degree of endemism, and conversely included few cosmopolitan OTUs. Our data were consistent with a latitudinal gradient of OTU richness. We observed a positive relationship between the relative OTU abundances and their range of occupation, i.e. cosmopolitans had the largest population sizes. Although OTU richness differed among locations, the distributions of the major taxonomic groups represented in the communities were analogous, and all local communities were similarly structured and dominated by a few OTUs showing variable taxonomic affiliations. The observed patterns of OTU richness indicate that similar evolutionary and ecological processes structured the communities. We conclude that marine bacterioplankton share many of the biogeographical and macroecological features of macroscopic organisms. The general processes behind those patterns are likely to be comparable across taxa and major global biomes.