Biogeographical diversity among marine bacterioplankton

The phylogenetic diversity of bacteria isolated on solid media from the Baltic Sea, Mediterranean Sea, Southern California Eight, Skagerrak, Weddell Sea (ice) and Andaman Sea was investigated by means of 16S rRNA gene sequence analysis. Of the 128 sequenced isolates, 52% showed similarity on the species level to previously reported bacteria, while as many as 18% showed a sequence similarity below 93%, which in the closest case would represent difference at the genus level. A majority of the isolated gamma-Proteobacteria could be assigned to known species, while half of the alpha-Proteobacteria were only identified to the genus level. Bacteria affiliated with the Flexibacter-Cytophaga-Bacteroides phylum showed the lowest levels of similarity to previously sequenced bacteria, mainly representing novel genera. Closely related isolates most often originated from the same geographic area. Nevertheless, our data also demonstrated that most genera have closely related representatives widely distributed between different sea areas. Isolates related to environmental clones, with a sequence similarity above the tentative genus level, were found in 51 cases, of which 17 were more similar to clones than to cultured bacteria. From this result we concluded that a large proportion of the great species richness of marine bacteria, found by culture-independent techniques, is likely to be verified through information from live and functional bacteria.