BACTERIAL DIVERSITY IN AN ARCTIC LAKE - A FRESH-WATER SAR11 CLUSTER

We used molecular techniques to assess the phylogenetic affinity of cu ltured and uncultured microorganisms from Toolik Lake, an oligotrophic lake in arctic Alaska, USA. The phylogenetic positions of cloned cult ures of bacteria were determined by sequence analysis of PCR amplified ribosomal RNA genes. The Toolik Lake bacterial isolates showed a high degree of similarity, 0.94 to 0.99, to a wide variety of phyla that a re well represented in the ribosomal RNA database. The occurrence of s pecies normally associated with a terrestrial habitat (Arthrobacter gl obiformis and Burkholderia solanacearum) or a more nutrient-rich envir onment (Cytophaga aquatilis and Zoogloea ramigera) suggests a particle -associated origin for these cell types, consistent with the fact that we used an unfiltered sample. In contrast, the analysis of rRNA genes cloned from a complex natural DNA community indicated the predominanc e of beta-proteobacteria closely related to the rRNA homology group II pseudomonads alcaligenes eutrophus and Pseudomonas pickettii. However , 2 of the rRNA gene clones are deeply branching relatives (similarity = 0.88) of the alpha-proteobacteria SAR11 cluster, previously detecte d only in marine environments. This finding indicates a widespread aqu atic distribution for this recently described group.