Bacterial diversity in an arctic lake: A freshwater SAR11 cluster

We used molecular techniques to assess the phylogenetic affinity of cultured and uncultured microorganisms from Toolik Lake, an oligotrophic lake in arctic Alaska, USA. The phylogenetic positions of cloned cultures 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 are well represented in the ribosomal RNA database. The occurrence of species normally associated with a terrestrial habitat (Arthrobacter globiformis and Burkholderia solanacearum) or a more nutrient-rich environment (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 predominance 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 detected only in marine environments. This finding indicates a widespread aquatic distribution for this recently described group.