Phylogeny of actinobacteria small subunit (SSU) rRNA gene clones recoveredfrom marine bacterioplankton

Recent analyses of PCR-generated SSU rRNA gene clone libraries have reveale d the presence of previously unidentified members of the class Actinobacter ia (high G+C cram positive bacteria) in a diverse array of natural environm ents. From terrestrial soil samples, Actinobacteria-related gene clones for m several phylogenetic clusters, distantly related to their nearest cultiva ted relatives. Actinobacteria-related clones have also been discovered in S SU rRNA gene clone libraries from sea water, though a lack of representativ e clones with complete sequences has lead to uncertainties regarding their potential affiliation with terrestrial soil environmental gene clones, as w ell as uncertain phylogenetic relationships with reference strains from the class Actinobacteria. Herein, we show the phylogenetic position of PCR-gen erated SSU rRNA gene clones related to the class Actinobacteria recovered f rom three diverse populations of marine bacterioplankton, including a sampl e from a depth of 80 m in the Sargasso Sea and samples from a depth of 10 m over the eastern and western continental shelves of the United States. Emp loying phylogenetic methods which take into consideration potential bias du e to unequal nucleotide frequencies between taxa, the nearly complete SSU r RNA gene sequence data presented here place the closely related marine clon es (>97.4% similar) as deep branching members of the class Actinobacteria, distantly related to their nearest cultivated relatives. In addition, the c lade of marine Actinobacteria gene clones did not demonstrate phylogenetica lly robust affiliations with any environmental gene clone sequences from te rrestrial soil habitats. Based on their early divergence from the base of t he Actinobacteria radiation and phylogeneric distance from other members of this class, microorganisms which possess SSU rRNA genes belonging to the m arine Actinobacteria clade probably represent a previously unidentified sub class or order within the class Actinobacteria.