Comparative 16S rRNA analysis of lake bacterioplankton reveals globally distributed phylogenetic clusters including an abundant group of actinobacteria

In a search for cosmopolitan phylogenetic clusters of freshwater bacteria, we recovered a total of 190 full and partial 16S ribosomal DNA (rDNA) seque nces from three different lakes (Lake Gossenkollesee, Austria; Lake Fuchsku hle, Germany; and Lake Baikal, Russia). The phylogenetic comparison with th e currently available rDNA data set showed that our sequences fall into 16 clusters, which otherwise include bacterial rDNA sequences of primarily fre shwater and soil, but not marine, origin. Six of the clusters were affiliat ed with the alpha, four were affiliated with the beta, and one was affiliat ed with the gamma subclass of the Proteobacteria; four were affiliated with the Cytophaga-Flavobacterium-Bacteroides group; and one was affiliated wit h the class Actinobacteria (formerly known as the high-G+C gram-positive ba cteria), The latter cluster (hgcI) is monophyletic and so far includes only sequences directly retrieved from aquatic environments. Fluorescence in si tu hybridization (FISH) with probes specific for the hgcI cluster showed ab undances of up to 1.7 x 10(5) cells ml(-1) in Lake Gossenkollesee, with str ong seasonal fluctuations, and high abundances in the two other lakes inves tigated. Cell size measurements revealed that Actinobacteria in Lake Gossen kollesee can account for up to 63% of the bacterioplankton biomass, A combi nation of phylogenetic analysis and FISH was used to reveal 16 globally dis tributed sequence clusters and to confirm the broad distribution, abundance , and high biomass of members of the class Actinobacteria in freshwater eco systems.