Cosmopolitan distributions were reported for several narrow phylogenetic groups of free-living bacteria.For instance the species-like subcluster C (PnecC) of the Polynucleobacter cluster (Betaproteobacteria),which is characterized by a maximal divergence of 16S rRNA gene sequences of 1.2%,was detected in freshwater habitats located all over the world.The revealed distribution includes all continents and all climatic zones.Furthermore,this species-like group of bacteria inhabits an ecologically broad diversity of freshwater habitats ranging from acidic to alkaline,from lowland to high-mountain lakes (up to 5000 meters altitude),and from oligotrophic to hypertrophic lakes.Besides the ubiquitous presence in stagnant freshwater habitats,this species-like group of bacteria harbours strains completely differing in lifestyle.One group of strains live as free-living organisms in the water column of lakes,ponds and streams,while the other group of strains lives as endosymbionts in ciliates affiliated with the genus Euplotes.