'Candidatus Planktophila limnetica', an actinobacterium representing one of the most numerically important taxa in freshwater bacterioplankton.

Actinobacteria often constitute a large fraction of the bacterioplankton in freshwater systems. Cultivation-independent methods have revealed that the so-called acI lineage frequently represents the most numerous taxon among assemblages of freshwater Actinobacteria and even among total freshwater bacterioplankton. Bacteria affiliated with this uncultivated lineage have been detected in freshwater habitats located in various continents and climatic zones but have never been found among terrestrial or offshore marine systems. So far, this ecologically important lineage of freshwater Actinobacteria is not represented by a recognized taxon. In this study, we established a stable mixed culture containing a strain affiliated with the acI lineage from a freshwater lake in Austria. The proportion of the strain in the culture could be increased by manipulation of the medium composition by more than one order of magnitude, however all subsequent attempts to isolate this strain into pure culture were unsuccessful. Some of the phenotypic traits of this acI strain were determined and its taxonomic position within the Actinobacteria was analysed. Phylogenetic analysis of this organism's 16S rRNA gene revealed a distant relationship with cultivated organisms and recognized species (89 % gene sequence similarity with the latter). Furthermore, this analysis did not support a clear assignment of the strain to any of the recognized families within the phylum Actinobacteria. It is suggested that a candidate taxon, 'Candidatus Planktophila limnetica' is established to represent this strain.