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

¹ Institute for Limnology, Austrian Academy of Sciences, Mondseestrasse 9, A-5310 Mondsee, Austria
² Biology Centre of the AS CR, v. v. i., Institute of Hydrobiology, Na Sádkách 7, 37005, eské Budjovice, Czech Republic
³ Department of Biochemistry and Molecular Biology, Dalhousie University, 5850 College St., Halifax, Nova Scotia B3H 1X5, Canada

Correspondence
Jan Jezbera
jan.jezbera{at}email.cz

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.

The GenBank/EMBL/DDBJ accession number for the 16S rRNA gene sequence of ‘Candidatus Planktophila limnetica’ strain MWH-EgelM2-3.acI is FJ428831.

Three supplementary figures, a supplementary table and details of methods used to isolate strain MWH-EgelM2-3.acI are available with the online version of this paper.