Actinobacteria comprise a substantial fraction of the bacterioplankton in freshwater lakes and streams. Numerous cultivation-independent investigations have retrieved actinobacterial 16S rRNA gene sequences from such habitats. The taxa detected in freshwater habitats are usually absent from terrestrial and marine systems. So far, none of the indigenous freshwater lineages is represented by a taxon with a validly published name. The seven organisms for which status is described here were isolated from freshwater lakes and ponds located in tropical, subtropical and temperate climatic zones. Phylogenetic analyses demonstrated that they are affiliated with one of the actinobacterial lineages indigenous to freshwater bacterioplankton. The seven novel taxa could only be cultivated to date as mixed cultures that also contain non-actinobacterial strains. Due to the lack of pure cultures, I propose to establish the candidate species ‘ Planktoluna difficilis’, ‘ Aquiluna rubra’, ‘ Flaviluna lacus’, ‘ Rhodoluna limnophila’, ‘ Rhodoluna planktonica’, ‘ Rhodoluna lacicola’ and ‘ Limnoluna rubra’ for these taxa.


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Growth of ' Limnoluna rubra' strain MWH-EgelM2-3 and an accompanying non-actinobacterial strain on NSY agar plates. The colonies of the candidate species are red-pigmented and those of the other strain are unpigmented. The plated liquid culture contained about 10 cells ml and was dominated by the actinobacterial strain (>90 % of cells). The photo shows a section of a plate that received an inoculum of about 10 cells. The strain of the candidate species grows well when in direct contact with the non-pigmented colonies; however, the size of the actinobacterial colonies decreases with distance from the non-pigmented colonies and, beyond a crucial distance from these colonies, no actinobacterial growth occurs.


Neighbour-joining tree based on almost full-length 16S rRNA gene sequences ( positions 47–1465). The tree reconstructs the phylogenetic relationships between the novel candidate species and environmental sequences of uncultured actinobacteria. [PDF](510 KB)


[PDF file of Supplementary Tables S1 and S2](68 KB)

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