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‘Candidatus Aquirestis calciphila’ and ‘Candidatus Haliscomenobacter calcifugiens’, filamentous, planktonic bacteria inhabiting natural lakes

Institute for Limnology, Austrian Academy of Sciences, Mondseestrasse 9, 5310 Mondsee, Austria

Correspondence
Martin W. Hahn
martin.hahn{at}oeaw.ac.at

Filamentous bacteria frequently occurring in the pelagic zone of natural freshwater lakes and ponds were previously identified as being related to Haliscomenobacter hydrossis based upon their 16S rRNA gene sequences. These bacteria exhibit a specific morphology characterized by the formation of straight, stick-like filaments of variable length (5 to >100 µm) and quite stable, but narrow, width (0.25 to 0.35 µm). Bacteria with these morphological characteristics form a monophyletic but broad phylogenetic group with a maximal divergence of 16S rRNA gene sequences of 12.0 %. This monophyletic group consists of at least three monophyletic subclusters. H. hydrossis is affiliated to one of these subclusters and represents the sole recognized species affiliated to the broad monophyletic group. ‘Candidatus Haliscomenobacter calcifugiens' and ‘Candidatus Aquirestis calciphila’ are uncultured representatives of the other two subclusters and have 16S rRNA gene sequence dissimilarities of 5.4 % and 8.2 %, respectively, with the type strain of H. hydrossis. ‘Candidatus H. calcifugiens' and ‘Candidatus A. calciphila’ have a 16S rRNA gene sequence dissimilarity of 8.5 %. These large ribosomal divergences justify the classification of these environmentally important bacteria as a novel species and a new genus, respectively. Intensive attempts to cultivate these filamentous bacteria have resulted in the establishment of mixed cultures, however, attempts to establish pure cultures have failed.

The GenBank/EMBL/DDBJ accession number for the 16S rRNA gene sequences of ‘Candidatus Aquirestis calciphila’ and ‘Candidatus Haliscomenobacter calcifugiens' are AJ786341 and AJ786327, respectively.