Strains of the cyanobacterial genera Calothrix and Rivularia isolated from the Baltic Sea display cryptic diversity and are distantly related to Gloeotrichia and Tolypothrix.

Strains of the cyanobacterial genus Calothrix display pronounced tapering filaments. These cyanobacteria are benthic, have a worldwide distribution and are among the most easily recognizable cyanobacterial genera. However, it is not clear whether cyanobacterial strains assigned to the genus Calothrix constitute a natural monophyletic group. We sequenced 16S rRNA genes from 42 cyanobacterial cultures and environmental samples belonging to the genus Calothrix, and the morphologically similar genera Rivularia, Gloeotrichia and Tolypothrix. Phylogenetic analysis of the 16S rRNA gene identified large sequence diversity among the Calothrix morphotype strains. Our results demonstrate that Calothrix, Gloeotrichia and Tolypothrix do not form a monophyletic group but instead display a high level of genetic diversity. The evolutionary distances between cyanobacteria, morphologically identified as Calothrix, suggest that they belong to at least five different genera. Our results also suggest that the genus Gloeotrichia is distantly related to the genus Calothrix. We found correlations between genetic grouping and morphology in redundancy analysis. However, morphology alone was not sufficiently reliable to distinguish strains from different 16S rRNA gene clusters. The high level of diversity that we observed confirms the hypothesis that the Rivulariaceae are species rich.