Probing environmental DNA reveals circum-Baltic presence and diversity of chlorophyll a/b-containing filamentous cyanobacteria (genus Prochlorothrix)

Cyanobacteria that possess phycobilisomes, light-harvesting antenna, have been well studied. In contrast, more rare green cyanobacteria (four genera/five species) that instead make use of chlorophyll–protein complex are poorly studied. In particular, the genus Prochlorothrix is represented by a small environmental DNA database and reports of only two cultured species from Northern Europe. In this work, marine and freshwater habitats of Northwestern Russia were investigated. PCR with Prochlorothrix 16S rRNA gene specific primers, FISH analysis with a Prochlorothrix 16S rRNA-targeted probe, Prochlorothrix culture isolation, and phylogenetic analysis of Prochlorothrix diversity were carried out. We identified Prochlorothrix 16S rDNA in samples from the St. Petersburg region and corroborated this finding by FISH. Attempts to isolate PCR- and FISH-detected Prochlorothrix strains were unsuccessful. Phylogenetic analysis revealed that the Prochlorothrix 16S rDNA sequences identified were very similar and formed a single cluster with high bootstrap support. Some of these sequences represent environmental strains of the species Prochlorothrix hollandica and P. scandica, while the others belong to new Prochlorothrix species or even to a new Prochlorothrix-related genus. Our results suggest a broader distribution and greater diversity in Prochlorothrix than previously thought.