Аннотация:Free-living and symbiotic diazotrophic cyanobacteria are among the key nitrogen-fixing organisms of boreal forests. Here, we report on the ultrastructural and taxonomic diversity of cyanobacteria inhabiting the pleurocarpous moss Hylocomium splendens based on the natural samples. The diversity of the morphology and ultrastructural organization of the cyanobacterial cells, trichomes, and microcolonies detected at the moss surface was indicative of the cyanobacterial taxonomic diversity which was confirmed with 16SrRNA gene metabarcoding. The peculiar features of the cyanobacterial cells forming the spherical juvenile thalli on the moss shoots included the non-parallel arrangement of their cell division planes, cell wall-reduced forms of vegetative cells, and the assembly of microtubular complexes in the septa formation zone of both daughter cells. The cyanobacterial cells harbored abundant inclusions rich in phosphorus and various metals. For the first time, the DNA metabarcoding of the gene 16SrRNA was used to characterize the biodiversity of moss cyanobionts in situ. According to the phylogenetic analysis of the obtained ASVs obtained, the moss cyanobionts included representatives of the families Nostocaceae, Leptolyngbyaceae, Stigonemataceae, and Tolypotrichaceae, dominated by the former. Six taxonomic variants of Nostoc sp. were detected. The composition and abundance of the variants of Nostoc and other less represented cyanobacteria differed between the years 2022 and 2023, likely due to weather fluctuations. Our study extends the current knowledge of the associations of mosses and cyanobacteria, that represent a convenient model for studies of plant-cyanobacterial symbioses and a suitable bioindicator for monitoring the condition of the environment.
