Abstract
Cyanobacterial expansion is harmful to the environment, the ecology of Lake Baikal and the economy of nearby regions and can be dangerous to people and animals. Since 2011, the process of colonisation of the lake with potentially toxic cyanobacteria belonging to the genus Tychonema has continued. An understanding of the mechanism of successful expansion of Tychonema requires scrutiny of biological and genomic features. Tychonema sp. BBK16 was isolated from the coastal zone of Lake Baikal. The morphology of BBK16 biofilm was studied with light, scanning electron and confocal microscopy. The biofilm is based on filaments of cyanobacteria, which are intertwined like felt; there are also dense fascicles of rope-like twisted filaments that impart heterogeneity to the surface of the biofilm. Genome sequencing, intergenomic comparisons and phylogenetic analyses indicated that Tychonema sp. BBK16 represent a new species related to planktic cyanobacterium Tychonema bourrellyi, isolated from Alpine lentic freshwater. Genome investigation revealed the genes possibly responsible for the mixotrophic lifestyle. The presence of CRISPR-Cas and restriction modification defence mechanisms allowed to suggest the existence of phages infecting Tychonema sp. BBK16. Analysis of CRISPR spacers and prophage-derived regions allowed to suggest related cyanophages. Genomic analysis supported the assumption that mobile elements and horizontal transfer participate in shaping the Tychonema sp. BBK16 genome. The findings of the current research suggest that the aptitude of Tychonema sp. BBK16 for biofilm formation and, possibly, its mixotrophic lifestyle provide adaptation advantages that lead to the successful expansion of this cyanobacterium in the Baikal's conditions of freshwater lake environments.