Abstract
Endemic freshwater sponges (demosponges, Lubomirskiidae) dominate in Lake Baikal, Central Siberia, Russia. These sponges are multicellular filter-feeding animals that represent a complex consortium of many species of eukaryotes and prokaryotes. In recent years, mass disease and death of Lubomirskia baicalensis has been a significant problem in Lake Baikal. The etiology and ecology of these events remain unknown. Bacteria from the families Flavobacteriaceae and Oxalobacteraceae dominate the microbiomes of diseased sponges. Both species are opportunistic pathogens common in freshwater ecosystems. The aim of our study was to analyze the genomes of strains Janthinobacterium sp. SLB01 and Flavobacterium sp. SLB02, isolated from diseased sponges to identify the reasons for their joint dominance. Janthinobacterium sp. SLB01 attacks other cells using a type VI secretion system and suppresses gram-positive bacteria with violacein, and regulates its own activity via quorum sensing. It produces floc and strong biofilm by exopolysaccharide biosynthesis and PEP-CTERM/XrtA protein expression. Flavobacterium sp. SLB02 utilizes the fragments of cell walls produced by polysaccharides. These two strains have a marked difference in carbohydrate acquisition. We described a possible means of joint occupation of the ecological niche in the freshwater sponge microbial community. This study expands the understanding of the symbiotic relationship of microorganisms with freshwater Baikal sponges.