Coaggregation Occurs between a Piliated Unicellular Cyanobacterium, , and a Filamentous Bacterium, .

Cyanobacteria are widely distributed in natural environments including geothermal areas. A unicellular cyanobacterium, , in a deeply branching lineage, develops thick microbial mats with other bacteria, such as filamentous anoxygenic photosynthetic bacteria in the genus , in slightly alkaline hot-spring water at ~55 °C. However, strains do not form cell aggregates under axenic conditions, and the cells are dispersed well in the culture. In this study, sp. NK55a and NBF, isolated from Nakabusa Hot Springs (Nagano, Japan), were mixed in an inorganic medium and incubated at 50 °C under incandescent light. Small cell aggregates were detected after 4 h incubation, the size of cell aggregates increased, and densely packed cell aggregates (100-200 µm in diameter) developed. Scanning electron microscopy analysis of cell aggregates found that filaments were connected with sp. cells via pili-like fibers. Co-cultivation of with a pili-less mutant of sp. did not form tight cell aggregates. Cell aggregate formation was observed under illumination with 740 nm LED, which was utilized only by . These results suggested that filaments gather together via gliding motility, and piliated cyanobacterial cells cross-link filamentous cells to form densely packed cell aggregates.