Coaggregation Occurs between a Piliated Unicellular Cyanobacterium, Thermosynechococcus, and a Filamentous Bacterium, Chloroflexus aggregans

Cyanobacteria are widely distributed in natural environments including geothermal areas. A unicellular cyanobacterium, <i>Thermosynechococcus</i>, in a deeply branching lineage, develops thick microbial mats with other bacteria, such as filamentous anoxygenic photosynthetic bacteria in the genus <i>Chloroflexus</i>, in slightly alkaline hot-spring water at ~55 °C. However, <i>Thermosynechococcus</i> strains do not form cell aggregates under axenic conditions, and the cells are dispersed well in the culture. In this study, <i>Thermosynechococcus</i> sp. NK55a and <i>Chloroflexus aggregans</i> 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 <i>C. aggregans</i> filaments were connected with <i>Thermosynechococcus</i> sp. cells via pili-like fibers. Co-cultivation of <i>C. aggregans</i> with a pili-less mutant of <i>Thermosynechococcus</i> sp. did not form tight cell aggregates. Cell aggregate formation was observed under illumination with 740 nm LED, which was utilized only by <i>C. aggregans</i>. These results suggested that <i>Chloroflexus</i> filaments gather together via gliding motility, and piliated cyanobacterial cells cross-link filamentous cells to form densely packed cell aggregates.