Photophobotaxis of single celled and filamentous cyanobacteria

Phototaxis refers to an organism’s movement toward a light source, while photophobotaxis involves movement into illuminated regions. Although phototaxis in cyanobacteria has been widely studied, photophobotaxis has been investigated in only a few species. In this study, we examined 18 cyanobacterial species and found that all single-celled and filamentous species exhibiting twitching motility performed photophobotaxis. In contrast, immobile Nostocales and a pilus-free Synechocystis did not display this behavior. The widespread occurrence of photophobotaxis aligns with the universality of photosystems, suggesting photosystems act as light sensors for this phenomenon. The photosystem II inhibitor DCMU disrupted photophobotaxis in single-celled cyanobacteria at a concentration of 10 µM, while filamentous species required ≥100 µM DCMU for inhibition. Despite the higher required concentration in filaments, the inhibition appears specific to photosystem II electron transport. This increased threshold may reflect heightened sensitivity due to collective sensing by multiple cells in the filament. Previous studies on spectral sensitivity and the cyanobacteriochrome PixJ in Phormidium lacuna identified PixJ as a negative regulator of photophobotaxis. In pixJ mutants, light sensitivity was increased compared to the wild type. Dual-wavelength experiments confirmed that yellow light induces PixJ to downregulate photophobotaxis. A paradox finding emerged in long-distance movements between two light sources: filaments moved faster than predicted by a random movement model, suggesting inter-filament communication. These results provide new insights into the mechanisms and regulation of photophobotaxis in cyanobacteria.