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 photophobotaxis of 7 single-celled and 11 filamentous cyanobacterial species, among them 3 Nostocales (filaments with heterocysts) and 5 Oscillatoriales and 1 Desertifiliales member. All single-celled species and all Oscillatoriales/Desertifiliales exhibited photophobotaxis, whereas no evidence of photophobotaxis was found for the Nostocales and two other species. A pilus-free mutant of Synechocystis sp. PCC 6803 did not display this behavior. The photosystem II inhibitor DCMU disrupted photophobotaxis in single-celled and filamentous cyanobacteria at a concentration of 10 μM; only the filamentous Phormidium lacuna (P. lacuna) required 100 μM DCMU for inhibition. This points to PS II as a sensor of photophobotaxis. The widespread occurrence of photophobotaxis aligns with the universality of photosystems. Previous studies on spectral sensitivity and the cyanobacteriochrome PixJ in P. lacuna identified PixJ as a negative regulator of photophobotaxis. In pixJ mutants, light sensitivity was increased compared with the wild-type. Dual-wavelength experiments confirmed that yellow light induces PixJ to downregulate photophobotaxis. Our experiments also show that P. lacuna moves faster in darkness than in light and that a temporal change of light intensity from light to dark can induce a change of movement direction. Both findings support the light trap model which is based on random movement and a change of movement direction at the light-dark border.