Equal-quantum action spectra indicate fluence-rate-selective action of multiple photoreceptors for photomovement of the thermophilic cyanobacterium Synechococcus elongatus

Unicellular thermophilic cyanobacterium Synechococcus elongatus displayed p hototaxis on agar plate at 55 degreesC. Equal-quantum action spectra for ph ototactic migration were determined at various fluence rates using the Okaz aki Large Spectrograph as the light source. The shapes of the action spectr a drastically changed depending on the fluence rate of the unilateral monoc hromatic irradiation: at a low fluence rate (3 mu mol/m(2)/s), only lights in the red region had significant effect; at a medium fluence rate (10 mu m ol/m(2)/s), four major action peaks were observed at 530 nm (green), 570 nm (yellow), 640 nm (red) and 680 nm (red), At high fluence rates (30-90 mu m ol/m(2)/s), the former two peaks remained, while red peaks at 640 nm and 68 0 nm disappeared and, interestingly, an action peak around 700-740 nm (far- red) newly appeared. These results indicate that two or more distinct photo receptors are involved in the phototaxis and that suitable photoreceptors a re selectively active in response to the stimulus of light fluence rates. F ar-red or red background lights irradiated vertically from above drasticall y inhibited phototaxis toward red light or far-red light, respectively. The se results indicate involvement of some phytochrome(s).