Phototaxis in a dinoflagellate: Action spectra as evidence for a two-pigment system

Action spectra were determined in the UV region of the spectrum for the first phase of the phototactic response (stop response) and for the phytochrome pigment associated with this response in the dinoflagellate Gyrodinium dorsum Kofoid. Differences between these action spectra indicate the participation of two pigments in phototaxis. Following R (620 nm) irradiation of the phytochrome, the stop response maxima occur at 470 and 280-nm; after FR irradiation they shift to 490 and 300-310 nm. These maxima suggest that the photoreceptor pigment for phototaxis is a carotenoprotein. The action spectrum shift following the different phytochrome conversions may represent a trans to cis isomer change by the carotenoid. The absorption maximum of PR in the UV appears to be at 320 nm, which is consistent with the shift of the R absorption maximum to shorter wavelengths (620 nm) as compared to higher plants. The PFR absorption maximum appears as a broad band between 360 and 390 nm. Comparison of PR to PFR conversions by different intensities of 620-nm and 320-nm light indicates that at lower intensities the logarithm of the threshold for the stop response is inversely proportional to the logarithm of the intensity of the sensitizing light. The ratio of response activation by R and UV light is about 4:1.