Photoinhibition and its wavelength dependence in the cyanobacterium Anabaena variabilis

In the cyanobacterium Anabaena variabilis the dependence of photoinhibition on fluence rate, duration and wavelength of irradiation were studied by measurements of oxygen production and fluorescence emission spectra. The analysis of the photosynthetic activity revealed that photoinhibition affects exclusively photosystem II (PS II), whereas photosystem I (PS I) remained largely unimpaired. Furthermore, PS II fluorescence emission decreased much faster in bleached than in unbleached controls.Studying the wavelength dependence of photoinhibition it was found that only radiation between 520 and 680 nm causes photoinhibition. This is about the same range of wavelengths which causes photobleaching. Fluorescence emission spectra of samples exposed to high fluence rates of 582 and 662 nm, respectively, essentially agree with those samples exposed to high fluence rates of white light, whereas the fluorescence emission spectra of samples exposed to blue light resemble those exposed to dim white light.NaN₃, a substance which prevents photobleaching, inhibits the photosynthetic O₂ production of Anabaena and, hence, enhances the photoinhibitory effect.