New trends in photobiology: Photosensitized reactions as primary molecular events in photomovements of microorganisms

Photomotile responses in microorganisms are aimed at escaping harmful light intensities and obtaining the best illumination conditions for growth and metabolism. In some colored microorganisms, molecules such as nucleic acids and proteins can be damaged not only by UV light, like in colorless cells, but also by near-UV and visible light absorbed by endogenous photosensitizers, which generate singlet oxygen and/or other noxious oxygen species. These photodynamic reactions can, in principle, initiate and/or regulate the transduction chain for photomotile responses, as in the case of Anabaena, which shows negative or positive phototaxis depending on the internal concentration of photodynamically produced singlet oxygen. This is not always true however, as shown by the case of the ciliates, Blepharisma and Stentor, whose photomotile responses, even though triggered by pigments which also act as endogenous photosensitizers, are not mediated by photodynamic reactions.