Photodynamically induced behavioural responses of the colourless flagellatePolytomella magna: Effects of methylene blue

Abstract

The colourless, stigma-free flagellatePolytomella magna which does not show natural responses to light becomes sensitive to light in the presence of photodynamically active dyes, such as riboflavin, methylene blue, toluylene blue, thionine, toluidine blue, neutral red and rose bengal, provided that oxygen is present. This reaction resulting in a light avoiding response depends neither on the absorption characteristics nor on the redox properties of the dyes. Detailed studies with methylene blue have shown that this response depends on the photon fluence rate and the dye concentration as well. The action spectrum of the photodispersal calculated on the basis of photon fluence rate-response curves is essentially identical with the absorption spectrum of methylene blue,i.e. no responses occurred below 530 and above 700 nm. Pre-irradiation experiments have shown that methylene blue is photobleached and, hence, inactivated in the presence but not in the absence of oxygen. Moreover, it has been demonstrated that the methylene blue mediated responses are chemotactic rather than photophobic ones. These experiments confirm the results of earlier investigations with riboflavin.