Phototaxis is a common phenomenon among algae and pigmented procaryotes. Some, like Euglena gracilis, have a prominent, carotenoid-containing stigma near the flagellar basis at the anterior cell pole. This seemed to suggest that the stigma is a photoreceptor, though two observations were contrary. In 1882 noticed T. W. ENGELMANN that the cells show a light-induced reaction even if not the stigma itself but only the flagellar basis is exposed to light. On the other hand exist mutants that lack the stigma and do still display phototactic behaviour. This means that the receptor is localized elsewhere. It was found in the parabasal body, a swelling at the flagellar basis. The stigma functions as a sun-shade, throwing a shadow on the parabasal body. Since the cells do permanently rotate around their own axis during movements, is the parabasal body periodically exposed to light or shaded from it. The cell is thus able to detect the direction of the light and to react to it.
The phototactic reaction is mainly induced by light of short wave lengths. Its action spectrum does not directly lead to the absorption spectrum of the photoreceptor, though, since part of the light is selectively absorbed by the stigma - it is also spoken of modulated light. In many other cases has the light to pass plastids with all their different pigments. This is, for example, true for Volvox species, where one colony consists of a large number of single cells able to move in a co-ordinated fashion. The chloroplasts of Volvox cells are characterized by a specific, bowl-like shape and a posterior position within the cell. As a consequence is light that enters the cell via the posterior pole always filtered. The direction of the light can thus be determined by each cell.
The decisive question is: how is a light signal transformed into a movement? Energy is required that can be supplied by photosynthesis. The driving unit or motor is the flagellum (if present; some algae glide or crawl). The clearing unit between incoming signal and energy supply that could also be called processor or effector is still a black box to us. In 1973 drew B. DIEHN (University of Toledo) a flow chart consisting of the smallest possible number of system elements necessary to explain the light-induced movements observed in Euglena gracilis.
The input of measured physiological data and of the radiant energy into a mathematical model proved that DIEHN’s model was a good working hypothesis. It was indeed possible to simulate a course of movements largely identical with the phototactic reaction of Euglena.
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de