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the motor reaction of motile microorganisms in response to the stimulus of light; a form of taxis. The reaction of zoospores to light, and the slow shift of chloroplasts within a cell in response to light, are also called phototaxis.
There are two main types of phototaxis, classified according to the type of movement: topotaxis (tropism) and phobotaxis. In topotaxis, cells move either toward the source of light (positive topotaxis) or away from it (negative topotaxis). In phobotaxis the moving cell reverses its direction at the boundary of areas with a different illumination in reactions known as the shock reaction and the alarm reaction. Positive phobotaxis inhibits movement into a darker area, which leads to a congregating of cells moving without any order in a spot of light, a phenomenon known as the light-trap effect. Negative phobotaxis results in the congregating of cells in darker areas.
The reaction in both types of phototaxis depends on the intensity of light: positive phototaxis is typical when the intensity is low, and negative phototaxis is typical when it is very high. Consequently, phototaxis assures optimal conditions of illumination for photosynthesis and for the vital activity of cells, and may be considered an important adaptive reaction of microorganisms.
The mechanism of phototaxis involves three principal stages: the absorption of light and the initial reaction in the photoreceptor, the conversion of the stimulus and the transmission of an ensuing signal to the motor apparatus, and change in the movement of the flagella. Phototaxis may be specialized or nonspecialized, depending on the mechanism of the reaction. In nonspecialized phototaxis, which is typical of photosynthesizing bacteria and a number of algae, the photosynthetic mechanism within the chloroplasts and chromatophores functions as the photoreceptor. The appearance of the signal results from a change that occurs in the rate of the primary processes of photosynthesis (electron flow and photophosphorylation) when there is a change in the intensity of light owing to the organism’s movement.
Specialized phototaxis takes place by means of specialized structures. In the Euglena, such a structure consists of a paraflagellar body attached to the flagellum, and a colored stigma situated on the organism’s side. When in movement, whether in the dark or in light, the cell rotates around its longitudinal axis. Consequently, when the side of the organism is illuminated, the stigma periodically darkens the paraflagellar body, which, it is assumed, functions as the photoreceptor. This process results in the generation of the signal that causes a change in the direction of movement. The mechanism causing the generation of the signal in the photoreceptor apparently results from the generation of an electric potential. The stimulus persists until the cell turns in a direction parallel to that of the light flow and reaches a position in which the photoreceptor does not become dark. The specialized structure, which is a few microns in size, directs the cell toward or away from the light source with great precision and is an example of an automatically regulated biological microsystem. Specialized phototaxis is also manifested as topotaxis, phobotaxis, and the stop reaction.
The term “phototaxis” sometimes refers to certain reactions of multicellular animal organisms to light, but these are complex reactions that are mediated by a nervous system and that belong more properly to the field of behavioral physiology. The nature of phototaxis, a fundamental process intermediate between photosynthesis and vision, is in many respects still unclear, but it is evident that phototaxis belongs to a new and promising field which combines elements of biophysics, molecular biology, bionics, mechanochemistry, and cell physiology.
REFERENCESSineshchekov, O. A., and F. F. Litvin. “Fototaksis mikroorganizmov, ego mekhanizm i sviaz’ s fotosintezom.” Uspekhi sovremennoi biologii, 1974, vol. 78, issue 1 (4).
Feinleib, M. E., and G. M. Curry. “The Nature of the Photoreceptor in Phototaxis.” In Handbook of Sensory Physiology. Berlin-Heidelberg-New York, 1971.
Diehn, B. “Phototaxis and Sensory Transduction in Euglena.” Science, 1973, vol. 181, no. 4104.
Nultsch, W., and D. P. Häder. Über die Rolle der beiden Photosysteme in der Photosysteme in der Photo-phobotaxis von Phormidium uncinatum. Berlin, 1974.
F. F. LITVIN