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Movement of a motile organism or free plant part in response to light stimulation.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



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.


Sineshchekov, 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.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Phototaxis and vertical migration of queen conch (Strombus gigas Linne) veliger larvae.
Phototaxis is a common phenomenon in many organisms including zoosporic fungi (chytrids) but appears undocumented for the fungal-like oomycetes.
However, the findings of LaRochelle & Dimock (1981) and the fact that the induction of mite negative phototaxis can occur in water modified by mussels clearly suggest that it can be mediated by distance chemoreception as well.
acuminata in relation to light that is high photoorthokinesis and positive phototaxis. In experiments where no light stimulus is given (control) there was no movement of snails towards the center of aquarium.
ChR2, an algal phototaxis receptor that uses light to depolarize the plasma membrane [32], acts as a light-gated cation channel when expressed in animal cells [12].
Species can be attracted to, or disoriented by, sources of artificial light through positive phototaxis (Verheijen 1985; Longcore and Rich 2004).
Microbial rhodopsins function as phototaxis receptors (sensory rhodopsin), light-driven proton or chloride ion transporters (bacteriorhodopsin and halorhodopsin) [2, 3, 5, 6, 8].
Chlamydomonas shows two behaviors in response to light: phototaxis and photophobic or avoidance response.
The neural wiring thus appears to be more complicated than if each behavior were driven by a single spectral type of receptor." When perceived by an insect, UVA radiation could trigger a wavelength-dependent response to light attraction similar to what has been measured in frogs: "If the tendency of most species of frogs to jump towards a light is measured as in a forced choice experiment, short wavelengths ([A.sub.max] 480 nm) stimulate positive phototaxis and longer wavelengths inhibit [phototaxis]." [Goldsmith 1994:303].
Temperature changes also produce measurable alterations in the directional responses to light (phototaxis) and gravity (geotaxis), and the activity of crustacean larvae in general [10].