Phototaxis

Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves towards or away from a stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis. Phototaxis is called positive if the movement is in the direction of increasing light intensity and negative if the direction is opposite. Phototaxis has been described in microorganisms and algea, insects and other invertebrates, and vertebrates. Typically nocturnal insects can show positive phototaxis, while nocturnal mammals often show negative phototaxis.

Phototaxis in bacteria and archea

Phototaxis can be advantageous for phototrophic bacteria as they can orient themselves most efficiently to receive light for photosynthesis. Phototaxis is called positive if the movement is in the direction of increasing light intensity and negative if the direction is opposite. Two types of positive phototaxis are observed in prokaryotes (bacteria and archea). The first is called "scotophobotaxis" (from the word "scotophobia"), which is observed only under a microscope. This occurs when a bacterium swims by chance out of the area illuminated by the microscope. Entering darkness signals the cell to reverse flagella rotation direction and reenter the light. The second type of phototaxis is true phototaxis, which is a directed movement up a gradient to an increasing amount of light. This is analogous to positive chemotaxis except that the attractant is light rather than a chemical. Phototactic responses are observed in a number of bacteria and archae, such as Serratia marcescens. Photoreceptor proteins are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are bacteriorhodopsin and bacteriophytochromes in some bacteria. See also: phytochrome and phototropism. Most prokaryotes (bacteria and archaea) are unable to sense the direction of light, because at such a small scale it is very difficult to make a detector that can distinguish a single light direction. Still, prokaryotes can measure light intensity and move in a light-intensity gradient. Some gliding filamentous prokaryotes can even sense light direction and make directed turns, but their phototactic movement is very slow. Some bacteria and archaea are phototactic. In most cases the mechanism of phototaxis is a biased random walk, analogous to bacterial chemotaxis. Halophilic archaea, such as Halobacterium salinarum, use sensory rhodopsins (SRs) for phototaxis. Rhodopsins are 7 transmembrane proteins that bind retinal as a chromophore. Light triggers the isomerization of retinal, which leads to phototransductory signalling via a two-component phosphotransfer relay system. Halobacterium salinarum has two SRs, SRI and SRII, which signal via the transducer proteins Htr1 and Htr2 (halobacterial transducers for SRs I and II), respectively. The downstream signalling in phototactic archaebacteria involves CheA, a histidine kinase, which phosphorylates the response regulator, CheY. Phosphorylated CheY induces swimming reversals. The two SRs in Halobacterium have different functions. SRI acts as an attractant receptor for orange light and, through a two-photon reaction, a repellent receptor for near-UV light, while SRII is a repellent receptor for blue light. Depending on which receptor is expressed, if a cell swims up or down a steep light gradient, the probability of flagellar switch will be low. If light intensity is constant or changes in the wrong direction, a switch in the direction of flagellar rotation will reorient the cell in a new, random direction. As the length of the tracks is longer when the cell follows a light gradient, cells will eventually get closer to or further away from the light source. This strategy does not allow orientation along the light vector and only works if a steep light gradient is present (i.e. not in open water). Some cyanobacteria (e.g. Anabaena, Synechocystis) can slowly orient along a light vector. This orientation occurs in filaments or colonies, but only on surfaces and not in suspension. The filamentous cyanobacterium Synechocystis is capable of both positive and negative two-dimensional phototactic orientation. The positive response is probably mediated by a bacteriophytochrome photoreceptor, TaxD1. This protein has two chromophore-binding GAF domains, which bind biliverdin chromophore, and a C-terminal domain typical for bacterial taxis receptors (MCP signal domain). TaxD1 also has two N-terminal transmembrane segments that anchor the protein to the membrane. The photoreceptor and signalling domains are cytoplasmic and signal via a CheA/CheY-type signal transduction system to regulate motility by type IV pili. TaxD1 is localized at the poles of the rod-shaped cells of Synechococcus elongatus, similarly to MCP containing chemosensory receptors in bacteria and archaea. How the steering of the filaments is achieved is not known. The slow steering of these cyanobacterial filaments is the only light-direction sensing behaviour prokaryotes could evolve owing to the difficulty in detecting light direction at this small scale.

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