What is "phototaxis"

Phototaxis \Pho`to*tax"is\, Phototaxy \Pho"to*tax`y\, n. [NL. phototaxis; photo- + Gr. ? an arranging.] (Biol.) The influence of light on the movements of low organisms, as various infusorians, the zo["o]spores of certain alg[ae], etc.; also, the tendency to follow definite directions of motion or assume definite positions under such influence. If the migration is toward the source of light, it is termed

positive phototaxis; if away from the light,

negative phototaxis. -- Pho`to*tac"tic, a. -- Pho`to*tac"tic*al*ly, adv.

n. (context biology English) The movement of an organism either towards or away from a source of light

Two types of positive phototaxis are observed in prokaryotes. 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 many organisms such as Serratia marcescens, Tetrahymena, and Euglena. Each organism has its own specific biological cause for a phototactic response, many of which are incidental and serve no end purpose.