Phototaxis

Phototaxis is a kind of taxis that occurs when a whole organism moves in response to the stimulus 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 light, and negative if the direction is opposite.

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 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.