phototaxis

Phototaxis is the directional movement of an organism in response to light. In phototaxis, cells or organisms orient themselves and swim toward or away from a light source, a behavior classified as taxis rather than growth or movement driven by another stimulus. Positive phototaxis describes movement toward light, while negative phototaxis describes movement away from light. The response helps organisms locate suitable light levels for photosynthesis or avoid damaging radiation.

Many unicellular protists exhibit phototaxis. Euglena and some algae possess light-detecting organelles called eyespots or photoreceptive systems that help steer their locomotion. Chlamydomonas, a unicellular green alga, uses a pair of eyespots and a light-sensitive signaling cascade to bias flagellar beating toward or away from light. Other free-living ciliates and various bacteria and microfauna also show phototactic behaviors under appropriate conditions.

Mechanistically, phototaxis relies on photoreceptors that sense light intensity or wavelength, coupled to signaling pathways that control the orientation and beating pattern of motile structures such as flagella or cilia. By integrating light cues with internal motor control, the organism adjusts its swimming trajectory to minimize or maximize light exposure. Some organisms can detect the direction of light and reorient accordingly, while others respond to overall light levels.

Phototaxis is studied for insights into sensory perception, signal transduction, and the control of locomotion. It is often contrasted with phototropism, a growth response to light in plants and other organisms. Related taxis responses include chemotaxis, gravitaxis, and thermotaxis, illustrating how motile life interprets and navigates its environment.