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Demonstrating Algal Phototaxis in Your Classroom

Cells or unicellular organisms that move in response to light exhibit phototaxis. Phototaxis is easily demonstrated using Euglena, Chlamydomonas, or Volvox. The following 2 demonstrations can be done in your classroom with inexpensive materials you probably already have on hand. If you’re pressed for time, check out our Euglena Phototaxis Kit (shown right). It’s got all the materials you need to demonstrate algal phototaxis—plus the Carolina™ Protozoa and Invertebrates Manual.


Because light is essential for photosynthesis, many algae have developed phototactic responses. An algal cell can have either a positive phototactic response (it moves toward light) or a negative phototactic response (it moves away from light). Many algae respond negatively under bright light and positively under dim light. Interestingly, Euglena displays a positive phototactic response in daylight, but at night, even when light is introduced, it is unresponsive.

Demonstration 1


  • Concentrated motile algae (e.g., Euglena)
  • Large test tube
  • Black construction paper
  • Scissors
  • Clear tape
  • Procedure

    1. Cut narrow slits in the black construction paper.
    2. Tape the paper to the outside of the test tube.
    3. Pour a concentrated culture of algae into the test tube.
    4. Allow the tube to remain in normal classroom light undisturbed for 10–15 minutes.
    5. Remove the paper without jarring or moving the tube, and observe that the algae are clustered where light filtered through the slits in the paper.


    To establish at what light intensity algae react negatively and to what wavelength they respond, perform the demonstration using several different light intensities and wavelengths (colors).

    Demonstration 2


  • Square petri dish
  • Black paint
  • Volvox culture
  • Stereomicroscope
  • Two microscope point lights or penlights
  • Procedure

    1. Paint the outside of a square petri dish black except for a horizontal slit on each of 2 opposing sides.
    2. Place the painted dish on the stereomicroscope's stage, and set up 2 microscope point lights (or penlights) for illumination.
    3. Dim the room lights and add Volvox and medium to the dish until it is about a quarter full.
    4. Move the point lights so that one is focused on one slit and the other on the opposing slit. Keep one light close to the dish and the other several centimeters away to set up a light gradient, i.e., one side of the dish at high light intensity and the other at low.
    5. Using the microscope, study the movement of Volvox and its response to the lights. Intersect the light paths, vary the distance between the lights, and note any changes in the algae's response.

    Additional resources

    Further your students’ knowledge of Euglena and other protozoans with the following kits, cultures, and equipment.

    Protozoa Survey Set

    Protozoa Demoslide Set

    Carolina™ Protozoa Slide Sets (Euglena)

    Euglena BioKit®

    Euglena Phototaxis Kit

    Carolina™ Minipond Ecosystem Kit

    Protist Observation System

    Carolina™ Deep-Well Slides

    Demoslide Stage Adapter

    Carolina Biological Supply

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