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.
Background
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 Materials Concentrated motile algae (e.g., Euglena) Large test tube Black construction paper Scissors Clear tape Procedure - Cut narrow slits in the black construction paper.
- Tape the paper to the outside of the test tube.
- Pour a concentrated culture of algae into the test tube.
- Allow the tube to remain in normal classroom light undisturbed for 10–15 minutes.
- 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.
Optional: 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 Materials Square petri dish Black paint Volvox culture Stereomicroscope Two microscope point lights or penlights Procedure - Paint the outside of a square petri dish black except for a horizontal slit on each of 2 opposing sides.
- Place the painted dish on the stereomicroscope's stage, and set up 2 microscope point lights (or penlights) for illumination.
- Dim the room lights and add Volvox and medium to the dish until it is about a quarter full.
- 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.
- 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 
