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Mussel Your Way Through Photosynthesis

By Bob Culler

Type of Activity:

  • Intro hands-on lab activity

Target Audience:

  • Biology
  • Life Science 9 & 10
  • Environmental Studies


Using zebra mussels (Dreissena polymorpha), elodea and an indicator dye, it is possible to study the role of light in photosynthesis. A segment of the carbon cycle will be studied by setting up a closed system between plants and animals. You will test for the presence of carbon dioxide using an indicator dye. Bromthymol blue is an indicator dye. It is yellow in an acid solution, green at its transition point of pH 6.0 to 7.6, and blue in an alkaline solution. Carbon dioxide (CO2) forms a weak acid when dissolved in water. Here the indicator will be used to indicate, indirectly, the presence of CO2.

Background Information:

In light, a green plant appears to play the role in exchange of CO2 and O2 opposite to that of animals.But respiration goes on continuously in both green plants and animals. In light, green plants immediately reuse (in photosynthesis) the CO2 released in respiration, so it does not accumulate in the environment. In darkness, however, bromthymol blue changes from blue to green in a culture tube containing elodea because CO2 from respiration is accumulating in the water, acidifying it. When, on exposure to light, photosynthesis begins again, CO2 is used more rapidly than it is released, and the plant starts to extract CO2 from the water. As the amount of CO2 in the water decreases, the indicator changes back to blue.

Teaching Hints:

Start lab on Wednesday, switch on Thursday, then switch back on Friday, leave for weekend, observe and record Monday. I use this activity to introduce the pH, without using pH paper or a meter. The following 10-0-10 scale of green to blue color can be used to indicate, indirectly, the amount of CO2 produced. Lab groups must collectively decide what happened each day and what should occur as tubes are switched from light to dark.

Observation for day:
Tube #:
Blue 10 9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10 Green
Explain why the indicator was this color?
Unexplained problems?
Hypothesis for day:
Blue 10 9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10 Green

The lesson from this lab becomes apparent when students realize that plants produce small amounts of carbon dioxide in the dark. From their past studies, they believe that plants only produce oxygen in the light. This helps to explain the relationship of energy and matter to an organism and how the interrelationships of organisms relate to the non-living world.

Another intriguing lesson involves the death of organisms. This often leads to a good discussion about the use of live materials in biological research.

Preparation: One hour

Class time: Four days, approximately 20 minutes each day

Acknowledgments and References:

Free information and a low-cost video "Zebra Mussels in the Great Lakes" can be obtained from Ohio Sea Grant College The Ohio State University, 1314 Kinnear Road, Columbus, OH 43212-1194. This lab activity was modified by Bob Culler from "Biological Science an Ecological Approach" Houghton Mifflin Co.



  • Florescent light
  • Cardboard box for a dark location
  • 8 screw cap culture tubes
  • 4 zebra mussels
  • 4 pieces of elodea
  • Bromthymol blue solution
  • Dechlorinated water (aquarium or pond water)
  • Labels and white card


  1. Prepare two sets of 4 culture tubes, label them A1 to A4 and B1 to B4.
  2. Pour dechlorinated water into each tube approximately 1cm to the top.
  3. Add 6 drops of bromthymol blue to each tube. Add nothing more to tube A1 and B1.
  4. To tubes A2 and B2, only add a zebra mussel.
  5. To tubes A3 and B3, only add elodea.
  6. To tubes A4 and B4, add both zebra mussels and elodea.
  7. Put caps on all tubes and seal tightly.
  8. Put A1 through A4 in the light; put B1 through B4 in the dark.
  9. After 24 hours, put a whit card behind each set of tubes, observe color change, recor dinformation.
  10. Reverse tubes, place "A" series in dark and "B" series in the light.
  11. Form a new hypothesis, record date.
  12. After 24 hours, repeat your observations. Record information, form new hypothesis.
  13. Switch the "A" series back to light and "B" series back to the dark.
  14. After several days, (weekend) observe the tubes again. Record information.
  15. Write lab report.


  1. Does the data support all of your daily hypotheses? Explain.
  2. Explain the color change in each tube, each day. Why did the indicator change color?
  3. Did any organisms die? Explain when and why.
  4. List the life cycle and background of zebra mussel in the Lake Erie ecosystem.
  5. What does the indicator show? Explain the 10-0-10 blue/green scale.
  6. Did the indicator change color in the tubes without elodea or zebra mussels?
  7. List the formulas for photosynthesis and respiration. Explain the role of light and the relationship between elodea and zebra mussel.
  8. If you were to repeat this lab, what would you change?
  9. List a few problems in this lab that you were not able to regulate or control.

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