Bioassay Investigations with Daphnia

Developed by Kathy CahillWantagh High School

Wantagh, New York

Inquiry Process Skills

• Hypothesizing

• Preparing solutions

• Designing and conducting experiments

• Recording experimental data

• Analyzing experimental data

• Drawing conclusions

• Considering further research

Environmental Science Concepts

Use bioassay to develop students’

understanding of:

– The effect of toxicant concentration on organisms

– The use of model organisms as indicators of potential health hazards for humans

Chemical Risk?

What is the chance that human health problems will result from exposure to a particular toxicant?

• Is it safe to eat fruits or vegetables containing traces of pesticide residues?

• Should fluoride be added to drinking water? • Is diet soda safe to drink?

Toxicant

• A chemical or mixture of chemicals that is harmful to living organisms.

• It’s the dose (concentration) that makes the poison.

Identifying Risks

To identify possible risks from exposure to a particular toxicant, scientists use simple plants or animals, rather than humans, as test subjects.

Bioassay

A procedure that uses living organisms to determine the toxicity of a chemical.

1. Expose living organisms to different concentrations of a potential toxicant

2. Observe the effects on the organisms’ behavior and survival

3. Determine if, or at what concentration, a chemical has harmful effects

Model Organisms

• Why use model organisms?

• What other kinds of organisms might be used for bioassays?

Dose-Response Curve

Dose (mg/kg body weight)

0

25

50

75

100

0 10 20 30 40 50 60 70 80 90 100

No Effect

All Affected

Half Affected

% R

es

po

ns

e

LD50

Lethal Dose 50 is the concentration of a substance which

causes the death of 50% (one half) of a group of test organisms.

Dose (mg/kg body weight)

0

25

50

75

100

0 10 20 30 40 50 60 70 80 90 100

No Effect

All Affected

Half Affected

% R

es

po

ns

e

Why Use LD50

Toxicant LD50 (mg/kg)Ethyl alcohol 10,000Salt (sodium chloride) 4,000Iron (Ferrous sulfate) 1,500Morphine 900Mothballs (paradichlorobenzene) 500Aspirin 250DDT 250Cyanide 10Nicotine 1Black Widow Spider venom 0.55Rattle Snake venom 0.24Tetrodotoxin (from fish) 0.01Dioxin (TCDD) 0.001Botulinum Toxin 0.00001

Bioassay Investigations with Daphnia

Part 1

Introduction to Bioassays

Part 2

Investigating the Effects of a Toxicant on Daphnia

Inquiry

Version

Guided

Version

Timeline

Part 1: Introduction to Bioassays

• Two 40 minute class periods– Introduce – Do procedure and make observations (absolute

minimum of 40 minutes and may need to adjust observation time)

– Analyze data and answer questions

Introduction to Bioassays

Materials for each pair of “students”

• Jar containing living Daphnia

• Plastic exposure chamber (or 6 test tubes or small cups)

• One large plastic pipette

• Two small plastic pipettes (note 1 mL marking near the top)

• 10 mL Graduated cylinder

• 6 small beakers or cups for mixing solutions

• Spring water (0% salt)

• Salt solutions (0.2 – 2%)

Introduction to Bioassays

Complete through step 4.

Do not add the solutions to the exposure chambers until instructed to do so.

Introduction to Bioassays

Complete steps 5 and 6

Your teacher will notify you when to make the 5, 10, and 20 minute observations.

Introduction to Bioassays

Work with your partner to complete the:

– Data table– Graph– Data analysis and Interpretation

questions

Investigating the Effects of Toxicants on Daphnia

Two versions:

Guided Version or Inquiry Version

Select ONE of these

Guided Version

Investigating the Effects of Toxicants on Daphnia

Similar to the first experiment but students will:– Investigate other toxicants

– Construct their own data table and graph

– Discuss application to humans

– Share and draw conclusions based on information from other groups

Guided Version Timeline

• Two 40 minute class periods

– Introduce

– Do procedure and make observations (absolute minimum of 40 minutes and may need to adjust observation time)

– Analyze data and answer questions

Inquiry Version

Students apply their understandings from the introductory laboratory activity by:

• Designing an experiment to investigate the effects of other toxicants.

• Constructing their own data table and graph

• Drawing conclusions and discuss application their applications to humans

• Optional: Sharing and drawing additional conclusions based on information from other groups

Inquiry Version Timeline

• One period for designing procedure

• At least one period for conducting experiment

• One period for analyzing group data

• Optional additional period for class sharing and discussion of results.

Work with three others

• Select a toxicant

• Design an experiment to investigate the effect of the toxicant on Daphnia

• Complete the exposure chamber graphic and the experiment planning sheet for your proposed experiment.

Record your work on the transparencies so that you are prepared to share your plan with others.

Extensions/Modifications

Talk with others at your table and make a list of ways that these experiments could be extended or modified.

Record your work on the transparency to that you are prepared to share your plan with others.