How Many Plants Make a Future? The Carbon Dioxide Challenge
The story of Biosphere 2 is introduced to the students in the context of a discussion of the carbon cycle, climate change, and CO2 accumulation in the atmosphere. The lab activity focuses on the role of photosynthesis in a sustainable future. Students explore the effect of photosynthesis and respiration in one liter 'closed systems' containing plankton, marine plants, and fish. By calculating carbon dioxide uptake and production in these systems, students predict a plant: animal ratio sufficient to maintain a system in carbon dioxide 'balance' for one hour. Building and testing the system, then extending it to a longer time period, demonstrates to the students that carbon dioxide accumulation, even in a simple system for a short time period, is not a trivial matter. Students apply concepts from biology, solution chemistry, and exercise considerable quantitative reasoning from real data as they design and fine-tune their systems.
This activity focuses on the role of photosynthesis in a sustainable future. By calculating carbon dioxide uptake and production in these systems, they predict a plant: animal ratio sufficient to maintain a system in carbon dioxide 'balance' for one hour. Accounting for the effects of confounding variables in their systems also develops student understanding of the challenges of applying the scientific method to complex situations. This activity demonstrates to the students that carbon dioxide accumulation, even in a simple system for a short time period, is not a trivial matter.
Context for Use
During the course of the activity, the students learn about the chemistry of carbon dioxide dissolution in water, make chemical and technologically- based measurements of water parameters, and use these measurements to calculate CO2 uptake (by phytoplankton and marine plants)and production (by plankton, plants, and fish) in their closed systems. The mathematical skills employed are basic, but the quantitative reasoning required is quite sophisticated, and this simple model serves to demonstrate the complexity of this "real-world" problem.
A data-based prediction employs quantitative skills using the student's own data to design a closed system where the ratio of photosynthetic mass to respiring mass is sufficient to balance CO2 for a short time period. When this system has been constructed and tested by the students, a class discussion frequently reveals that the initial predictions of plant/animal mass have failed to balance CO2 in the system over a one hour period, providing an opportunity to discuss some of the confounding variables involved in the experimental set-up. An extension to the activity has students design a system that will remain balanced for 24 hours, incorporating a 12 hour dark period where respiration continues but photosynthesis ceases.
Note: This activity was designed as part of a marine science unit, but translates well into freshwater systems.
Description and Teaching Materials
Scaffolding Activities: Delivered as Lecture/Discussion
- Discuss global warming, and the buildup of excess atmospheric carbon dioxide as a primary contributing factor.
- Review the carbon cycle, stressing the roles of respiration and photosynthesis. Emphasize that the uptake of carbon dioxide by photosynthetic organisms is being increasingly reduced by deforestation, and potentially reduced by a decline in photosynthetic plankton if the oceans become more acidic. Pose the problem: how much plant mass is required to balance respiration in our closed global system? We sometimes show the video Acid Test (see resources) at this juncture.
- Point out that this is not a trivial problem, and that so far our best efforts to balance the uptake and production of carbon dioxide in a complex system have failed. Tell the story of Biosphere 2, an 'earth in a bottle' experiment that cost upwards of $200 million, and involved nine years of focused planning by NASA scientists and engineers. The 3.15 acre system choked on carbon dioxide after two years, following adverse affects to humans, other vertebrate animals, pollinating insects and trees. This dramatic story is beautifully illustrated on a number of websites (see resource section for recommendations). Inform the students that in the laboratory activity to follow, they will be challenged to balance carbon dioxide uptake and production in a 1 liter beaker, with one species each of plant and animal.
- Students need to know the following content:
- Equations for photosynthesis and respiration, noting that carbon dioxide is the product of respiration and a reactant in photosynthesis. They should be aware that all organisms respire continuously, whereas photosynthesis is limited to phytoplankton and plants in the presence of light.
- The solution chemistry of carbon dioxide in water, specifically the relationship between CO2 concentration and pH. At a minimum, students need to be aware that dissolving carbon dioxide in water results in formation of both bicarbonate and carbonate ions, thus dissolved carbon dioxide cannot be measured directly. However, dissolving CO2 gas in seawater also leads to increased concentration of hydrogen ions (H+) and thus increases acidity and decreases pH. Thus a decrease in pH is to be expected as CO2 is dissolved in water, and an increase as CO2 is consumed. Therefore, changes in pH indicate CO2 production and consumption.
Obtaining a quantitative estimate of dissolved CO2 requires knowing pH and also kH. kH or carbonate hardness (aka carbonate alkalinity) is a measure of presence of carbonate (CO32-) and bicarbonate (HCO3-) ions resulting from the dissolution of CO2 gas. It is usually expressed either as parts per million (ppm or mg/l), or in degrees KH (from the German "Karbonathärte"). kH is easily measured with an inexpensive carbonate hardness kit available from any store catering to marine aquaria.
Possession of a pH and kH measurement for a water sample allows calculation of the CO2concentration of that sample. The necessary formula is: CO2 (in PPM) = 3 * KH * 10( 7-pH ) Alternatively, students can input their pH and kH values into the online calculators contained in aquarists' websites (see resources for a suggestion).
As with other solutes, the solubility of CO2in water is related to temperature (we remind the students of the tendency of soda pop to go 'flat' at room temperature). Thus we attempt to retard temperature changes in the activity by keeping the experimental beakers submerged in a water bath. We check on the success of this by measuring temperature before and after. If the temperature has changed by more than 1 degree centigrade, the results are to be interpreted with caution: we inform the students that they will be required to research and reason out the possible effects on their data if a larger temperature change occurs.
Materials required per groupThree one liter beakersThermometerPlastic wrap/rubber bands to seal beakersSea water to fill beakers to one literDigital ScaleElectronic pH meterKH (carbonate hardness) testing kit (obtainable from aquarium stores)Aquarium or cooler sufficient to hold three 1 - liter beakers in water bathGrow - light (to place atop aquarium or cooler and stimulate photosynthesis)Marine plant material (up to enough to half-fill one liter beaker). Eelgrass suggested if available. Plant material should be in good condition, and without roots or non-photosynthetic parts.One marine fish (a tide pool fish small enough to survive one hour in a beaker is needed: we have used gunnels and sculpins)
Lab Directions to StudentsThis activity challenges you to design and build a simplified version of Biosphere 2 - in a 1-liter lab beaker. That is, you are to produce a balanced system in terms of photosynthesis and respiration over a one hour period. Your system will contain only one species of animal and one species of plant. As this is a marine system, your system will also contain plankton. Some plankton are photosynthetic, others not. At any rate, the presence of plankton will have an effect upon the system, in terms of either taking up or producing carbon dioxide. The net effect of plankton upon the carbon dioxide concentration will be apparent from the control beaker.
- Collect initial data re typical CO2 uptake and production for three closed marine systems (Figure1):
- Control (seawater including plankton)
- Photosynthesizing (marine plant matter in seawater)
- Respiring (marine organism in seawater)
- Use your data from Part 1 to make a prediction about the ratio of plant/animal matter likely to produce no increase in CO2 in one hour. Build and test your system.
- Set up three one liter beakers to represent three closed marine systems:
- Control (contains seawater and plankton only)
- Photosynthesizing system (seawater and a known mass of marine plant material)
- Respiring system (seawater and a known mass of a marine fish)
- Record masses of plant and animal material (Figure 2). Be as careful as possible that the fish does not become unnecessarily stressed (Figure 3) and that you choose plant material that is in good condition. Remove roots and browning parts of leaves. Enter species of plant/fish and their relative masses in Table 1.
- Place plants and fish in separate beakers, add seawater to 1 liter
- Set up third beaker as control (plankton)
- Let three systems 'settle' (Figure 1). After 5 minutes, record initial pH, kH, and temp . Enter parameters in Table 1
- Seal beakers (Figure 1).
- Place all three beakers under grow-light or in sunlight for 60 minutes
- Keep beakers in water (to retard temperature changes )in an aquarium/cooler for one hour. (Figure 4)
- Remove the grow-light. Record pH, kH, and temp. at 60 minutes. Think before doing this: how are you to take these measurements while minimizing the time that the beaker is not sealed? Enter parameters in Table 2
- Calculate carbon dioxide concentration for each system at t=0 and t=60 (Table 2)
- Calculate the change in carbon dioxide for each system over one hour. For each system, state if carbon dioxide was produced or consumed in the system. Enter changes in Table 2.
- For the photosynthesizing and respiring systems only, calculate mg/l carbon dioxide produced or consumed per gram of living matter in the system. Enter in Table 2.
Why did we wait 5 minutes before recording initial parameters?
Did the temperature change significantly (more than 1 degree Celsius)? If so, how will this affect the carbon dioxide concentration in the system?
Why is it important to keep the systems sealed?
Is the plankton only system a photosynthetic or respiring system, overall?
Why did we not calculate the carbon dioxide produced or consumed per gram of plankton? What would we need to do in order to do this?
What confounding variables were operating in this experiment, and what effects might these have had upon carbon dioxide uptake or production?
Part 2: Designing and Testing Your Balanced System.
- Use the data in Table 2 to predict the ratio of plant: animal matter you would need in a given system to balance carbon dioxide uptake and production by photosynthesis and respiration respectively. Use the following to guide your prediction:
Net CO2 consumed per gram plant matter: _____
Net CO2 produced per gram animal: _____
Net CO2 produced or consumed by plankton: ______
Consider how to compensate for the Net CO2 produced or consumed in the control system. Then calculate the ratio of plant/animal mass such that CO2 produced/consumed = 1: _____
- Check your calculations and reasoning with the lab instructor.
- Build your system using the ratio of plant to animal mass that you have predicted will result in a sustainable system for one hour. Adjust the mass of plant matter selected to the mass of fish selected. Place both plants and fish in the same beaker, and follow the same procedure as in part 1: allow the system to settle for 5 minutes, take initial parameters (Table 3), place in a water bath under a grow-light for 60 minutes, then take final parameters (Table 3) while disturbing the system as little as possible. Think before you act at this stage! Should you use the same fish or one which has not been tested previously? Do you need to use the same species of plant and fish? Why or why not?
- Calculate your net carbon dioxide production/uptake in the system, and enter in Table 3.
Explain your design. If you changed the source or species of plant and/or fish, justify this.
How well did you succeed in creating a balanced system? Explain any problems that might have contributed to the inability of the system to achieve a net carbon dioxide production of zero.
Report your findings to the remainder of the class. Use the class results to make a summary statement/report about the subtleties of creating a system that remains in carbon dioxide balance for one hour.
Part 3: Extension
Extend your system such that it is designed to balance over a 24hr period that includes a 12 hour dark period. Recall that in the absence of light, both plants and animals will be respiring (producing CO2), but no photosynthesis will be taking place.
Teaching Notes and Tips
Invertebrates, such as clams and oysters, can substitute for fish if only the mass of the soft tissue is considered. In this case, take the mass of the animal after the conclusion of the experiment and after removing the shells. Be aware that the respiration rate of invertebrates tends to be considerably lower than that of fish.
This activity serves as a good springboard for ideas for small scale student projects suited to elementary biology classes. Students may become interested in the effects of plankton photosynthesis and respiration at different times and in different places, how photosynthetic rates vary with plant species, and the factors affect respiration rates in fishes.
Assessment and FeedbackCompletion of tables, necessary calculations, and design of a 'sustainable' system from experimentally derived data provide a mechanism that ensures understanding of the biological and chemical processes involved, and indicates that the student's quantitative reasoning is sound.
As a summative assessment, we suggest having each group distribute their results to others, including observations and comments. The plant: animal mass ratios that proved reasonably successful may vary. Have students review that results of others, and produce a report or short presentation on their thinking about class results.
Finally, we encourage students to extend their thinking. What about the activity interested or perplexed them? If they were to do a follow-up independent project, what would they choose to investigate? What ideas about methodology do they have?
References and Resources
Carbon Dioxide: http://www.lenntech.com/carbon-dioxide.htm
Ocean Acidification: http://en.wikipedia.org/wiki/Ocean_acidification
"Acid Test: The Global Challenge of Ocean Acidification." National Resources Defense Council. 22 minute video available online at http://www.nrdc.org/oceans/acidification/aboutthefilm.asp
Wurts, W.A. and Durborow, R.M. 1992. "Interactions of pH, Carbon Dioxide, Alkalinity and Hardness in Fish Ponds." Southern Regional Agriculture Center. http://www.ca.uky.edu/wkrec/InteractionspHEtc.PDF
Website to help calculate the CO2 Concentration Based on pH and kH: http://www.aquascapingworld.com/magazine/July-2008/July08/Understanding-the-pH-KH-Relationship.html
Tropical Deforestation: http://earthobservatory.nasa.gov/Features/Deforestation/deforestation_update3.php
DOE. 1994. Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water; version 2, A.G. Dickson & C. Goyet, eds., ORNL/CDIAC-74. See Chapter 2: "Solution Chemistry of Carbon Dioxide in Seawater." http://cdiac.ornl.gov/ftp/cdiac74/chapter2.pdf