Carbon Cycle. Seeing. the. by Pamela Drouin, David J. Welty, Daniel Repeta, Cheryl A. Engle- Belknap, Catherine Cramer, Kim Frashure,and Robert Chen.

Transcription

1 Seeing the Carbon Cycle by Pamela Drouin, David J. Welty, Daniel Repeta, Cheryl A. Engle- Belknap, Catherine Cramer, Kim Frashure,and Robert Chen. The most important biochemical reactions for life in the ocean and on Earth are cellular respiration and photosynthesis. These two reactions play a central role in the carbon cycle. The ocean-based carbon cycle is highly relevant to today s students because of its key role in global warming. The Earth s atmosphere maintains the temperature of the Earth within a relatively narrow range that can support life. The atmosphere is made up of gases such as carbon dioxide, methane, water vapor, and others that allow radiant energy to pass through, but prevent heat loss back into space. As the composition of the atmosphere changes due to more carbon dioxide emissions produced by humans generating power, the insulating capac- Explore carbon cycle at Enter code SS Pamela Drouin is a seventh-grade physical science teacher at Hastings Middle School in Fairhaven, Massachusetts. David J. Welty (dwelty@fairhavenps.org) is a science teacher and a 6 12 science and technology academic coordinator for the Fairhaven Public Schools in Fairhaven, Massachusetts. Daniel Repeta is a senior scientist in the Marine Chemistry and Geochemistry Department at the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts. Cheryl A. Engle-Belknap is an instructional curriculum specialist for the Atlantis Charter School of Greater Fall River, Massachusetts. Catherine Cramer (cramer50@adelphia. net) is a writer and communications manager for the Center for Ocean Sciences Education Excellence New England in Boston. Kim Frashure is a PhD fellow and Robert Chen is an associate professor, both in the Department of Environmental, Earth, and Ocean Sciences at the University of Massachusetts in Boston. 14 s c i e n c e s c o p e J a n u a r y 2006

2 ity of the atmosphere increases and more heat is trapped. As carbon dioxide levels increase, so does the average temperature of Earth. Besides the obvious solution of cutting carbon dioxide emissions, which does have economic drawbacks, the possibility of sequestering carbon dioxide in long-term storage stages of the carbon cycle is an experimental option (see Figure 1). The following lessons outline a classroom experiment that was developed to introduce middle school learners to the carbon cycle. The experiment deals with transfer of between liquid reservoirs and the effect has on algae growth (Figure 2). It allows students to observe the infl uence of the carbon cycle on algae growth, explore experimental design, collect data, and draw a conclusion. Teaching the carbon cycle This carbon cycle lesson is designed for a seventhgrade physical science class. In the activity, students observe how different levels of carbon dioxide in the atmosphere affect the growth of algae. Before getting started, students review the steps in an experiment and write an If, then prediction with a because clause on how the carbon cycle might influence growth of algae. For example, If there is more, then there will be less algae growth, because too much will kill the algae. Or, If increases, then algae growth increases, because it is used by algae during photosynthesis to make sugar. Experimental design This single experiment is set up by the teacher for multiple classes to observe and analyze because the amount of equipment (Figure 3) required is impractical for multiple setups; the experiment is simple enough to set up for a science teacher, but too complex for a middle school student to handle; there is a 0.4 molar potassium hydroxide (KOH) solution in the negative control that presents a safety hazard to students; and the amount of algae transferred to each beaker needs to be consistent, as large differences in algae settling time could infl uence how much algae is in each group. The students responsibilities during the experiment are to make observations, collect data, reach a conclusion on how different levels of affect algae growth, and then apply that knowledge to determine how greenhouse gases might be tied to global warming. FIGURE 1 Causes of increased emissions and effects/results is naturally found in: respiration, decomposition, natural forest fi res. is human-made by: burning fossil fuels for transportation, electricity, and human-made forest fi res for clearing land. Contributes to the greenhouse effect causing increased warming of the Earth. Change in weather patterns increases precipitation. Melting polar ice caps and alpine glaciers. Wet places become wetter. Dry places become drier. Heat waves Rising sea levels cause fl ooding, changes in habitats for plants and animals. Changes in habitats for plants and animals Increases the rate of evaporation and causes bursts of increased rainfall. J a n u a r y 2006 s c i e n c e s c o p e 15

3 FIGURE 2 Experimental design s relationship to photosynthesis FIGURE 3 Materials needed for the experiment in the air in the water 6 in the water SUN 6 H 2 O in the water 6 O 2 in water Closterium algae Sugar C 6 H 12 O 6 6 CO2 + 6 H 2 O C 6 H 12 O O 2 Photosynthesis two liters of distilled water one culture of actively growing Closterium algae (Carolina Biological Supply #HT , $ 4.95) tube of 50X Alga-Gro (Carolina Biological Supply #HT , $27.60) Schultz, Plant Food Plus ($2.95) three half-gallon Rubbermaid screw-cap containers four 600 ml Pyrex beakers (Ball canning jars work as a substitute, but must be able to fit inside the half-gallon container; plastic containers may have impurities that inhibit algae growth; beakers are cleaned and rinsed twice with distilled or bottled water) 0.5 liter of carbonated water (seltzer water) 0.6 liter 0.4 Molar potassium hydroxide solution (obtain from a chemistry teacher), wear safety glasses when handling 500-mL graduated cylinder 50-mL graduated cylinder rubber band plastic wrap or parafilm glass-marking pen two fluorescent lights electric on-off timer The experiment consists of a set of three half-gallon containers partially filled with solutions that establish different levels. Inside each container is a culture of actively growing freshwater Closterium algae (see Figure 4). Each half-gallon container is a closed system that prevents gas exchange with the outside. The beaker of growing algae sits in a liquid reservoir within the container. The reservoir solution controls the amount of in the air (Figure 5). The Closterium algae is grown under a fluorescent light on a 12-hour light/dark cycle. The experiment consists of three groups: (1) the normal control with a tapwater reservoir; (2) the experimental group with a carbonated water reservoir; and (3) the negative control with a 0.4 M KOH reservoir. The tap water represents the natural condition of dissolved gases in the environment. The experimental group is a 1:2 mixture of bottled carbonated water to tap water, which is enriched with gas. reacts with water to form carbonic acid (H 2 ). (g) + H 2 O (l) H 2 (aq) Because 0.4 M KOH reacts with to form K 2 (s), KOH effectively depletes from the atmosphere and water of the closed system. 2 KOH (aq) + (g) K 2 (s) + H 2 O (l) Depending on the level of your students, you can explain the reactions occurring inside each container, or simply label them as Depleted, Regular, and High. Students should be instructed that as the algae culture grows the cells will become more crowded and the culture 16 s c i e n c e s c o p e J a n u a r y 2006

4 FIGURE 4 Growing closterium algae 1. Add 50X-Alga-Gro to 1 liter of distilled water to make 1X-Alga-Gro. 2. Transfer to the glass beaker 400 ml of 1X-Alga-Gro. 3. Add 1 drop of plant food. 4. Transfer half the volume of the Closterium algae culture to the beaker. 5. Cover with plastic wrap and secure with a rubber band. 6. Punch six holes in the plastic wrap. 7. Place in a sunny window or under a fluorescent light for about a week. As the culture grows, the green color will intensify. will become a darker green. This is called cell density and indicates a higher algae number from greater growth. After the teacher has set up the experiment, students observe the systems over the next two weeks and record their observations in a data table (Figure 6). At this point in the lesson, students start making connections between carbon dioxide levels and algae growth. Specifically, they begin to realize that algae depend on photosynthesis for survival, which is why algae live within the sunlight zone in the ocean. To further student understanding, the following equations for photosynthesis and cell respiration are introduced and explained with the use of guided questions. FIGURE 5 Experimental setup H 2 O Photosynthesis C 6 H 12 O O 2 C 6 H 12 O O 2 Cell respiration H 2 O 1. Transfer into each half-gallon plastic container 600 ml of one type of reservoir liquid: normal: 600 ml of tap water experiment: 400 ml of tap water and 200 ml of carbonated water negative: 600 ml of 0.4 M potassium hydroxide 2. Label beakers Normal, Experiment, or Negative with a marker. 3. Transfer 350 ml of 1X-Alga-Gro to each beaker. 4. Transfer 50 ml of actively growing algae into each beaker. 5. Resuspend the algae after each transfer. 6. Carefully insert beakers into half-gallon containers. 7. Close and wrap tightly with plastic wrap or parafilm. half-gallon screwcap container 600 ml beaker or flask reservoir water (tap water, carbonated water, or 0.4 M KOH) algae culture What type of gas will collect in the container as photosynthesis occurs? How would photosynthesis be affected if too little or too much carbon dioxide were present? How would respiration be affected if too little carbon dioxide were present? In which container would you expect to find the greatest amount of algae growth? Why? In which container would you expect to find the smallest amount of algae growth? Why? More advanced students can be introduced to the role of carbon, hydrogen, and oxygen in the carbon cycle. They can also study the atomic structure, electron orbitals, valence electrons, and the molecules of cellular respiration and photosynthesis: carbon dioxide, glucose, oxygen, and water. The relationship between the products and the reactants can also be discussed and the balancing of the equation examined. Results During this experiment, students will observe that the experimental group with carbonated water grows better than the normal control. For this reason, the experimental group has more algal cells and is a darker green. The negative control grows less well than the normal control; consequently, the resulting culture has less algal cells and is a lighter green. These results demonstrate what would be predicted from the photosynthesis reaction: When there is more carbon dioxide, the algae grow better. When there is less carbon dioxide, the algae grow poorly. The negative control of 0.4 M KOH will show little detectable algae growth. Students should compare these observations J a n u a r y 2006 s c i e n c e s c o p e 17

5 FIGURE 6 to their predictions about how the levels would affect the algae growth. As a follow-up, ask students the following questions to assess what they learned about the process: What is the reaction that this experiment demonstrates? What part of the reaction does the experiment test? What would you expect to be the outcome for the algae in the experimental group? What would you expect to be the outcome for the algae in the negative control? What is going on in each chamber of the experiment? Identify ecological/environmental conditions that are similar to the conditions found in each container. Conclusion Student data table Day Observation Depleted Regular High 1 Darkness (1 10) 3 Darkness (1 10) 7 Darkness (1 10) 10 Darkness (1 10) 14 Darkness (1 10) Through this experiment students learn that has a positive effect on algae growth and, in fact, is essential to the growth of algae. At the end of the activity, students should be able to make the connection that algae growth is dependent on photosynthesis. Through a discussion of deforestation and consumption of fossil fuels, students should be led to the concept of long-term carbon reservoirs being depleted and an increased amount of carbon being put into the atmosphere. (One way to remove more carbon dioxide from the atmosphere is to 18 s c i e n c e s c o p e J a n u a r y 2006 stimulate algae and plant growth.) Finally, the concept of food chains built upon photoautotrophic organisms that use sunlight, water, and carbon dioxide to make sugar should be introduced. The sugars then support the energy demands of the herbivore heterotrophs and carnivore heterotrophs. Students should be able to grasp the concept that since most life is directly or indirectly dependent on photosynthesis, if photosynthesis stopped due to sunlight being blocked, then all life on Earth would be in jeopardy. However, if animal respiration stopped, which is only one of several sources of carbon dioxide for plants, plants could survive from carbon dioxide released by volcanoes and the ocean. Extensions You can supplement this activity by measuring the amount of oxygen released by the algae and performing serial dilutions and filtrations to further quantify the amount of algae growth. By performing serial dilutions, it is possible to determine the algae concentration per ml, so students can compare cells per ml to cells per ml. This will allow them to find the dilution where the algae cells limit out following dilution. Filtration allows all of the algae to be captured on a solid substrate for densitometry or better photographic documentation. These activities can be found with the online version of this article, available at Acknowledgments This experiment grew out of collaborative work in the Ocean Science Education Institute (OSEI), which develops and implements high-quality ocean science education for middle school students through projects that connect with existing district curricula and effective science educational practices. OSEI is a project of the Center for Ocean Science Education Excellence New England (COSEE-NE), an NSF-funded partnership between the New England Aquarium, the University of Massachusetts/Boston, and the Woods Hole Oceanographic Institution (WHOI). The OSEI format includes a five-day workshop, numerous classroom visits, and two follow-up days. During the school year, researchers and Massachusetts middle school teachers, district science coordinators, and facilitators teamed up to produce districtwide, inquiry-based science curricula for middle school students based on current ocean science research. To find out more about OSEI and other COSEE-NE programs, please visit our website at