Teaching the Carbon Cycle with a Consideration of Chemosynthesis (FeMO)

Shawn Doan, Sehome High School Bellingham (WA) based on an original unit by Shawn Doan, Donald Shepherd and Steve Ruthford.
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Initial Publication Date: March 1, 2008


Carbon Cycle

This series of activities introduces students to the concept of tracking carbon atoms using a geochemical budget model. The model is constructed using information from "Our Changing Planet" by Fred T. Mackenzie, 1998. The model is then altered by the introduction of human fluxes and evaluated using information from the National Geographic Society magazine article "The Case of the Missing Carbon". Finally, the model is re-evaluated considering the biomass and fluxes of chemosynthetic microbes as described by FeMO research at Loihi, Hawaii. This activity introduces the hypothesis that chemosynthesis accounts for the missing carbon

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Learning Goals

Students will begin to think of carbon quantitatively and apply those quantitative concepts when considering changes to a model geochemical carbon cycle. Students will synthesize ideas from a variety of sources including the FeMO to modify their model of the carbon cycle. Students will demonstrate their new understandings through diagrams and technical writing.

Context for Use

This activity is designed to fit into a larger unit on global change. Students benefit most if they have a good knowledge of photosynthesis and respiration chemical reactions and some sense of the age of the Earth. This lesson addresses the following National Science Education Standards: - Students should develop understanding of Geochemical cycles - Students develop ability to use technology and mathematics to improve investigations and communication. - Students should develop understanding of Energy in the earth system - Students should develop understanding of Interactions of energy and matter - Students should develop understanding of Natural and human-induced hazards - Students should develop understanding of Science and technology in local, national, and global challenges

Description and Teaching Materials

Duration and Prerequisites

The unit will take at least 10 hours if students do the NGS and FeMO reading in class (recommended). Students should have been introduced to the concepts of atoms and elements, the reactions for photosynthesis and respiration, food chains, the age of Earth, and have some knowledge of global change.

Suggested Sequence of Carbon Cycle Unit

1. Students construct a Budget using their allowance. (Specify that students need to show at least 5 reservoirs one source of money in their budget and spend it in 3 places. All the boxes must be connected by arrows that are labeled by the activity, e.g., "buying lunch". Give extra credit if students can link where thy spend there money with their source. See example.) Students should write a short explanation of how they gain or loose money in their budget; look for the concept of dynamic equilibrium. Finally, students explain their budget to a neighbor and record final version in their journal.

2. Teacher draws, and students copy, a Carbon Cycle Diagram Without Labels into their journals. Students put their own numbers on the arrows so that all the reservoirs balance. Their numbers must be different than their neighbor's numbers. Exchange budget with neighbor to double check balance and make corrections.

3. Students write a paragraph explaining how to balance a budget diagram in their journals. (this can be homework)

4. Review chemical reactions for photosynthesis and respiration. Emphasize the role of individual atoms. I use simplified forms of the photosynthesis and respiration equations - see teaching tips.

5. Hand out carbon budget handout. Instruct students to read labels and mark photosynthesis arrow with a "P" and respiration arrow with an "R". Students circle human caused arrows. Students double check their answers with neighbors.

6. Students write 2 questions about carbon budget in their journals. Teacher answers questions and allows discussion.

7. Teacher provides rates for 5 naturally occurring arrows (human fluxes are left blank). Students calculate rates for 3 remaining natural arrows.

8. Students write a paragraph that describes the movement of one carbon atom through the 4 reservoirs of the carbon cycle. (this can be homework)

9. Class is questioned on why the human rates were not included in the balanced budget. Discussion should emphasize age and chemical stability of Earth and very recent rise of human industry as a force of nature. (review geologic time)

10. Teacher draws a simplified cross section of Earth showing the 4 reservoirs of carbon. Students copy the diagram in their journals then guess the percentage of carbon in each reservoir by writing it in their journals. Teacher provides quantities for the 4 carbon reservoirs. Students are stunned to learn that to a first approximation 100% of carbon is in the lithosphere.

11. Teacher provides rates for human fluxes of carbon to the atmosphere. Emphasize that these fluxes are very well known compared to the others because humans keep track of the amounts by paying for coal, oil natural gas, cement and land. All human fluxes add carbon to the atmosphere. Ask students to estimate the time to double the amount of carbon in the atmosphere if the removal fluxes stay the same. (Coal, oil, natural gas and cement are from the lithosphere. A small change in the lithosphere makes a huge difference in the atmosphere.)

12. Students read The Case of the Missing Carbon February 2004 National Geographic Magazine. (The hard copy magazine has pictures and diagrams that are very helpful explaining the concepts in the article. The website does not have these.) Students take notes and for every page of notes write one or two questions. (this can be homework)

13. Students introduced to FeMO activities and chemosynthesis. A question sheet or guided note taking should emphasize the chemosynthetic nature of the microbes, the biomass and their source of carbon.

14. Once students have worked with FeMO information and discussed it they should write and answer the following questions in their journals: "Has FeMO found the missing carbon from the NGS article? Why or why not? If the presence of chemosynthesizers explains the missing carbon how will the carbon budget need to be modified?"

15. Class discusses which reservoir is increased by chemosynthetic biomass and which fluxes should be increased. Students should make changes on a class copy (on overhead or chalkboard) then agree. When all agree students then change their copies to match.

16. Students write and illustrate a story about the carbon cycle.

  • What is a "carbon cycle" or "carbon budget"?
  • How have humans changed the Earth's carbon cycle?
  • How does the presence of chemosynthesizers change our understanding of the carbon cycle?
  • How does adding chemosynthesizers into the carbon cycle change the build up of carbon in the atmosphere?
  • Resources

    Carbon Cycle Unit Understanding By Design Template
    Example Student Budget
    Blank Carbon Cycle
    Carbon Cycle with Data
    Example Web Search Questions for Searching the FeMO Site

    Teaching Notes and Tips

    Reservoirs vs. fluxes: it is useful to have students visualize the diagram as a series of 4 tanks of water connected by 11 hoses. In a balanced budget the flow through the hoses can be fast or slow provided the water level in the tanks stays constant. This is an example of dynamic equilibrium.

    Students will find it is easiest to balance the budget if they start with a number for the photosynthesis arrow and balance the biomass reservoir first, then the hydrosphere, then lithosphere and finally atmosphere. It is also much easier if they start with a number like 4 or 8 rather than 56 or 137. Tell students that their numbers should not be like their neighbors. When a student gets it balanced they should raise their hand so you can check the math. Check that their numbers don't match their neighbors and work through the cycle starting with input to the biosphere - input should balance outputs - then progress to the hydrosphere, the lithosphere and finally the atmosphere. Seeing this is a clue to the student to work through it this way.

    When introducing the actual values for the fluxes and reservoirs: Students should have a sense that the geochemistry of the Earth was self regulating prior to man's industrialization. To achieve this, have the students circle those arrows they think are man made, then have them balance the budget without the man made fluxes. Ask students why we have not included these arrows and you will get answers referring to industrial revolution, population explosion, and age of the Earth. Be prepared to discuss how our perception of steady state (or dynamic equilibrium) is influenced by the length of time we consider, i.e., that which appears to be increasing over a short period my just be an oscillation over a longer period.

    It is simplest if students are aware they can use the geochemist's shorthand for photosynthesis and respiration, e.g. CO2+H2O ==> CH2O+O2 and O2+CH2O ==> H2O+CO2

    I teach the simplified forms of the photosynthesis (CO2+H2O ==> CH2O+O2) and respiration (O2+CH2O ==> H2O+CO2) chemical equations to my 9th grade students. The simplified forms are accepted geochemical shorthand for generalizing many reactions and are easier for students to learn. The relationship between 6(CO2)+6(H2O)=>C6H12O6+6(O2) and CO2+H2O ==> CH2O+O2 is that the chemical formula for glucose is divided by 6 so that single molecules of CO2, H2O and O2 may be used.

    Students should be made aware that the arrows, with one exception (burial), represent chemical reactions. For example the photosynthesis flux arrow in the diagram is the same as the arrow in the reaction CO2+H2O ==> CH2O+O2

    The quantitative nature of these reactions can be reinforced by using plastic or wooden molecular models of CO2 and H2O. Tell students to watch the carbon atom and disassemble them and reassemble them into CH2O and O2 so that they see that the carbon in CO2 is the same atom as in CH2O.

    Review the calculation for the excess of carbon entering the atmosphere via the human fluxes and have students write this in their journals.


    Several assessment stages are built into the unit. The first is the student's allowance budget and their explanation of personal cash flow. Students should be able to quantify their own income and spending. Check to see that students roughly balance their income and expenses.

    Students should be able to balance the carbon cycle fluxes using their own numbers. I walk around the class and check them as they finish. This is followed by a written explanation. The explanation should include some example math. Prompt students to include examples with numbers.

    Students should be able to find photosynthesis and two respiration fluxes on the diagram. I often have them mark 4 "respiration" arrows (incl. deforestation and fossil fuel combustion) to spark a discussion. Walk around and evaluate. The 2 student questions allow informal assessment during class discussion.

    The student paragraph describing the movement of a single carbon atom through the 4 reservoirs assesses student understanding of the transformations inherent in the fluxes between reservoirs and reinforces the quantitative nature of the diagram. Make sure students explain that the carbon atom is combining with other atoms to make new molecules as it moves from reservoir to reservoir (exceptions are burial and dissolution in water).

    Careful attention to student statements during the discussion concerning the pre-industrial carbon budget will ensure that students understand that the human fluxes are a perturbation on a system that was once in balance. It is important to accept that the pre-industrial carbon cycle was not always balanced. Questions regarding episodic events such as volcanoes and meteor impacts suggest the students are uncomfortable with carbon cycle balance on longer timescales.

    Questions taken during the reading of "The Case of the Missing Carbon" allow the teacher to assess understanding and give the student an opportunity to refine their understanding.

    Class review of the questions answered during the FeMO web search allows students to correct their own misunderstandings.

    If students are to be successful on the final paper, the questions to be answered should be provided before the discussion of the changes to the carbon budget suggested by chemosynthesizers. It helps to get students to write their first response before it is discussed and to modify that response after they have participated in the discussion.

    References and Resources