Understanding the Impact of Carbon Dioxide on the Formation of Coral Reefs

By Kate Fraser on Apr 05, 2015

Animals of the coral reef need calcium carbonate to build their skeletons. In this activity, students compare the formation of coral reefs today when there is still an abundance of carbonate present in the ocean to bond with the calcium to coral reefs built in a possible future ocean where there is not enough carbonate available.  Calcium carbonate is the combination of two ions dissolved in sea water, Calcium which is Ca+2 and carbonate which is CO3 -2 to form CaCO3.

“With ocean acidification, corals cannot absorb the calcium carbonate they need to maintain their skeletons and the stony skeletons that support corals and reefs will dissolve. Already, ocean acidification has lowered the pH of the ocean by about 0.11 units (SCOR 2009). Moving the ocean's pH from 8.179 to a current pH of 8.069, which means the ocean is about 30% more acidic now than it was in 1751 (SCOR 2009). If nothing is done to reduce carbon dioxide emissions into the atmosphere, ocean acidification will increase and more and more corals will be damaged or destroyed.”   See How Does Climate Change Affect Coral Reefs?


Legos and duplos

  1. Label both tubs in large print and Braille. One tub will say “Today’s Ocean”.  The other tub will say “Future Ocean”.
  2. Place about 300 of the smaller bricks in each of the two plastic containers.  One bin represents the ocean today while the other represents the ocean in a high-CO2 future. Each bin contains the same number of smaller Legos, which represent calcium ions. Rising levels of CO2 do not directly affect ocean Ca2+ concentration.
  3. Place 20 of the larger Lego pieces representing COions in the tub labeled “Future Ocean”
  4. Place 40 of the larger Lego bricks in the tub labeled “Today’s Ocean”  

Note: These ion concentrations are exaggerated for the purpose of illustrating the impact on calcifying organisms.

  1. Move the bins to two separate tables, and place towels nearby.
  2. Fill the bins with water to approximately 2 inches below the rim of the bin.
  3. Divide the group into two teams, one for each ocean.  Each team will be responsible for assembling the compounds of CaCO3 by attaching the smaller calcium ion to the larger carbonate ion them fastening the calcium carbonate to the Lego base which represents the coral skeletons upon which new coral build their colonies.
  4. The teams may decide how they wish to accomplish this task. For example, one person might choose the task of attaching the completed compound to the base and other members build the compounds. For a large group one student at a time might build, and then signal the next student to come. Or for small groups, all could work together at one tub. 


  • LegosTM (2 colors/ sizes) (how many of each) in contrasting colors.  We recommend the 2 by 4 dot sized bricks to represent the carbonate (as the larger ion) and the 2 by 2 size to represent the calcium ions which are smaller.
  • Larger bricks can be used for students with limited hand skills or DuplosTM, which are even larger but have the same ratio of sizes. 
  • 2 plastic rectangular bins, approximately 9” by 14” by 5 inches deep or larger (Larger bins will be necessary if using Duplos.)
  • 2 Lego base plates size (The 16 by 32 dot size would work well.)
  • Timer
  • Paper Towels or regular towels for the spills!


  1. Set the timer for 3 minutes
  2. Begin to assemble the compounds
  3. Place finished compounds on bases.
  4. Stop building when the timer goes off.
  5. Count the number of compounds on each base.
  6. Which team has the most CaCO3? Discuss why.
  7. Check if there are remaining CO3 ions in either tub.  If so an additional building session may be necessary. 


Potential Discussion Points:

  • Comparing/ contrasting with the real world: In reality, it takes many years, decades and even centuries for corals to reach their full size and build extensive reef systems. We have also exaggerated the difference between present and future carbonate ion concentrations. In reality, the difference is much smaller, but because corals are building their skeletons all the time and over longer time frames, the impact eventually shows.
  • Reefs are made of many different types of corals; corals are colonies made of many individuals.  Each team member contributed to building the CaCO3 reef, which is exactly what happens on a real coral reef; not only are there many different types of corals on a reef (e.g., brain corals, branching corals) and calcifying organisms (e.g., calcareous algae), but each coral is made up of many individual-yet-connected, little anemone-like polyps that contribute to building the coral’s CaCO3 structure.
  • Variability among coral organisms and reef systems: Not all corals respond the same way to OA. While some are highly sensitive others seem unaffected. Each team member may have a different approach to locating and putting together their pieces of CaCO3. The amount of reef each team was able to build was due not only to the availability of each ion building block, but also how each team member approached the task of calcification. 


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