Plants consume atmospheric carbon dioxide (CO ₂ ) during photosynthesis. Oceanic phytoplankton account for almost half of all photosynthesis on Earth. CO ₂ is a major contributor to global warming. How could these facts tie together to change global temperature?

In 1990, scientist John Martin claimed that iron supplies in the Southern Ocean (also known as the Antarctic Ocean) limit the growth of phytoplankton — tiny, plantlike organisms, including certain algae, that live in aquatic ecosystems. Martin suggested that higher iron levels would encourage more phytoplankton to grow in the ocean. The phytoplankton would remove greater quantities of CO ₂ from the atmosphere during photosynthesis, therefore contributing to a reduction of global warming.

A group of scientists conducted an experiment called SOIREE (Southern Ocean iron release experiment) to test Martin's theory. As part of the experiment, the scientists "fertilized" an area of the Southern Ocean with an iron compound. As predicted, an increase occurred in both the phytoplankton levels and rates of photosynthesis near the ocean surface. The scope of the experiment was not sufficient to determine if carbon was drawn to the deep ocean, a process that would be necessary in order to cause a reduction in the atmospheric CO ₂ .

• Three articles describing the results were published in the October 12, 2000 edition of the journal, Nature. Students can read about the study in the ENN.com article, "Helping ocean algae could beat greenhouse effect.

Teaching the Problem

(The following Logal SimLibrary activities require Logal Express. Sign up for a free trial subscription.)

1. John Martin determined that iron is the limiting factor for the growth of phytoplankton in the Southern Ocean. Students can learn about the role of limiting factors for photosynthesis in different environments in the Biology Explorer activities, Limiting Factors I and Limiting Factors II.
   
   
2. The SOIREE study is based on the fact that the photosynthetic process consumes carbon dioxide. Students can learn more about the process in the Biology Gateways activity, Fueling Photosynthesis with Carbon Dioxide and Water.

Analyzing the Problem

In a response published in the same edition of Nature, Sallie Chisholm of MIT in Cambridge, Massachusetts, wrote:

Although seductive in its simplicity, in practice this idea would threaten the ocean ecosystem. Artificial fertilization with iron would probably have many unintended side effects, such as deoxygenating the deep ocean and generating greenhouse gases that are more potent than CO ₂ . We know that it would change the structure of the marine food web... Moreover, we cannot expect to change the flux of carbon from the atmosphere to the oceans — a single 'arrow' in this complex, self-organized system — without changing other features of the system in undesirable ways. It is likely that we would not recognize these changes until it was too late to reverse them.

Ask students to evaluate Chisholm's statement. Have them list at least three possible unintended outcomes and evaluate how likely each one might be. Ask them to justify their opinions.

1. Zooxanthellae are tiny oceanic algae living on coral reefs. They give the reefs their brilliant colors. Global warming is causing a rise in ocean temperature, which kills the zooxanthellae and leads to "coral bleaching." Students can learn more about coral reefs at the following sites:
   
   
   • World Wildlife Fund's Coral Reefs
     
     
   • NOAA's Coral Reef
     
     
   • Riverdeep's "Reef Awareness Week"
     
     
   • National Wildlife Federation's report Coral Reefs and Climate Change 2000
     
     
2. Students can learn more about global warming at the following sites:
   
   
   • American Museum of Natural History's Global Warming: Focus on the Future
     
     
   • EPA's Global Warming
     
     
   • CNN.com's our changing climate
     
     
   • National Geographic's Riddles of a Changing Climate
     
     
3. For general ocean studies, students can visit the following sites:
   
   
   • Smithsonian Institute's Ocean Planet
     
     
   • NOAA's Year of the Ocean (1998)
     
     
   • ThinkQuest's Ocean Adventure