Dee Breger, Drexel University

Assumptions that tiny diatoms such as the ones shown above could fix carbon from the air and sink it to the bottom of the ocean have been hard to prove.

The US Department of Energy has taken an interest in carbon sequestration, but a grand scheme to induce thick blooms of carbon-fixing algae has yet to bear fruit in early studies. The DOE directs a large share of its global warming budget to carbon-sequestration research, drawing on biologists in hopes of enlisting algae, microbes, or plants to fix and store excess carbon created by the fossil-fuel economy. It has awarded tens of millions of dollars in biology-based research grants to geneticists, bioinformaticians, and cell biologists across the country.

By far the most ambitious and most expensive part of the program is a plan to seed the ocean with iron dust...


Yet, most ocean scientists side with Caldeira. The results of another large-scale experiment, the Subarctic Ecosystem Response to Iron Enrichment Study1 didn't do much to foster optimism. While the diatoms did respond dramatically to the iron additive, only 8% of the carbon material they produced fell below 50 meters, which is the minimum depth necessary for permanent sequestration. In addition, only four days passed between the end of the iron seeding and the expiration of the bloom. Scientists had been hoping that the bloom would endure much longer, but the silicic acid needed to produce the organisms' carbonate shells became depleted.

The results of SOFeX, also failed to show a massive flux in the carbon cycle.2 As a form of purposeful carbon sequestration, the authors pointed out that they were able to sink only 900 tons of carbon for the 1.26 tons of iron used in the experiment.

In addition, some question whether any of that carbon actually gets to the ocean floor. "There are entire ecosystems of microbes on each particle, and we really have no idea how much of it they consume as it falls from 50 meters to the sea floor, which can be 2000 meters below," says Kathy Barbeau, a marine chemist at Scripps Institution of Oceanography in La Jolla, Calif. The expedition was scientifically successful to a great degree. It proved Martin's original hypothesis again. It also was the first such expedition to get phytoplankton to bloom in an area of ocean that had low silicic acid content. "We expected that region to not bloom at all, and when it did everyone was surprised," says Rik Wanninkhof, an oceanic scientist at the National Oceanic and Atmospheric Administration who helped to tag the iron flakes with traceable chemicals. "It just goes to show how little of this process we understand."


The original idea behind iron seeding as a carbon mitigation strategy was to find areas of the ocean that are high in nitrates but low in chlorophyll, in other words, regions where nitrogen-fixing ocean bacteria are active but where little photosynthetic activity occurs. Approximately 20% of the ocean can be considered to fit these high-nitrate, low-chlorophyll criteria, according to Coale. The hypothesis is that little photosynthesis occurs in those regions, because they lack a suitable resource for the electron-transporting minerals necessary to make photosynthesis efficient.


© 2000 Nature Publishing Group

Above is a satellite view of the Southern Ocean Iron Enrichment Experiment (SOIREE) done in 1999 far off the southern coast of Australia. The algal bloom that resulted can be seen as a light semi circle.

Iron, even in trace amounts, is an ideal electron transporter. What hadn't been understood well, however, was the interplay between iron and other necessary nutrients, especially phosphates. "We knew that diatoms need phosphorous, but we thought there would be enough in the environment," says Caldeira. The natural supply appeared to expire quickly, however. The only corrective, he says, is to seed with phosphates in addition to iron, but the economics of such a system wouldn't work.

Even if someone were to figure out how to make algal blooms sequester carbon economically, very little research has been done on how such a project would affect the rest of the ecosystem. For instance, phytoplankton ecologist Elena Litchman at Georgia Institute of Technology in Atlanta ponders how the next level on the food ladder would react to a long-term artificially incurred algal bloom. She points out that mesozooplankton feed off of diatoms and might create a counter-bloom, producing as much carbon dioxide as the diatoms sequester. Such a change would then drastically affect the rest of the ecosystem. "It's a tangled mess of interactions that we have such a lack of knowledge about," says Litchman. "By altering one pathway, it could impact biological events in ways that nobody can imagine."

Sam Jaffe can be reached at sjaffe@the-scientist.com.

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