Phytoplankton, microorganisms found in the oceans, fix carbon from CO₂ by photosynthesis and thus play a key role in regulating the carbon dioxide level in the atmosphere. The fixing of carbon by these microorganisms removes significant amounts of CO₂ from surface waters and, because CO₂ in surface waters is in equilibrium with CO₂ in the atmosphere, the amount of atmospheric CO₂ is ultimately decreased by the phytoplankton.

The question of why phytoplankton are so scarce in certain regions of the oceans has puzzled oceanographers for decades. In something of an oceanic tour de force, a group of scientists led by Kenneth S. Johnson of the Moss Landing Marine Laboratories seeded a 72 km² expanse of surface waters of the Pacific Ocean with acidic iron sulfate to give a surface iron concentration of 1-2 nanomolar (1-2 x 10^-9M). (1) These scientists found that a massive phytoplankton bloom and a consequent increase in fixed carbon resulted. Iron produces this result because a chlorophyll molecule complexed to iron is an essential component of the carbon-fixing photosynthetic system of the phytoplankton. The scientists believe that phytoplankton growth is therefore limited by the availability of iron. They further theorize that enrichment of waters with iron, known to occur during the last ice age, could well have reduced global warming by enhancing CO₂ fixation.

In recent years, scientists have wondered whether seeding the oceans with iron might help reduce atmospheric CO₂, which has grown to alarmingly high levels because of fossil-fuel combustion (3); high levels of CO₂ may be promoting global warming. However, various modeling experiments eventually suggested that this tantalizing idea was not feasible in practice.

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