Anemic Phytoplankton Absorb Less Carbon than Thought

Phytoplankton in the Pacific Ocean are starved for iron, and as a result these microscopic plants soak up less of the greenhouse gas carbon dioxide than was previously thought, researchers have found. Although the difference in carbon uptake is not enough to perturb climate predictions significantly, the research should lead to an improved understanding of how climate changes will affect phytoplanktons ability to take up carbon.

The worlds oceans tend to absorb carbon dioxide in the form of carbonate, but the Pacific Ocean actually emits CO₂ in areas of cold, upwelling water that warms as it reaches the surface, releasing the gas. Phytoplankton thrive on this CO₂, using it to drive their photosynthesis. But the plankton dont grow very fast given the amount of nitrogen and phosphorus at their disposal. To find out why, marine researchers took a zigzagging 12-year journey through the Pacific, collecting tens of thousands of plankton samples. Fluorescence imaging gave them a measure of the planktons photosynthesis.

Too little iron is the problem, the group finds. Phytoplankton living in water with lower iron concentrations perform less photosynthesis than those in iron-rich conditions, even though they both make the same amount of chlorophyll. When these little ocean plants are starved for iron they produce more chlorophyll than they need, says lead researcher Michael Behrenfeld of Oregon State University. That way, if iron levels rise, they can then take advantage of the increase immediately, he explains. Prior studies of ocean photosynthesis relied on satellite images, which measure chlorophyll levels alone, so they wouldnt have revealed this distinction.

The group identified three regions of the Pacific where iron is holding phytoplankton back: the south near Antarctica, the north below Alaska, and at the equator. Taken together, the iron deprivation in these areas means that estimates of global ocean carbon uptake are probably 2 to 4 percent too high, the group reports in the August 31 Nature. In the tropical Pacific that change is a significant amount, comparable to changes resulting from El Nio transitions, says Behrenfeld.

Mapping ocean nutrients fills in another gap in climate change models. A major source of iron for the oceans is dust blown in from the deserts (as in the dust storm pictured above). As the climate changes, new wind patterns may alter the oceans iron content, which may in turn alter carbon uptake, Behrenfeld notes.