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A long-standing puzzle in ocean photosynthesis was why phytoplankton failed to grow fast in parts of the Pacific Ocean; after all, the microscopic plants have access to plenty of carbon dioxide thanks to upwelling water. Lack of iron is the answer, marine scientists conclude. Photosynthesis is reduced among phytoplankton in water with poor iron concentrations as compared with those in iron-rich conditions, even though phytoplankton in both conditions 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 environmental changes cause iron levels to rise, they can then take advantage of the increase immediately. Prior studies relied on satellite images, which measure chlorophyll levels alone, so they would not have revealed this distinction. The iron deprivation means that estimates of global ocean carbon uptake are probably 2 to 4 percent too high, the group reports in the August 31 Nature.