The amount of carbon absorbed by plant plankton in large segments of the Pacific Ocean is much less than previously estimated, researchers say.
Nutrient-stressed plant plankton had been mistaken as healthy
US scientists said the tiny ocean plants were absorbing up to two billion tonnes less CO2 because their growth was being limited by a lack of iron.
Iron deposits provide nutrients for the microbes, which in turn grow by absorbing atmospheric carbon dioxide.
The findings have been published in the science journal Nature.
About 50 billion tonnes of carbon dioxide was estimated to be absorbed by the world's oceans, so the reduction could mean up to 4% less CO2 being sequestered than previously thought, the team of US researchers said.
Phytoplankton (tiny plants) play a key role in the world's carbon cycle, as they are involved in about half the Earth's photosynthesis; along with zooplankton (tiny animals), they form the base of the whole ocean food web.
The paper's lead author Michael Behrenfeld, from Oregon State University, said that when stressed by a lack of iron, the phytoplankton created additional pigments that glowed green, unlike normal pigments.
But satellite imagery used to monitor the oceans' plankton blooms could not distinguish the difference, he said: "That green colour was not an indication of health, it was an indication of stress from the lack of iron."
Dust storms deposit iron particles in the oceans
Professor Behrenfeld and his colleagues examined 12 years of data gathered from 36,000 miles (57,900 km) of ship tracks through the tropical Pacific Ocean.
This allowed them to establish a "fluorescence fingerprint" of what parts of the ocean were experiencing iron stress, as well as areas suffering from a lack of nitrogen - another key element for ocean productivity.
Professor Behrenfeld said: "Nitrogen and phosphorus are nutrients that come up from the ocean bottom to feed the upper water column.
"Iron, on the other hand, can come from the deep, but it also enters the ocean through dust deposited by the wind. Windstorms blowing sand and dust off large deserts are a major source of iron for the world's oceans," he added.
The researchers identified three large areas of the Pacific where phytoplankton appeared to be suffering from a lack of iron - the southern ocean around Antarctica, the sub-arctic north below Alaska, and a vast area in the tropical Pacific centred on the equator.
Professor Peter Burkill, from the National Oceanography Centre, Southampton, UK, said the study was a welcome contribution to the growing understanding of the oceans' role in the global carbon cycle.
"We use oceanographic research vessels but they are very limited in their operational capabilities and are also very costly.
"So we are turning to other techniques, such as satellite imagery. These are not perfect but they do have the advantage of allowing us to look at great swaths of the oceans," he added.
"Studies such as this help to calibrate these models but there are many other questions out there that still need to be answered, such as how the oceans' physics work and what happens to the carbon that is drawn down."
Professor Behrenfeld has also been involved in previous experiments in which iron was added to the ocean in an attempt to boost productivity.
The studies showed that it did boost phytoplankton growth, but it did not deliver the results that models had predicted.
Professor Behrenfeld said introducing iron was complex: "When you first do it, there is an explosion of growth.
"Then you add a bit more iron, and the phytoplankton respond a bit more," he said. "But at the same time you are promoting plankton growth, the grazers that feed on them come to life because they suddenly have a more abundant food supply."