In the plant kingdom, greener does not always mean healthier. Scientists have found that microscopic ocean algae called phytoplankton actually get greener when they are stressed. A new study finds this is why the tiny plants in a large region of the Pacific are failing to absorb as much of the greenhouse gas carbon dioxide as they normally do, with implications for global warming.
Plants thrive on carbon dioxide just as animals do on oxygen. This is good for the atmosphere because plants soak up much of the carbon dioxide emitted by cars, industry, and forest burning. This prevents it from becoming a greenhouse gas that traps the sun's heat to contribute to global warming.
Phytoplankton have a particularly big role to play in absorbing carbon dioxide, acting as the lungs of the planet. This is especially true in the tropical Pacific, the largest natural source of carbon dioxide to the atmosphere.
Yet these single-celled plants may not be as effective in the region as once thought. Oregon State University botanist Michael Behrenfeld says 12 years worth of data gathered from research ships in the region show less than expected phytoplankton abundance because of a nutritional deficiency.
"There are actually lots of nutrients up at the surface, nitrogen and phosphorus, and they don't seem to be taking it up," he noted. "Why is that? What we found is that these tiny little plants are starved for iron."
The iron for sea plants generally comes down from the dust blown away from arid areas like deserts.
Behrenfeld and colleagues assessed phytoplankton iron levels with a technique that measures their glow. The glow is an emission of some of the solar energy they take in. The more phytoplankton glow, the less iron they have.
Lacking enough iron, phytoplankton do not take in as much carbon dioxide as healthier plants. Behrenfeld's team estimates that up to 2.5 billion more tons of carbon may escape into the atmosphere each year than once believed.
You would not know this by looking at color satellite pictures of the tropical Pacific. They show very green algae, a condition considered until now a sign of its robustness, but Behrenfeld says his findings show the opposite.
"When phytoplankton are stressed by iron, they actually appear greener," he explained. "Normally what we think of is that when plants are really green, that means they are really healthy, they are growing really fast, but in the tropical Pacific, that is often not the case."
Behrenfeld says the green stress response occurs when the algae, rather than wither, add more plant cells filled with green chlorophyl. It apparently is an emergency effort to collect more iron.
A scientist not involved with the study, ocean biologist Paula Bontempi of the U.S. space agency NASA, says the findings expand understanding of the ocean's role in Earth's carbon cycle.
"The health of the ocean's ecosystems overall play a pretty important role in this cycle and support what is a multi-trillion dollar ocean economy," he said. "Better understanding of these life processes enables the researchers to better predict the impact of things like climate variability and change on the Earth's systems."
The phytoplankton study appears in the journal Nature.