One-Celled Ocean Plants Key to Stable Atmosphere, Climate

by Monte Basgall

These varied organisms, collectively known as phytoplankton, have not only generated and sustained most of the oxygen we breathe, but they also "play a profound role in regulating atmospheric carbon dioxide," in part by sequestering vast amounts of the gas deep in the ocean when they die, wrote oceanographers from Rutgers and Duke universities and a German institution.

Because carbon dioxide traps solar heat, phytoplankton growth also helps regulate Earth's climate in an intricate interplay with ocean currents, wind-blown dust, nutrient discharges from rivers, solar radiation levels and other factors, their paper added.

For instance, the extra carbon dioxide could warm the atmosphere and seas in a way that shifts ocean currents, while changing rainfall patterns over land masses could alter the supply of vital phytoplankton nutrients. The result could be as drastic as "a net efflux of CO2 from the oceans to the atmosphere; that is, a positive feedback," the authors warned. But they also stressed "such an analysis is greatly oversimplified, perhaps even naive."

Richard Barber, a professor at Duke University, said their paper's purpose is not to predict runaway carbon dioxide levels but, rather, to show that phytoplankton have evolved in step with a complex and changeable environment to achieve a surprising atmospheric stability.

"It is very important to know what has stabilized Earth's heat, carbon dioxide and water budgets," Barber said in an interview. "These cycles have stayed within bounds over geologic history, though they have never been constant. Earth has always been getting colder or warmer. The climate is always changing, so it's out of steady state.

"But it stays within bounds, and that's an unusual system," Barber added. "Phytoplankton that have evolved in the ocean belong to a number of different plant phyla and even different kingdoms. This diversity is one of the reasons there has been so much stability. Different kinds of organisms kick in when conditions are right for them.

"They don't pile up too much oxygen, carbon dioxide, nitrate or phosphate. Those are kept under some kind of biological control. That's how the past has been. The big issue is, will the future work that way?"

The paper was coauthored by Paul Falkowski of the Rutgers Institute of Marine and Coastal Sciences in New Brunswick, N.J., Barber, and Victor Smetacek of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany.

Living in the sea's sunlit "euphotic zone," they release oxygen into the atmosphere and take in carbon during their very short lives. Some are consumed by small grazing animals while others die before they can be eaten and sink to the depths.

All in all, phytoplankton take in enough atmospheric carbon dioxide to form a staggering 45 billion metric tons of organic carbon per year. And an estimated 16 billion of those metric tons find their way to the deep ocean, where they can remain for long periods.

Such a large carbon output by such tiny organisms implies that the world's phytoplankton population must recycle itself quickly, on the order of about once a week, the paper estimates. And that quick a turnaround time also means phytoplankton are exceptionally sensitive to change, the authors noted.

Whether or not phytoplankton grow in abundance depends on the availability of nutrients that arrive at the whim of ocean currents and seawater mixing that may turn on and off. Climatic alterations onshore also can affect the nutrient pipeline. A dry year, for instance, can reduce the output of silica from rivers. But an arid spell, coupled with unusually windy conditions, can increase the supply of airborne dust that makes its way from land to sea.

During past ice ages, the authors wrote, evidence suggests there was at least 10 times more wind-blown dust, which could supply phytoplankton with iron, a nutrient that is often in especially short supply in the ocean. The extra iron could have fomented an explosion in plant growth, and helped draw down atmospheric carbon dioxide to the exceptionally low levels recorded in ancient ice cores.

The fact that carbon dioxide increased rapidly when glacial periods ended is also consistent with the argument that rapidly recycling phytoplankton had at least some role, the authors wrote.

If carbon dioxide-caused global warming melts some polar ice and increases precipitation, some computer models suggest the effect might be a weakening of nutrient-bearing currents and mixing and a reduction in wind-borne dust. That could provide a profound challenge to phytoplankton despite their billions of years of evolution, and exacerbate the release of carbon dioxide, the authors speculated.

"My colleagues and I want to start people thinking about the phytoplankton in the ocean and the carbon dioxide in the atmosphere as an evolutionary problem in the context of the organisms that are doing the cycling. We think that's a big factor in climate regulation," Barber said.