Public Release: 

Iron Weighs Heavily In Ocean Plankton Growth, CO2 Absorption

DOE/Brookhaven National Laboratory

Upton, NY - Phytoplankton are the lungs < and the lifeline< of the planet. But without a balanced diet that includes iron, these tiny ocean plants cannot exhale the oxygen we humans breathe, nor inhale the carbon dioxide we, our cars and our factories spew out.

So report scientists from the U.S. Department of Energy's Brookhaven National Laboratory and their colleagues, in two papers in Nature: one in the October 10 issue, and one that appeared August 29. Both describe results collected on two 1995 research cruises < one near the Galapagos Islands, the other in the subarctic Pacific. The studies' results add to what is known about an important cornerstone of the global ecosystem.

On the Galapagos cruise, described in the Oct. 10 article, a BNL-designed instrument helped prove that underachieving phytoplankton could be revved up to consume more carbon dioxide if they got an iron supplement.

The August article discussed a BNL-developed molecular detection method that showed that subarctic phytoplankton are underproductive because they're stressed for iron.

Both studies answer longstanding questions about phytoplankton. But the scientists adamantly state that fertilizing the ocean with iron would not be a "quick fix" for the potentially climate-changing effects of humankind's carbon dioxide production.

"Too many factors would erase any increase in carbon dioxide absorption, not to mention the risks of such a huge environmental manipulation," said BNL oceanographer Michael Behrenfeld.

Phytoplankton are tiny single-celled plants, trillions of which have floated freely on the ocean's surface for nearly 3.5 billion years. Together, they process about 40 percent, or 43 billion tons, of the world's carbon annually, much of it atmospheric carbon dioxide.

When phytoplankton do not get all their necessary nutrients, say the scientists, they cannot absorb as much carbon dioxide as usual. The two cruises studied areas of ocean where phytoplankton productivity is "limited" by lack of iron, so that they cannot take full advantage of abundant nutrients such as nitrogen and phosphorous.

Underwater Overachievers

In the Galapagos study, scientists fertilized an underproductive patch of the South Pacific with iron three times over eight days, while taking measurements to see how phytoplankton activity in the 25-square-mile area changed.

The result was dramatic: Within 32 hours of the first application, an area 6 miles wide and 81 feet deep turned green with phytoplankton. The effect lasted two weeks.

Said Behrenfeld, "Our boat towed BNL's submersible instrument, which repeatedly dove down to 200 feet and returned to the surface, taking readings all the while. We could see immediately how the phytoplankton responded to the iron < their photosynthetic efficiency increased tremendously in the first 48 hours." A second instrument inside the boat showed that the phytoplankton began making much larger "antennae" to capture sunlight, in order to take advantage of the iron-rich diet.

The team calls the effect "bottom up" control of phytoplankton, to contrast it with the long-held "top-down" control theory that says that tiny animals called zooplankton were eating the phytoplankton as fast as they grew. The BNL scientists show that it is nutrients, not grazing, that restricts the population. But they stop far short of saying that adding iron to underproductive patches of ocean could slow global climate change.

The experiment was led by scientists from Moss Landing Marine Laboratories in California. The researchers who designed and built Brookhaven's instruments with funding from the U.S. Department of Energy and NASA also include BNL's Zbigniew Kolber and Paul Falkowski, and scientists from Britain's Plymouth Marine Laboratory.

Lack of Iron Stresses Them Out

It may be easy to tell when humans are stressed, but what about phytoplankton? BNL biochemist Julie LaRoche and her colleagues reported in Nature on August 29 that they have devised a phytoplankton stress test and used it in the waters off of Vancouver, British Columbia.

The test takes advantage of the fact that phytoplankton living in iron-poor oceans "make do" by substituting an iron-free protein, flavodoxin, for their usual ferredoxin, which has to be made with iron. "So, by detecting whether the phytoplankton cell is making flavodoxin, we can tell if it's stressed for iron," LaRoche explained.

In May and September of 1995, LaRoche and her colleagues sampled populations of diatoms, a kind of phytoplankton known to be particularly sensitive to iron deficiency, at five stations in the ocean. Their 560-mile course ran from just off Vancouver to the center of the Pacific Ocean.

After collecting the samples, they mixed phytoplankton cells with antibodies that bound to flavodoxin and revealed how much there was. They found that phytoplankton taken from off-shore, iron-poor waters had nearly 200 times more flavodoxin than those from fertile waters close to shore. The test may help oceanographers with their experiments, since it can indicate phytoplankton iron-stress levels in water samples carried home from a research cruise, without sophisticated equipment or large-scale fertilization.

The study, which included scientists from the University of British Columbia, the University of Delaware and the Plymouth Marine Laboratory, was funded by DOE, the National Science Foundation and the Canadian Joint Global Ocean Flux Study.

Brookhaven National Laboratory carries out basic and applied research in the physical, biomedical and environmental sciences and in selected energy technologies. Brookhaven is operated by Associated Universities, Inc., a nonprofit research management organizations, under contract with the U.S. Department of Energy.

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