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Growth in the sea comes down to a struggle for iron

28.02.2005


Scientists know that injecting iron into some major regions of the oceans can stimulate the growth of diatoms and other phytoplankton, but something odd occurs as these tiny marine plants continue to grow. They begin to starve in the midst of plenty, acting as though iron, an essential nutrient, still is in short supply. Why this happens is unclear, but the answer could be that iron sets off a kind of chemical warfare in the marine ecosystem, according to University of Maine oceanographer Mark Wells. And diatoms may not always come out on top.

In collaboration with a large Japanese research program, Wells and a team of scientists from UMaine and other universities are studying the fate of iron in marine waters. Their findings could help determine the fate of something else, a controversial proposal to address the threat of global warming. The National Science Foundation and the U.S. Department of Energy are providing financial support.

Field work got underway in July, 2004, when Wells served as chief scientist on the Kilo Moana, a research ship that left Hawaii, bound for the sub-Arctic waters of the western Pacific. Owned by the Office of Naval Research and operated by the University of Hawaii, the ship was loaded with equipment and supplies, ready for an extended stay at sea. Its subsurface pontoon hull give it more stability in choppy seas, a benefit to Wells who normally gets seasick on these voyages.



For nine days, she steamed northwest to rendezvous with a research vessel from the University of Tokyo, the Hakuho Maru, carrying a large group of Japanese scientists. The Japanese project is known as SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study), an ongoing effort that began in 2001 to study the ecological consequences of injecting iron into the ocean.

For more than a century, oceanographers puzzled over the low abundance of phytoplankton in some ocean waters that seemed to hold all the basic nutrients for life. Speculation that iron provided part of the answer goes back to the 1930s. However, it wasn’t until 1989 in the journal Nature that oceanographer John Martin reported the first series of iron experiments that indicated iron was indeed the missing ingredient. And he went further than that. Since growing plants absorb carbon dioxide, a powerful greenhouse gas, he suggested that natural increases in iron inputs to the oceans during the geologic past may have removed enough carbon from the atmosphere to affect global climate, perhaps even contributing to the onset of ice ages. Martin died in 1993 just as tests of his ideas were getting underway, but scientists have embarked on a series of experiments in three major areas of the world’s oceans.

"The three areas are the Southern Ocean around Antarctica, the equatorial Pacific out to about 140 degrees west and the sub-Arctic Pacific," says Wells. "The North Atlantic has a big spring bloom, just like we have on our coast, but these other areas have a persistent excess of nutrients because phytoplankton growth is reduced."

Many scientists from around the world are now studying iron effects in these regions. Though scientific efforts are aimed primarily at understanding these natural systems, there has been some speculation that iron fertilization might be useful in reducing the threat of global warming. However, this idea remains highly controversial in scientific circles given the very limited understanding of these unique systems, says Wells.

Wells specializes in chemical oceanography and has worked in places as varied as Rhode Island’s Narragansett Bay and Antarctica. Joining him on the July voyage were two UMaine graduate students -- Eric Roy and Lisa Pickell -- and postdoctoral researcher Jennifer Boehme. UMaine scientist Mary Jane Perry collaborates on the project but was unable to join the trip.

Also participating were professors Charles Trick of the University of Western Ontario and William Cochlan at San Francisco State University, along with their graduate students. Several members of the Japanese research team were onboard the American vessel. The Americans’ interest stems in part from the first SEEDS experiment in 2001. Japanese scientists had recorded the largest phytoplankton bloom of any of the iron fertilization tests conducted to that date. The question that remains unanswered is why the diatoms showed signs of nutrient stress before the iron and other nutrients were used up.

Wells and his colleagues think they may know. For clues, they have looked to a discipline that is far removed from the sea. Soil contains lots of iron, but most of it stays locked up in minerals, as accessible to microorganisms as the gold in Fort Knox. Bacteria and fungi have learned to scavenge some of this iron by building a trap. They create molecules called siderophores that are able to lock up iron. And in many cases, only the organism that built the molecule in the first place has the key to unlock it, says Wells.

"It’s basically chemical warfare by the bacteria in soils, trying to get the iron. They specifically target iron with these molecules. Siderophores don’t complex other metals very well. The idea is that it (the molecule) is like a magic bullet. They release it, it binds iron, and then only they have the key to unlock it and get the iron out of it. "Now in reality it’s warfare. In some cases other bacteria have figured out ways to get the iron from molecules that they didn’t produce. So, they can pirate that iron. It’s beginning to look like the same thing may be happening in the ocean," Wells explains. In July, by the time Wells and his colleagues arrived at their appointed location in the northwestern Pacific, the Japanese team had already injected iron into the water and were monitoring the growing phytoplankton patch, roughly six by eight kilometers in size.

"The patch changed quickly and constantly, breaking into two patches and streaming out into a long, narrow strip. The major night-time task was to steam in crisscrossing transects to re-do the map of the patch because of this rapid change," says Wells.

The two vessels worked together in the patch for 12 days but operated independently most of the time. New security guidelines prohibit scientists from boarding or even sharing samples or equipment with a vessel from another country.

The American team went to work lowering equipment over the side to collect water samples. They analyzed water chemistry, nutrients, and microorganism diversity. Working to assist their Japanese colleagues on board both vessels, they made these measurements to characterize how the phytoplankton responded to the iron enrichment over a 32-day period. They also ran experiments on deck to learn how available the iron was in the patch, how diatoms were growing and coming together in multi-cellular aggregations. They studied the sinking rate, the slow process through which aggregations of diatoms sink into the deep sea, taking their carbon with them.

The Kilo Moana returned to Hawaii after spending 42 days at sea. Early results suggest that the struggle for iron may indeed follow something like what happens in the soil. Wells and his colleagues are evaluating the data they collected and conducting additional laboratory experiments.

UMaine scientists are planning to return to the Pacific in 2007, but they have a different destination in mind. Work by Japanese and Canadian researchers has already shown that the consequences of iron additions may vary from the eastern to the western North Pacific. Even with additional iron, large phytoplankton cells in the east do not appear to grow as readily. Since prevailing winds tend to blow iron rich dust off the Asian continent into the Pacific, it may be that phytoplankton closer to the source of that dust may be primed to respond differently.

Wells and his colleagues will be studying the possibility that small variations in iron throughout the world’s oceans may have important ecological consequences as well. Their results will contribute to the diverse oceanographic understanding needed to determine whether or not adding iron to ocean waters is a wise tool in any attempts to manage a changing climate.

Mark Wells | EurekAlert!
Further information:
http://www.maine.edu

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