Nitrogen in the air feeds the oceans

08/10/05

A USC oceanographer's long-term study shows that the marine food chain depends in large part on atmospheric nitrogen. The finding also demonstrates the oceans' massive absorption of greenhouse gas.

A decade-long USC study has written the ending to a long-standing mystery: Where do marine organisms in the tropical oceans get the nitrogen they need to grow?

In the process, the study also may help to explain how tons of carbon dioxide disappear into the ocean every day, slowing the progress of global warming.

Nitrogen is a building block of life and an essential nutrient for phytoplankton and other aquatic life. Biologists have long known that as dead organic matter decomposes in the depths of the ocean, nitrogen breaks free and drifts upward.

The problem is that not nearly enough nitrogen rises up to nourish all of the teeming life near the surface.

In a paper chosen for a commentary in the current issue of Nature, a team led by biological oceanographer Douglas Capone of the USC College of Letters, Arts and Sciences confirms that certain aquatic microorganisms draw huge amounts of nitrogen from the air.

Previous estimates suggested that nitrogen fixation from the atmosphere played only a very minor role in the oceans. The term "fixation" describes the process by which dinitrogen, an inert gas, is transformed into usable chemical forms such as nitrate, a compound of nitrogen and oxygen.

More recent geochemical estimates hinted at a larger role for nitrogen fixation. Capone's study provides direct evidence.

"Capone and colleagues now demonstrate, in the most exhaustive and comprehensive study so far, that over large regions of the tropical and subtropical Atlantic, biological N2 fixation is indeed substantial," writes Nicolas Gruber of the University of California, Los Angeles, in the News & Views section of Nature. "In a painstaking effort, they measured N2 fixation rates ... at more than 150 stations during six cruises.

"[N2 fixation] provides the ecosystem of the illuminated ocean with a source of new nitrogen that rivals the vertical supply of nitrate."

Capone said that his project, begun in 1994, has yielded "the most robust estimate" of the scale of nitrogen fixation.

"It's providing a rigorous assessment of how quantitatively important this process is," he said.

The study, published recently in Global Biogeochemical Cycles, focused on the marine organism Trichodesmium, the best-known "fixer" of atmospheric nitrogen.

Though it is only one of many nitrogen fixers in the ocean, Trichodesmium's contribution alone is nearly 10 times greater than previous estimates of oceanic N2 fixation worldwide.

"What makes Capone and colleagues' study particularly compelling is that they estimated N2 fixation rates using an array of independent methods, each with their own strengths and weaknesses. This results in an unprecedented level of confidence in the estimates obtained," Gruber writes in Nature.

The study has implications for climate science.

An old misconception, even within the scientific community, is that photosynthesis in the ocean removes carbon dioxide from the air. But Capone and others have pointed out that nitrogen rising from the depths brings with it enough carbon dioxide for photosynthesis by phytoplankton and other marine organisms.

Only photosynthesis that draws on nitrogen outside the ocean can cause a net removal of carbon dioxide from the air. External nitrogen comes from rivers, atmospheric deposition, and on a larger scale, N2 fixation, Capone said.

The new study's estimate of global N2 fixation is large enough to account for the uptake by photosynthesis of the 1.5 billion metric tons of carbon dioxide thought to enter the ocean each year. The amount represents 10 to 20 percent of annual carbon production, he said.

In theory, if Trichodesmium and other nitrogen fixers could be stimulated to grow, the oceans could increase their uptake of carbon dioxide.

Since N2 fixers are often limited by nutrients other than nitrogen - typically phosphorus or iron - seeding the ocean with such nutrients could lead to some reduction in greenhouse gases. In one venture chronicled in Nature, the musician Neil Young lent his yacht to a group that fertilized the waters off Hawaii with iron powder.

Capone counsels caution, citing studies that suggest large-scale ocean fertilization might eventually make the atmosphere more toxic.

But Capone's lifework has convinced research groups at the University of Maryland and Woods Hole Oceanographic Institution to incorporate N2 fixation as a variable in their models of carbon dioxide uptake and other biochemical cycles in the ocean.

It has been a long journey to substantiate the importance of nitrogen fixation, which was proposed decades ago by, among others, USC's Richard C. Dugdale.

"Dick first demonstrated in a small paper he published in Deep Sea Research in 1961 that there was some nitrogen fixation occurring in the sea with Trichodesmium," Capone said.

"I don't think many people believed him at the time."

Source: Eurekalert & others

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