A decade-long USC study has written the ending to a long-standingmystery: Where do marine organisms in the tropical oceans get thenitrogen they need to grow?
In the process, the study also may help to explain how tons ofcarbon dioxide disappear into the ocean every day, slowing theprogress of global warming.
Nitrogen is a building block of life and an essential nutrient forphytoplankton and other aquatic life. Biologists have long knownthat 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 nourishall 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 USCCollege of Letters, Arts and Sciences confirms that certain aquaticmicroorganisms draw huge amounts of nitrogen from the air.
Previous estimates suggested that nitrogen fixation from theatmosphere 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, acompound of nitrogen and oxygen.
More recent geochemical estimates hinted at a larger role fornitrogen fixation. Capone's study provides direct evidence.
"Capone and colleagues now demonstrate, in the most exhaustiveand comprehensive study so far, that over large regions of thetropical and subtropical Atlantic, biological N2 fixation is indeedsubstantial," writes Nicolas Gruber of the University of California,Los Angeles, in the News & Views section of Nature. "In apainstaking effort, they measured N2 fixation rates ... at more than150 stations during six cruises.
"[N2 fixation] provides the ecosystem of the illuminated ocean witha source of new nitrogen that rivals the vertical supply of nitrate."
Capone said that his project, begun in 1994, has yielded "the mostrobust estimate" of the scale of nitrogen fixation.
"It's providing a rigorous assessment of how quantitativelyimportant 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 thanprevious estimates of oceanic N2 fixation worldwide.
"What makes Capone and colleagues' study particularlycompelling is that they estimated N2 fixation rates using an array ofindependent methods, each with their own strengths andweaknesses. This results in an unprecedented level of confidence inthe estimates obtained," Gruber writes in Nature.
The study has implications for climate science.
An old misconception, even within the scientific community, isthat photosynthesis in the ocean removes carbon dioxide from theair. But Capone and others have pointed out that nitrogen risingfrom the depths brings with it enough carbon dioxide forphotosynthesis by phytoplankton and other marine organisms.
Only photosynthesis that draws on nitrogen outside the ocean cancause a net removal of carbon dioxide from the air. External nitrogencomes 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 toaccount for the uptake by photosynthesis of the 1.5 billion metrictons of carbon dioxide thought to enter the ocean each year. Theamount represents 10 to 20 percent of annual carbon production,he said.
In theory, if Trichodesmium and other nitrogen fixers could bestimulated to grow, the oceans could increase their uptake ofcarbon dioxide.
Since N2 fixers are often limited by nutrients other than nitrogen -typically phosphorus or iron - seeding the ocean with suchnutrients could lead to some reduction in greenhouse gases. In oneventure chronicled in Nature, the musician Neil Young lent hisyacht to a group that fertilized the waters off Hawaii with ironpowder.
Capone counsels caution, citing studies that suggest large-scaleocean fertilization might eventually make the atmosphere moretoxic.
But Capone's lifework has convinced research groups at theUniversity of Maryland and Woods Hole OceanographicInstitution to incorporate N2 fixation as a variable in their modelsof carbon dioxide uptake and other biochemical cycles in the ocean.
It has been a long journey to substantiate the importance ofnitrogen fixation, which was proposed decades ago by, amongothers, USC's Richard C. Dugdale.
"Dick first demonstrated in a small paper he published in Deep SeaResearch in 1961 that there was some nitrogen fixation occurring inthe sea with Trichodesmium," Capone said.
"I don't think many people believed him at the time."
Funding for Capone's project came from the National ScienceFoundation.
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