Of all Earth's elemental cycles, nitrogen may be the most complex and costly. After all, it takes a lot of energy to turn the gaseous coupled nitrogen atoms that make up nearly 80 percent of our atmosphere into a usable form (most often, one atom of nitrogen paired with three of oxygen, or nitrate). Bacteria known broadly as diazotrophs can perform the trick with the aid of a special enzyme and a ready supply of iron (or, using high pressures and temperatures, nitrogen can be fixed industrially). Such nitrogen fixation, as it is known, is a fundamental limit on plant growth and an essential nutrient for all life. Yet, this critically important cycle remains poorly understood. "We don't really know where it's happening," says biogeochemist Curtis Deutsch of the University of Washington in Seattle. "We don't really know what environmental factors govern the rate or distribution of the process."
Deutsch and his colleagues aim to fix that by providing a better understanding of where nitrogen fixation thrives in the oceans. Toward that end they used global circulation models showing how ocean waters mix as well as chemical signals in the water to reveal where nitrogen fixation is likely occurring, according to their report in the January 11 Nature.
At the same time that diazotrophs are fixing nitrogen, other, older microorganisms work in areas of low oxygen, such as sediments or dead zones, to turn nitrogen back into the inert gas that makes up the majority constituent of our atmosphere. "These two processes of fixation and denitrification control how much nitrogen is available as a nutrient in the environment," Deutsch explains, adding that "human beings fix about as much nitrogen as the rest of the biosphere on Earth combined."
Specifically, the opposing processes of fixation and denitrification leave marks in the global ratio of nitrate and phosphate, another nutrient, in the water. Where nitrate is produced by fixation, the ratio will rise above normal as the diazotrophs consume phosphate but add nitrate, whereas denitrification removes nitrate without impacting phosphate levels, thereby lowering the ratio. Scientists determined that areas of nitrogen fixation rest near those zones in which oxygen is at a minimum and denitrification can occur; they came to this conclusion by combining measurements of nitrate and phosphate levels throughout the global oceans (collected by scientists over decades) with a model of ocean currents.
"An analysis of nutrient observations with an ocean circulation model was used to infer what the fixation had to be to account for the change in nutrients along pathways of ocean circulation and mixing," notes geochemist Jorge Sarmiento of Princeton University.
This evidence flies in the face of Sarmiento's own previous work, which had argued that nitrogen fixation must occur predominantly in the North Atlantic due to the iron-rich dust from continents that settles there. But it gains credence from other studies, such as a satellite map of "blooms" from one nitrogen-fixing microorganism--Trichodesmium cyanobacteria, or sea sawdust--that matches as well as supporting studies of various nitrogen isotopes in the nitrate itself and lab cultures of nitrogen fixers that show the tiny organisms will do their work even when surrounded with relatively high concentrations of nitrates.
But one key piece of evidence is notably absent: actual measurements of fixation going on at these spots in the Pacific Ocean. "The community will have a much easier time accepting this if there is actual biological evidence, if someone goes out and measures direct rates," notes Angela Knapp, a marine geochemist at the University of Southern California. And it remains unclear where such nitrogen fixers working in the Pacific would get the necessary iron. "The iron supply from [nutrient-rich waters] below combined with whatever amount of iron is delivered by dust through the atmosphere appears to be adequate," Sarmiento says. The living loci of one elemental cycle may have been found, but with it, so have a host of other mysteries.