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Nitrogen cycle in oceans surprises researchers
Once thought to occur half a world apart, two key parts of the global nitrogen cycle are actually occurring side by side, according to a new study. Shrinking the distance between the two processes from thousands of kilometers to within the same ocean means nitrogen could cycle through the oceans much more quickly than previously thought — potentially speeding up how quickly the oceans can respond to global climate change, the researchers say.
Scientists studying ocean sediments from the past thousands of years have observed that when Earth comes out of an ice age, nitrogen loss from the oceans “ramps up sharply,” says Curtis Deutsch, an oceanographer at the University of Washington. That leaves less nitrogen available to the microscopic ocean plants that need it for photosynthesis, which in turn means that less carbon dioxide is pulled out of the atmosphere, leading to a warmer, greenhouse world.
For scientists to determine how much nitrogen might be lost during such a cycle, they need to know rates for both sides of the process: denitrification, in which specialized bacteria convert organic nitrogen to inorganic nitrogen gas; and the inverse, nitrogen fixation, where different types of bacteria transform inorganic nitrogen gas into its primary organic forms, nitrate and nitrite.
Scientists have a good grip on where denitrification is happening in the oceans, Deutsch says. Because denitrification tends to happen in low-oxygen conditions, it occurs primarily in the oxygen-poor Pacific and Indian oceans, rather than in the Atlantic.
Understanding nitrogen fixation has been much trickier, however, Deutsch says. It made intuitive sense that denitrification and nitrogen fixation would happen near each other, he says, because removing the nitrate from seawater during denitrification should give nitrogen-fixing bacteria a big competitive advantage: Unlike many other bacteria, they don’t need nitrate in order to thrive.
While examining the life cycle of the most common known nitrogen-fixing ocean bacteria, however, scientists have previously found that nitrogen-fixing bacteria need abundant iron in order to thrive. As iron is much more readily available in the relatively narrow Atlantic Ocean, where trade winds carry iron-rich dust from the coast of Africa, than in the far larger Pacific, scientists reasoned that nitrogen fixation must therefore be occurring more intensely in the Atlantic compared to the Pacific, Deutsch says.
To get a better sense of where most nitrogen fixation was occurring, Deutsch and his colleagues decided that rather than considering where the nitrogen-fixing bacteria might prefer to live, they would look instead for evidence of their activity — specifically, the ratio of nitrogen to phosphorus in the waters. During nitrogen fixation, the bacteria consume phosphorus while producing fixed nitrogen, altering the overall ratio of the two nutrients. The team examined the global distribution of these nutrients throughout the world’s oceans, looking for places where the phosphorus-to-nitrogen ratio in seawater decreased as it moved with the ocean currents.
To their surprise, these ratios revealed that more nitrogen fixation was occurring in the Pacific and Indian oceans, rather than the Atlantic, the team reported Jan. 11 in Nature. That means the two processes are actually occurring much closer to each other, suggesting that the feedback between the two, and their effect on climate change, could be significantly different than previously estimated.
“If you have a window in your house that keeps blowing open in the wind, and the thermostat is on the other side of the house, the room might get quite cold before the heat kicks in,” Deutsch says. “But if it’s right next to the window, the thermostat could respond much more quickly.”
In the same way, he says, if the nitrogen-fixing bacteria are living right next door to the denitrifiers, they can respond much more quickly if the nitrogen supply is reduced — providing less nitrate to fuel carbon dioxide-consuming photosynthesis, for example.
A link between denitrification and nitrogen fixation “makes eminent sense,” and suggests that the ocean nitrogen cycle may be in balance after all, if much more nitrogen fixation is occurring in large parts of the Pacific than had been thought, says Douglas Capone, a biological oceanographer at the University of Southern California. The discovery is just one of several that have shaken up the nitrogen world in recent years, including the discovery of an abundance of ancient nitrogen-fixing organisms called Archaea in oceans, and the discovery of a brand-new biological pathway for nitrogen removal, he says.
“We thought ten years ago that we understood the nitrogen cycle,” but now, Capone says, scientists have learned to “be more cautious.”