Tracking Nitrogen’s Elusive Trail in the Ocean

Humans often seem to be unable to fix a problem without creating a new one. We invented DDT to kill mosquitoes and stop the spread of malaria, but almost caused the extinction of bald eagles and other birds. For industry and transportation, we developed fossil fuels, whose greenhouse gas emissions are now causing global climate change. And we invented a way to extract nitrogen from the air to make fertilizers that bolstered agriculture and helped feed hungry populations. But ultimately that has upset the balance of nitrogen, especially in the oceans, causing a rash of new dilemmas that we now confront.

Fertilizers have leaked into the oceans through wastewater and runoff from farms and lawns, loading coastal waters with excess nitrogen that fertilizes the rampant growth of marine plants. The decomposition of these multitudes of phytoplankton removes oxygen from seawater, creating oxygen-poor “dead zones” where fish cannot live. The situation also may be leading to the increased production of nitrous oxide, a gas that traps heat 300 times more efficiently than carbon dioxide and also destroys ozone. (See Another Greenhouse Gas to Watch: Nitrous Oxide.)

Any attempt to restore the nitrogen balance on our planet requires far more understanding than we now have of how nitrogen moves through the environment. To find solutions to these problems, we first need to determine how much nitrogen is entering the ocean, what happens to it once it gets there, and how much eventually gets removed or recycled back to the atmosphere. But tracking nitrogen—as it gets incorporated into various chemical compounds in the air, in the ocean, and in organisms—has been a daunting challenge.

Nitrogen’s journey through the oceans is largely mediated by microbes. To live and grow, a variety of microbes use nitrogen in different ways, metabolically transforming it into diverse chemical compounds. They cycle it quickly and often concurrently in the same places in the vast ocean, so that we humans can’t easily measure the chemical processes that are taking place.

But we’re trying. As a graduate student in the MIT/WHOI Joint Program, I am working on a new method using a natural tag to follow nitrogen’s trail. The tag that I measure can record nitrogen’s chemical life history: where it came from, what chemical reactions it has undergone, and how and where it ends up.

Fixing nitrogen and food shortages

Nitrogen gas (N₂) makes up 78 percent of our atmosphere, but most organisms cannot utilize this source of nitrogen to grow, because the nitrogen atoms are held together by a tight triple bond so strong that it requires extreme temperatures and pressures to break.

In the early 20th century, German scientist Fritz Haber won the Nobel Prize for inventing a method to rip this bond apart and create other nitrogen-containing compounds that are more available for industrial use. He needed to use temperatures of 932° to 1,832°F (500° to 1,000°C) and pressures 100 to 250 times atmospheric pressure to break the triple bond.

The Haber process starts with so-called “non-fixed” forms of nitrogen, which aren’t easily usable—nitrogen gas (N_²), for example, or other nitrogen-containing gases such as nitrous oxide (N₂O) and nitric oxide (NO). The process converts them into easily usable “fixed” forms of nitrogen, such as ammonia (NH₃), nitrate (NO₃^–), or nitrite (NO₂^–), which are essential nutrients for organisms to grow.

The Haber process provided a new supply of “fixed” nitrogen to make fertilizers that are now responsible for sustaining one-third of the world’s population. In the future, however, with rising population and demands for food and energy, human-made sources of nitrogen in the environment are projected to double the natural input to the environment, reported two of the world’s leading nitrogen cycle experts, Nicolas Gruber of the Swiss Federal Institute of Technology and James N. Galloway of the University of Virginia, in the journal Nature in 2008.

Which brings us to our current predicament.

Agricultural runoff can overload coastal waters with nitrogen. Every year, for example, a mother lode of nitrogen drains into the Mississippi River, mostly from agricultural lands, and pours into the Gulf of Mexico. It stimulates a phytoplankton bloom and then an oxygen-depleted dead zone covering an area almost the size of New Jersey.

Nutrient-rich, oxygen-poor waters like these support a frenzy of nitrogen cycling, in which certain bacteria use nitrates as an energy source. As they eat the nitrates, they form nitrites as an intermediate chemical byproduct. What happens to the nitrites is key. They can either be converted back to nitrates and reused as food by other microbes. Or they can be converted to nitrogen gas and released back to the atmosphere. How can we follow what happens to the nitrites?

Isotopes to the rescue

With the help of my Ph.D. advisor, Karen Casciotti, I’m developing a way to use isotope ratios to track different chemical transformation processes involving nitrogen. Isotopes are atoms of the same element that have different weights. In my case, I study ¹⁴N and the heavier ¹⁵N, as well as two isotopes of oxygen: ¹⁶O and ¹⁸O. The key is that in chemical reactions, atoms with different weights behave a little differently.

Imagine your mom asks you to move 20 bricks from a pile of 100 bricks in the driveway to the basement. Some weigh 14 pounds, let’s say, and the others weigh 15 pounds. Which bricks will you choose to move? The light ones, of course! After you are done moving your 20 bricks, the pile in the basement will have a larger ratio of light to heavy bricks than the pile in the driveway.

Similarly, microbes moving nitrogen around with their biochemical reactions usually pick lighter or heavier isotopes of nitrogen. So different microbes using different chemical reactions will produce end products with different ratios of heavy and light isotopes. We call this phenomenon an “isotope effect,” and if we can measure these isotope effects, we can distinguish the reactions that produce them.

Sorting out the nitrogen cycle

In the Watson Laboratory at Woods Hole Oceanographic Institution, I’m conducting experiments with marine nitrite-oxidizing microbes first discovered in the 1970s by Stanley Watson (after whom the building is named) and John Waterbury, a scientist emeritus on the floor below me. Waterbury became an expert at culturing marine microbes like these and maintains a “library” of microbes in the Watson Lab that other scientists can use in their experiments. Nitrite-oxidizing microbes convert nitrite back into nitrate, and they are the only natural source of nitrate to the oceans.

In my experiments, I measured the isotope ratios of nitrogen and oxygen atoms in nitrate produced by these microbes from nitrite. We’ve discovered something unique about these bacteria—they actually prefer to move the heavy isotopes, ¹⁵N. That’s unexpected and interesting, but more important, it reveals something new about the oceanic nitrogen cycle. This information helps us track chemical pathways and determine how much nitrite is recycled naturally back into nitrate, which is reused by other microbes in the ocean, and how much is converted into nitrogen or nitrous oxide gas, which is released back into the atmosphere.

Our experiments offer the promise that we can perform similar isotopic analyses of other chemical reactions that are going on in the ocean and begin to reveal exactly what is happening to the extra nutrients we are adding to our oceans. Can we continue to “fix” nitrogen into fertilizers without disrupting the natural environment? We won’t know until we better understand exactly how human activities are affecting the nitrogen cycle. And that’s where I can help.

Carly Buchwald was supported by the J. Seward Johnson Fund and an MIT Presidential Fellowship. The research is funded by the National Science Foundation. This article was written during a science writing course for graduate students at WHOI, supported by funds from The Henry L. and Grace Doherty Professor of Oceanography.