A Rare Heavy-Nitrogen Fingerprint Reveals How Much Nitrogen Microbes Quietly Remove From Water
UC Santa Barbara and UCLA researchers report in Science that molecules with two nitrogen-15 atoms expose microbial nitrogen loss that standard methods miss, from Texas groundwater to the Bay of Bengal.
Nitrogen is a building block of life, but only in the right amounts. Too much in water can degrade water quality and slowly kill aquatic organisms. Microbes remove nitrogen from water by converting it into a gas that escapes into the air, yet scientists have struggled to measure how much is removed that way. A study published in Science now offers a natural fingerprint for that process.
Biogeochemists at the University of California, Santa Barbara, UCLA and collaborating institutions showed that a rare form of nitrogen gas can reveal microbial nitrogen conversion that conventional methods obscure. The technique requires sophisticated machinery, so it will not reach routine field monitoring soon, but it could improve water-quality assessments and estimates of the global nitrogen budget.
"The problem is that our water naturally has a huge amount of nitrogen gas that is dissolved from the air, so the gas that microbes produce can be very difficult to see," said first author Jiarui Liu, who did the work as a postdoctoral fellow at UCSB's Marine Science Institute and at UCLA. "The answer is written in the way nitrogen atoms are paired inside the nitrogen molecule."
Nitrogen gas, N2, is two nitrogen atoms joined together. They come in two weights: the common nitrogen-14 and the heavier nitrogen-15, which has one extra neutron. Most N2 molecules contain two nitrogen-14 atoms, some contain one of each, and very rarely both atoms are nitrogen-15. In atmospheric nitrogen, the two heavy atoms pair up more often than chance would predict. The nitrogen produced by microbes pairs nearly at random, so when it mixes with air nitrogen it dilutes the excess of heavy pairs. The abundance of the rare molecule therefore shows how much gas microbes produced.
The team measured nitrogen extracted from water and sediment with UCLA's Panorama mass spectrometer, whose unusually large size lets it separate molecules that differ in mass by tiny amounts. "At UCLA, we discovered the anomalous pairing of heavy nitrogen atoms in Earth's atmosphere and are now making use of this signature of nitrogen in air as a powerful and unique geochemical tool," said geochemist Edward Young, a co-author.
The researchers applied the approach to Texas groundwater, lakes in Antarctica and Minnesota, coastal basins off Southern California, the Bay of Bengal and deep-sea sediments offshore from Alaska. Fertilizer runoff and wastewater have made humans a growing force in the nitrogen cycle, and excess nitrogen can fuel harmful algal blooms. When the blooms die and decay, they consume oxygen and can create dead zones that threaten fish and other aquatic life.
"We want to understand whether microbes in groundwater can mitigate nitrate pollution, and how much nitrogen is removed along the way before it can fuel algal growth in lakes, rivers and coastal waters," Liu said. Co-author Alan Seltzer of University College Dublin noted that many routine groundwater programs do not measure the N2 gas produced within an aquifer, "which can create substantial biases in our accounting of where nitrogen comes from and where it goes." The broader goal is a clearer measure of the pace of Earth's nitrogen cycle, the balance of supply and removal that helps sustain food webs around the world.
