Hometown U: Nitrogen is a key to unlocking nature's back stories

Hometown UMay 18, 2013 

As sea ice retreats and ringed seal become harder to reach, more polar bears are feeding on bowhead whale bone piles left from successful subsistence hunts. This photo was taken along the Beaufort Sea coastline of Alaska. (Photo by Susanne Miller, U.S. Fish and Wildlife Service)

High school chemistry class was probably the last time you heard anyone mention the periodic table of elements, which lists 118 building blocks for everything in nature. Nitrogen is one of those elements.

If you are Matt Rogers, manager of the stable isotope lab at UAA, nitrogen is a key to unlock nature's back stories. Take a tiny piece of animal tissue or a few plant leaves, some water or dirt, and with the help of high-end technology, he can use nitrogen to decipher the ecological story behind any of them.

Was that polar bear eating ringed seal or bowhead whale leftovers? How much of a wolf's diet was caribou and how much was salmon? Exactly what nutrients were available in the soil where that shrub grew?

Is it like reverse-engineering a recipe, figuring out everything that went into a chocolate chip cookie? He humored me with a yes, but said that instead of just identifying ingredients like sugar, flour and eggs, his tools can tell him exactly what the chicken that laid the egg had been eating.

Rogers finds the answer by distilling the nitrogen left in samples, called its "isotopic signature." An isotope is just a variation of an element. Nitrogen has two stable isotopes. Both have seven protons, but one has seven neutrons and the other has eight. The more neutrons, the heavier the nitrogen.

In nature, the heavier one reacts differently from the lighter one. It bonds tighter so it stays in your body longer and it also ends up in ocean systems more, he explained.

The ratio of heavy to light nitrogen varies. On land it might be 98 percent light and 2 percent heavy. In the ocean, it's more like 94 percent light and 6 percent heavy.

So, in determining the diet of wolves, if they're getting more of their food from salmon, the nitrogen balance in their body will match the heavier nitrogen ratio of the ocean. Add that to geographic information from radio-collared wolves, and you know where they've been and what they ate. Those results come from recent work on the diet of wolves in the Lake Clark region.

As the lab manager, Rogers works on everybody's samples, from master's students to collaborative efforts among UAA researchers and state and federal agencies, and even other universities in the Lower 48 and around the world.

"If you're working up here from the University of Arizona, it's a lot easier to ship us your water samples than try and get them all the way home safely," he said.

Also, now that UAA professor Jeff Welker is back from a Fulbright year at Norway's University Centre in Svalbard, the lab will soon see polar bear samples from there. That excites Rogers, who co-wrote a paper on the diet of polar bears off Alaska's northern coast.

The paper is in review for a journal called Polar Biology. Using heavy-light nitrogen ratios similar to those in the wolf diet study, Rogers and co-authors from UAA and the U.S. Geological Survey are focusing on whether Alaska's polar bears have started to feed off bowhead whale bones left on shore after successful subsistence hunts, perhaps a new foraging strategy in light of retreating sea ice and fewer available ringed seals.

The USGS team darted and collared five polar bears. Researchers took hair, fluid and tissue samples from each. Rogers showed me how the isotopic data for polar bear blood, hair, serum, fat and seal muscle fell out on an axis measuring nitrogen and carbon. The long hairs from the back of their forepaws were most telling, with a fistful of data points dropping away from the rest. That indicated depleted nitrogen.

Those "outliers" were key. "What is it that Asimov said?" recalled Rogers. "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka,' but 'That's funny ...' "

The depleted nitrogen ultimately revealed that the polar bears were getting less food from ringed seals, with their heavy nitrogen signature, and more food from bowhead whale bones. Whales and seals are far enough apart on the spectrum that their isotopic signatures are revealing.

All this detective work happens mostly out of sight in German-made spectrometers that use high heat (think 3,000 degrees F) to vaporize and ionize samples, turning them into gas and electrically charged particles that accelerate through a magnetic field at 200,000 miles an hour.

Rogers, who calls the analyzers "the Ferraris of the research world," is good at using similes to explain the process. "You've got this big cloud of nitrogen gas and you're shooting it around this bend. The heavy nitrogen is like a clunky old school bus. The light nitrogen is like a MINI Cooper or an Indy car. Lighter and more agile, the Indy car makes it around the bend and now you've separated the heavy and light nitrogen, and you have their ratio."

After three years as the overseer of the million-dollar UAA lab, Rogers is smitten by the power of the tools he works with. He's expecting an incoming project looking at the diet of brown bears in Lake Clark, running their hair and other samples through the Ferraris in the isotope lab.

 


Kathleen McCoy is an electronic media specialist at UAA, where she highlights campus life through social media.

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