2 September 2016

An Arctic fixation

Posted by llipuma

This is the latest in a series of dispatches from scientists and education officers aboard the National Science Foundation’s R/V Sikuliaq. Read more posts here. Track the Sikuliaq’s progress here.

by Kim Kenny

An introduction to the research

Two teams will do separate but related scientific work aboard the Sikuliaq over the next month. The following is an overview of their proposed research and what they expect to find. A previous post summarized the work of the CEOAS team.

Dr. Rachel Sipler leads the six-person research team from the College of William & Mary’s Virginia Institute of Marine Science (VIMS) and the University of California, Santa Cruz (UCSC). They’ve set out to investigate how widespread nitrogen fixation – the process by which nitrogen is taken from the atmosphere to make living material – is throughout the Arctic. What they find could necessitate a reevaluation of Arctic biogeochemistry and change our understanding of global nitrogen and carbon cycles.

Is nitrogen fixation really happening in the Arctic?

Normally no one would even think to ask this question. Of course nitrogen fixation isn’t happening in the Arctic, because it’s too cold. Nitrogen fixation has always been thought to require warmth and a lot of light.

But turns out that’s not true. Surprisingly, nitrogen fixation is happening in the cold waters of the Chukchi Sea.

Sipler and VIMS colleague Deborah Bronk found this to be the case during an Arctic research cruise in the summer of 2011. Another research group at the Universite Laval simultaneously did a study that found nitrogen fixation occurring in the Canadian Arctic. And a research team headed by Margaret Mulholland at Old Dominion University found nitrogen fixation happening in the Atlantic Ocean, at temperatures comparable to the Arctic.

All these studies were conducted in marine, not freshwater, environments. Nitrogen fixation is common in freshwater, even Arctic freshwater, since proximity to land provides sources of terrestrial carbon. (That proximity may also play a role in marine nitrogen fixation.)

Is this a new phenomenon, or just newly noticed? Sipler thinks it’s probably the former, partly because another study (a 1961 dissertation by Dale Toetz from the University of Wisconsin) near Barrow, Alaska looked at the same thing at the same time of year, using the same methods, and got different results – he found no nitrogen fixation. It could be that it was happening and Toetz simply didn’t see it, but Sipler thinks that’s probably too big a coincidence to be true.

These studies are some of the only clues scientists have to the untold story of nitrogen fixation in the Arctic Ocean. They are meager data points the VIMS/UCSC team seeks to vastly increase on this trip.

With that information, they’ll be better able to say if nitrogen fixation is indeed happening in the Arctic, who’s doing it, how fast they’re doing it, and how these findings fit with other sources of nitrogen in the larger global nitrogen budget.

All life depends on nitrogen. It is the most abundant gas in our atmosphere. But we can’t use it unless nitrogen fixers make it accessible by changing it into different forms of nitrogen, like ammonium, which both our cells and plants use to grow.

Nitrogen is a limiting nutrient in many parts of the ocean, meaning it determines how much food our planet gets by limiting the growth at the bottom of the food chain. Nitrogen sets a “grow no further” bar for phytoplankton, which in turn limits food supply for most everything else in the ocean.

When it comes to climate change, a lot of attention is given to carbon budgets, and not as much focus on nitrogen. But nitrogen actually plays a huge role in the carbon budget. The amount of available nitrogen in the water limits the amount of carbon that can be taken out of the atmosphere and “put” into the water, a process called carbon sequestration. If it turns out there is more nitrogen in the Arctic, that means the ocean has a greater ability to sequester carbon than previously thought.

And that’s a big deal, because it changes our understanding of climate change. It means there might be less carbon polluting our atmosphere since polar oceans are buffers for more carbon dioxide. (Of course, more carbon in the oceans generally means increased ocean acidification. So there’s that. Welcome to the trade-offs and complexities of climate science.)

Of the carbon dioxide the oceans absorb, the polar regions play a huge part.

Polar oceans are known to be huge sinks for carbon dioxide. If Sipler’s team finds there is more nitrogen in the Arctic, polar oceans might be an even larger carbon dioxide sink than scientists thought.

Who’s doing the fixing?

Nitrogen won’t just fix itself. We need diazotrophs – nitrogen fixing bacteria and archaea – for that. The question is what type.

The diazotrophs Sipler and the members  of her team from UCSC (Jonathan Zehr and Kendra Turk-Kubo) are looking for in the Arctic could be cyanobacteria – which get their energy from light – or heterotrophs – which get their energy from organic matter. The differentiation is important because who they are determines how much they contribute to the global nitrogen budget.

If the diazotrophs fixing nitrogen in the Arctic turn out to be photosynthetic cyanobacteria and dependent on light, they’re fairly limited to surface waters and to the warmer summer season when they have more access to sunlight.(Remember: less sea ice generally means more sunlight can get through to organisms in the water.)

If it turns out the diazotrophs are using organic matter instead, they’re not necessarily limited to shallow water or limited by timing. That would mean nitrogen fixation is probably happening year-round in the Arctic. And that would mean there’s a way to get a lot more nitrogen into the ocean that we haven’t accounted for.

Is nitrogen fixation widespread throughout the Arctic? If the answer is yes, we need to reevaluate Arctic biogeochemistry, and perhaps global nitrogen and carbon cycles.

It’s important to note that the rates of nitrogen fixation Sipler and Bronk originally saw in the Arctic were small – a lot smaller than those demonstrated to be happening in warmer waters. But even really low rates can be significant. Based on what little data is currently available, 1-7.5% of global nitrogen fixation happens in the Arctic, which might not sound like a lot, but oceanographers recognize it as a huge portion.

Whether diazotroph contributions to the global nitrogen budget are substantial or limited, either finding would be a crucial piece of the story.

And so Sipler’s team, along with the CEOAS team, will set out to observe whatever truth the Arctic offers. And we’ll have some fun along the way.

This post is coming to you from Alaska Airlines flight 137, somewhere between Portland and Anchorage. Our next update will be posted from Nome, when our whole gaggle of scientists has assembled there tonight. After a night in Nome, our focus will turn to the business of final preparations before embarking on Sept 2. Stay tuned!

— Kim Kenny is a freelance journalist who specializes in science writing and multimedia. This post was originally published on thedynamicarctic.wordpress.com.