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Bereft of trees, the Arctic tundra unfolds across Alaska, Canada, Russia, Greenland, Iceland and Scandinavia in sweeping vistas. Here in Earth’s northern reaches, the frozen landscape thaws briefly in summer, enabling shrubs, grasses and other hardy plants to grow in shallow, nutrient-poor soils.

But for Ph.D. student Perrin Hagge, it’s not rooted vegetation that has fastened his attention. Hagge instead focuses on the mottled patches of lichen and moss that quilt this frigid region.

“I’m interested in how nitrogen moves through the Arctic, and particularly how that’s mediated by lichens and mosses, and what climate change means for that process,” says Hagge, who studies in the lab of Nicolas Cassar, a biogeochemist at the Nicholas School of the Environment.

Though evocative of plants, lichens instead consist of a fungus living symbiotically, or interdependently, with at least one type of photosynthetic alga or bacterium. By contrast, mosses are plants, but instead of true root systems, they grow hair-like strands called rhizoids that anchor them to rocks and other surfaces.

In the Arctic, lichens and mosses play outsized roles as conduits for nitrogen, an essential nutrient plants need to grow. Specifically, lichens and mosses associate with certain bacteria that convert atmospheric nitrogen — unusable by most animals and plants — into a life-sustaining form. This process is called nitrogen fixation.

With guidance from Cassar, Hagge has been at dozens of sites across the Arctic to study how environmental factors associated with climate change — namely, shifts in temperature, light and water availability — affect nitrogen fixation. The findings could ultimately shed light on the region’s capacity to store carbon, according to Hagge.

“The Arctic is undergoing some of the most rapid environmental changes on Earth, yet we still know surprisingly little about how nutrient cycles will respond. Hagge’s research is helping uncover one of the key mechanisms linking warming, nutrient availability and carbon sequestration in Arctic ecosystems,” says Cassar, who is also the Nicholas School’s Lee Hill Snowdon Bass Chair.

Rapid Arctic warming is having seemingly contradictory consequences. On the one hand, permafrost is thawing and releasing carbon into the air. But the Arctic is also greening, with new plant species moving into higher, formerly inhospitable latitudes — and plants absorb carbon from the air through photosynthesis.

“With melting permafrost, you have projected greater carbon release. But with the same kind of warmer temperatures that cause the melting permafrost, you have the possibility to sequester more carbon [because] increased temperatures ramp up photosynthesis,” Hagge says. He wants to know: “Will the amount of nitrogen fixation [that is occurring] be able to sustain carbon sequestration in the Arctic?”

Through a series of experiments, Hagge has been exposing his moss and lichen samples to changing ambient conditions and measuring rates of nitrogen fixation.   

“What we’re trying to capture is where their optimal temperature is, or their thermal performance curve. How do they ramp up their nitrogen fixation as the temperature increases, and at what point does temperature then begin to inhibit their fixation ability?” Hagge says.

So far, his results suggest that the rate of nitrogen fixation slowly increases until an optimal temperature, which depends on the sample’s native latitude. After that point, nitrogen fixation abruptly declines, likely because proteins involved in the process deteriorate after a certain temperature threshold.

Additionally, Hagge has found that a minimum amount of water is required to start nitrogen fixation, and that additional water after this threshold further increases the rate of nitrogen fixation.

Taken together, Hagge’s findings suggest there is a balance between temperature and water necessary for nitrogen fixation in lichens and mosses that could change over time. For instance, if the temperature is optimal but water is scarce, nitrogen fixation will be limited.

As he continues his Ph.D. program, Hagge will delve further into connections between temperature and nitrogen fixation in lichens and mosses.

“The more we can get a better sense of how nitrogen fixation could affect temperatures in the Arctic by supporting carbon sequestration, the better we can model carbon cycling, and the better climate projections we will have,” Hagge says.