University of Minnesota researcher digs into hidden world of lichen to measure the air we breathe

On a cool August morning at Fort Snelling State Park, the bright songs of crickets drifted up from tall grasses. In the background, the din of hundreds of cars vibrated on nearby Hwy. 5 and Hwy. 55, with the occasional airplane adding to the clamor.

The urban park, right next to a source of pollution, was the perfect spot for Natalia Mossmann Koch. The University of Minnesota researcher was looking for a tree to place air pollution monitors.

But the technology she is using is novel, for Minnesota: Mossmann Koch will rely on measurements taken from chunks of lichen, tied to the outside of two eastern cottonwoods in this park. And by analyzing the specimens over a year, she hopes to glimpse the heavy metals they may have absorbed from the surrounding air.

Mossmann Koch said she plans to do the same in urban parks around the state. Her study will focus both on examining which lichens are naturally present — giving an idea of whether the air is clean enough to support more sensitive types — and testing her transplants to see how many heavy metals they absorb.

A year of repeated intrusions of wildfire smoke in Minnesota and other parts of the country has heightened public awareness about what's in the air. Mossmann Koch hopes to develop a cheaper and more accessible way for people to measure air pollution by introducing them to the complex world of lichens.

Lichens are assemblages of fungi and algae that slowly propagate on tree bark, rocks and railings. With no roots, the plants are entirely reliant on air and the humidity drifting through it for water and nutrients. That means they can suck up toxic substances, too.

Scientific literature is full of examples of other researchers who have used lichen as a broad proxy for air quality. One 1997 paper from Italian academics compared a map of lung cancer rates in the Veneto region with areas where there was a low diversity of lichens. In a near-perfect correlation, fewer lichen species meant more cancer cases.

Mossmann Koch remembers the first lichen to stop her in her tracks. The researcher, 38, started her academic career in her native Brazil and migrated to the United States to work at the U in 2020.

As an undergrad, she was walking through a forest near the southern city of São Francisco de Paula with her professor when she spotted something bright red clinging to a tree.

The professor, an ecologist who was proficient in fungi, didn't know the name, and it was years later when Mossmann Koch finally learned the English common name for the species — the Christmas lichen. It was her opening into an incredibly complex world.

There are about 850 known lichens in Minnesota, with more still being discovered. The ones that thrive here have to survive an arid, arctic winter — so they can stay viable with 5% to 10% water content.

Ask whether a lichen can be considered its own organism and you "step right into deep metaphysics," said Daniel Stanton, an assistant professor who runs the lab where Mossmann Koch works as a postdoctoral researcher at the U.

Lichens are "the closest thing we have on land to corals," Stanton said. Corals, after all, are also dependent on a symbiotic relationship with algae — and when they become "bleached" from stress, it means a colorful layer of symbiotic algae has died off.

"Or," Stanton said, "we prefer to think of corals as the lichens of the sea."

Mossmann Koch is using samples of common green shield lichens, taken from the pristine Cedar Creek Ecosystem Science Reserve in Anoka County, as the air sensors in her study. The specimens, still affixed to the bark they grew on, look like a picture book definition of a lichen — flat, wavy, a silvery blue-green.

But hold up a magnifying lens to the surface of a lichen-covered tree — like the Fort Snelling cottonwood Mossmann Koch used to tie up her green shield lichens — and suddenly, a whole community appears.

The lens reveals orange waves and soft yellow strings and grayish tubes capped with black discs, like something from an alien planet. Even in an urban park near car and plane exhaust, at least four species are growing.

"I'm going to try to find a pattern comparing the heavy metals or pollution, and urbanization around the site," Mossmann Koch said, "and be able to communicate [that pattern] to the people."

And if four species sounds like a lot, take the example of a tree branch in Stanton's lab, taken from rural Itasca County — with a quick microscope examination, he and Koch found at least 12 different lichens on the 5-inch sprig.

At the six-month and one-year mark, Mossmann Koch will also take down her lichen monitors and test them in the lab for signs of vitality. She'll flash them with light and measure the UV rays the plants emit back after doing photosynthesis, and gust them with carbon dioxide in a special machine to see how much they absorb.