Wildfire Smoke is Alive with Microbes

Scientists are looking downwind of fire to understand smoke's full ecological footprint.

Texas high school student Sara Mims looked through her microscope at particles she had trapped from the air. She was trying to capture dust and fungal spores from a Saharan wind event across the globe. What she saw changed the trajectory of her high school science project. “I didn’t know what I was looking at. I was a high school student, not a microbiologist,” says Mims.

New Mexico's 2022 Hermit's Peak/Calf Canyon Fire

Pyroaerobiology — the study of smoke-borne bacteria, fungi, and other microbes — is growing more critical with the increasing frequency, size, and intensity of wildfires. Photo of New Mexico’s 2022 Hermit’s Peak/Calf Canyon Fire by Eric Splinter.

It was only after checking satellite images and seeing large smoke plumes arriving from fires across the Gulf of Mexico that she realized she was seeing charred material along with fungal spores. She found that she could trap and culture more fungal spores in smoky rather than clear air. This and other clever experiments led Mims and her research assistant — her dad — to propose that fire-generated air currents carry living microbes.

Surprisingly, Mims’s findings lay dormant for over a decade after she published them back in 2004. Nevertheless, her experiments are the first example of a growing field of research called pyroaerobiology, which lies at the intersection of fire ecology, atmospheric science, and microbiology. It’s the study of smoke-borne bacteria, fungi, and other microbes: who they are, where they go, and what they do when they get there. Pyroaerobiology stems from the observation that wildfire smoke is — in a sense — alive, which has implications for human health and the environment. And it’s growing even more critical with the increasing frequency, size, and intensity of wildfires.

Mims went down a different path in college, but fire scientist and prescribed burn manager Dr. Leda Kobziar has picked up where Mims left off. Kobziar was supervising a prescribed burn in Florida with her students back when she was an Associate Professor at the University of Florida. During the burn, she began to ponder an article she’d read on the use of bacteria to seed ice particles in snowmaking machines. She wondered if bacteria were similarly precipitating “super fog,” a dense fog that forms in the Southeast when fire smolders in wet fuels and the weather is cold. Digging deep into the research literature for evidence of bacteria in smoke, Kobziar found Mims’s paper, and only Mims’s paper.

“Finally I found the needle in the haystack, and it really was just one needle in a big haystack,” she commented in a NASA-sponsored presentation.

Now an associate professor at the University of Idaho, Kobziar says her path into fire science was sparked by her love of pine forests, her training as a wildland firefighter during college, and her desire to better understand the influence that fire has on ecosystems. “I had the realization that fire’s the most fascinating ecological force that I had ever encountered and how important it was for maintaining and conserving the ecosystems that I felt a strong affinity for.”

Kobziar and her team collaborated with scientists at the University of Idaho, the University of Florida, and the US Forest Service to confirm and build upon Mims’s findings with larger scale burns. And they did, using prescribed burns in longleaf and slash pine forests in Florida, along with experimental burns in a controlled lab setting. “I realized in that moment that I really needed to learn a lot more about microbiology,” Kobziar recalls, so she brought in additional expertise. They’ve since shown that smoke from both low- and high- intensity forest burns contains a higher number and diversity of microbes than does ambient air, and that most of the microbes are alive.

“There are so many implications, repercussions, and questions about what’s driving this phenomenon,” says Kobziar, especially given the kinds of microbes found in the smoke.

Their sampling of smoke from different fires often reveals multiple plant and human pathogens along with microbes that alter soil chemistry in ways that can stimulate plant growth. The smoke also contains endophytes, which are bacteria and fungi that live within plants and produce chemicals supporting plant growth, development, reproduction, communication, and defense. Also, many newly-identified bacterial species in the smoke are closely related to known endophytes. Some interesting species that can break down environmental contaminants are also along for the ride.

One significant concern that’s received a lot of attention is that smoke-borne microbes may cause respiratory disease in people. The potential implications for downwind ecosystems have gotten less of the spotlight.

Fire has influenced ecosystem evolution since plants appeared on land over 400 million years ago. The interactions and interdependence of plants and soil microbes shape ecosystems — and fire affects both plants and microbes. The impact of fire on land has traditionally been seen as local. For example, areas that burn regularly, or did prior to the modern era of fire suppression, contain fire-adapted plant and microbe species. But Kobziar’s research raises the hypothesis that smoke-borne microbes may extend the biological impact of forest fires well beyond the burn area.

One such impact may be the introduction of plant diseases to near and distant areas downwind from a fire, as postulated by Mims and Kobziar. The long-distance transport of pathogens in air currents has been documented. Coral and sugar cane infections in the Caribbean have been attributed to spores riding air currents from Africa, and evidence points to the airborne spread of a fungal disease from California to endangered white pines in New Mexico.

Mims speculates in her paper about a more positive impact: “A beneficial role for spores carried skyward with forest fire smoke could be reinoculation of fire-sterilized soil with mycorrhizal fungi and the dispersal of such symbiotic fungi to new locations.” Mycorrhizal fungi grow in association with roots, facilitating mineral and water uptake by the plant.

“I would say that there’s mounting evidence for that mechanism. I think it’s possible,” says Dr. David Vuono, a microbial ecologist at the Colorado School of Mines and part of Kobziar’s team. But, he adds, “making that link is going to be really difficult.” It’s one thing to model the impacts of incoming microbes in a controlled laboratory setup. Looking for downwind effects on an ecosystem level is much more challenging.

Taking on the challenge, Kobiar and collaborators are creating new ways to track microbes, hypothesizing that the smoke from wildland fires is a potential, and previously unrecognized, source of microbial and genetic diversity introduced into downwind ecosystems. This would be another potential positive impact, as higher species and genetic diversity in an ecosystem tends to increase its resilience to environmental shifts. How long different microbes survive in smoke plumes isn’t yet known, but the intense upward air currents of a large fire can launch particles high into the troposphere, with large smoke plumes crossing continents and oceans.

“There are countless examples around the planet that suggest that microbes can survive very challenging, harsh conditions for extended periods of time, at least long enough to be transported to a new ecosystem,” Kobziar notes. “My vision is that 30-to-50 years from now we’ll have a sort of atlas of what kinds of organisms tend to be aerosolized and survive in fires when they occur in a specific type of ecosystem with burn conditions of a particular fire regime.” (A regime refers to fire characteristics such as size, intensity, and frequency in a particular location.)

This atlas will enable scientists to understand the relative contribution of microbes emitted from wildfires to the overall dynamics of microbial exchange amongst terrestrial, atmospheric, and aquatic environments. Such exchange is a potential means of pathogen spread, but also an important source of biodiversity in ecosystems around the world.

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