Microbes Are Aerosolized and Moved Around by Wildfire Smoke

Craig Shoemaker, a meteorologist with the National Weather Service in Sacramento, told the New York Times, “Without the smoke, it would be a clear day. This is all generated from the fires.”

We know that prolonged exposure to smoke isn’t good for your health, and is associated with risk of asthma, strokes, respiratory infection, and more. Yet while smoke has long been considered harmful, it was, until very recently, thought of as an inert substance—made up of particulate matter, but not alive. 

That's no longer the case. In 2018, a paper published in Ecosphere reported that researchers who sampled smoke found the plumes teeming with microbial life. It was a first; microbes had never been directly measured in wildfire smoke before. This was the birth of the burgeoning field of pyroaerobiology, which seeks to study how microbial life is aerosolized and moved around by wildfire smoke, and what the potential implications are on ecology and human health. 

Now that we know that smoke is alive, or rather, contains life, there’s important work to do, said Leda Kobziar, a fire ecologist at the University of Idaho, and one of the lead figures in pyroaerobiology. Our exposure to smoke, and all its microbes, is increasing. The wildfire season in the western U.S. is now 105 days longer, burns six times as many acres, and has three times as many large fires, compared to the 1970s. This is exacerbated by the climate crisis and its increasing temperatures and longer drought periods. Over the past 40 years, the area burned by wildfires has quadrupled in the U.S. A new study this month found that wildfire smoke accounted for about half of the air pollution in the western U.S. during the worst wildfire years.

The microbes in smoke could have many impacts—from altering ecosystems they land in, potentially being pathogenic to plants or humans, or not making a difference at all. There are still a lot of unknowns, but one thing pyroaerobiolgy can say for sure is that the microbes are present in the smoke, and we should probably start paying attention to them. 

“All of the pieces started to fall together suggesting that this was not an unusual phenomenon," Kobziar said. "This is something that was happening across a variety of different types of ecosystems and in a wide variety of wildfire situations." 

She stumbled onto pyroaerobiology by accident. A graduate student needed a project to work on, and a friend had recently sent her an article about bacteria being used in snow-making machines in Hunter, New York, where she grew up skiing.

To create man-made snow, many ski facilities use products like Snomax, which contain a bacteria, Pseudomonas syringae. This bacteria is an ice nucleator, which means that it has proteins in its cell walls that help ice to form at higher temperatures. 

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It led Kobziar to wonder about smoke: Could there be microbes in smoke that could, potentially, be meaningfully changing their environments, the way that Pseudomonas syringae affected its surroundings? 

When she looked to see if anyone had studied microbial life in smoke before, she found only one related paper from 2003, by teenager Sarah Mims and her father. They didn’t directly measure the smoke, but found (using a smoke detector attached to a kite) that there were more fungal spores in the air when there also was more particulate matter floating around. Kobziar called their work a “great seed to start the creation of pyroaerobiology.” 

Kobziar and her colleagues' first experiments sampled the smoke from prescribed fires at the University of Florida's Austin Cary Forest in 2015, and also smoke from experimental lab fires. They found 70 aerosolized microbial strains, both pathogenic and non‐pathogenic. 

“It started as just a curiosity,” Kobziar  said. “But after our first study, we found that there were so many microbes that were being aerosolized in smoke that I took it upon myself to learn as much as I could about microbiology.”

The communities of organisms that were aerosolized were incredibly diverse and much different than what was present in the air when there wasn’t any smoke. And—unexpectedly— the majority of these microbes were alive.  

“That might be the biggest surprise for all of us,” Kobziar said. Another feature that the researchers have observed: when comparing high-intensity fire to low-intensity fire, there was a higher quantity of microbes and higher levels of diversity in the high-intensity fire.

The studies they’ve done since have started to compare different temperatures—from flaming to smoldering—and fires created from the burning of different kinds of materials. It could be that certain microbes survive in different smoke environments, or that the movement of the microbes into the air is more likely for one fire than another. These logistics are still unknown. 

“The potential for smoke to aerosolize and transport viable microbes is a virtually blank piece of the microbial biogeography puzzle with far‐reaching implications,” as they wrote in their 2018 paper. 

Because fire is extremely hot, there's long been an assumption made that it would kill any microbes it encountered. “Combustion is, after all, one way of sterilizing stuff,” said David Vuono, a microbiologist and research professor at the Colorado School of Mines and the Desert Research Institute. “It was just one of these things that gets overlooked.”

At the last large microbiology conference Vuono went to, in 2019, no one brought up microbes in smoke. “I went to an aerobiology session and not a single mention of this,” he said. 

How can microbial life survive in a smoke plume? “That’s a really good question,” said Rachel Moore, a postdoctoral researcher at Georgia Tech who started studying pyroaerobiology while working on her PhD at the University of Florida. “We actually don't know the answer to that yet.” 

Smoke plume temperatures can reach temperatures of greater than 290 degrees Celsius, but it may not be that hot throughout the plume, depending on what’s being burned, how much oxygen is available, and other environmental factors. 

Vuono said they’re working on a paper right now, still to be completed and submitted for peer review, that will hypothesize how exactly the microbes get picked up by the smoke, and how they survive. One theory is that the microbes act as hitchhikers onto smoke particles, which are essentially pieces of burning carbon. 

Another is that forest fires generate a lot of heat, creating a large volume of air that is rising, and cooler air rushing from high to lower pressure. “Basically, you have a lot of air that is moving around and will pick things up with it, like environmental microbes, which might then stick to smoke particles,” Vuono said. “Then the smoke particles become like a taxicab.”

The pyroaerobiology research detected a handful of different organisms that can act as pathogens in people, but Kobziar said they’re not sure if they’re present at high enough concentrations to cause infections. 

“We know that some of them are there, but we're not ready to say that a particular organism is capable of causing infection in people,” she said. 

Still, in December, Kobziar and George Thompson published a Perspective in Science, calling out wildfire smoke as a potential infectious agent. Thompson, an infectious disease doctor at the University of California, Davis, said he's noticed that with the increasing number of wildfires, there are more cases of valley fever, or coccidioidomycosis, in firefighters. Coccidioidomycosis is caused by an organism found in soils, and aerosolized by wind. “We have strong reason to suspect that it is aerosolized by wildfire as well,” Kobziar said. 

“I think that valley fever may represent the tip of the iceberg,” Thompson said. There are also higher cases of respiratory infection associated with smoke exposure, though any connection to microbes in smoke is not proven. 

For most healthy people, the immune system is able to fight off invasive microbial organisms, Thompson said. But he is concerned about people with compromised immune systems. “Breathing in all that particulate matter with bacteria and fungi, they could easily catch pneumonia and even have really severe consequences for them potentially,” Thompson said. 

Kobziar wants to start studying firefighters' health, and also tracking the relationship between rates of infection and the amount of time populations of people are inundated by wildfire smoke.

“I’d like to see an analysis of whether or not there's a relationship between those periods of time, particular individuals exposure to smoke, what amount of time they spend outside, and whether or not they developed any infection,” Kobziar said.

Burning has been used as a way to reduce plant diseases in agricultural systems, but could actually be a way of spreading fungal spores long distances. “You bet you can have a problem there,” Vuono said. 

Whenever microbes move from one location to another, where they possibly didn’t exist before, it has the potential to change the ecosystem they land in. Microbes can affect nutrient cycling, or what gases are absorbed or given off by soils; they can make plants sick, kill them, or, in other cases, help organisms in an ecosystem survive and grow. 

“What goes up must come down,” Moore said. “Eventually they will return to the surface. Can that affect the microbes that are there living there already?” 

Moore had been studying how fire affects ice nucleating microbes in the air—the same types of microbes that had prompted Kobziar to look into smoke. It was a moment of astonishment when she discovered that Kobziar and her co-workers had published on the same topic. They are now working together too; in October last year, Moore, Kobziar and their co-authors published in Nature that they found ice nucleating particles and microbes in smoke from prescribed wildland fires in North Florida.

The ice nucleating microbes found in smoke are a compelling example. If smoke is a mechanism for ice nucleating microbes to make it into the atmosphere, that could also have implications for the formation of ice in clouds, which affects snow and rainfall.

But just because a microbe lands somewhere, doesn’t mean it will necessarily flourish or overtake the ecology of where it was transported to. “In any given environment, you're going to have some sort of selective habitat filter,” Vuono said.

He joined the team of pyroaerobiology collaborators because of his expertise in sequencing the genetic material of microbes, like DNA and RNA. Many of the microbes in the environment can’t be cultured, or grown in the lab, so they have to be identified by comparing their genetic material to that of other already known microbes.  

For each environment, there are microbes that are metabolically fit to survive in that environment and proliferate, and others that don’t survive well there. If a microbe travels in smoke to an environment that already has a lot of microbial activity, there might not be a niche for the new microbe to colonize that environment. But, if there is an ecosystem with resources that are unconsumed, it’s more likely that a microbe could establish itself and start growing.

A microbe that ends up in an ecosystem that is already bustling might not disappear all together. It could become dormant, Vuono said, and wait for conditions to become just right to proliferate.

This mode of microbial transportation has probably been around for a long time, Kobziar said; we just didn’t know it. “We think of smoke as a potential conveyor belt of biodiversity that has probably both positive, negative and innocuous repercussions."

But it will take a lot of study to figure out how it works. Vuono doesn’t think of a smoke’s microbial contents as a microbial community like the gut microbiome which is a community of microbes all living together in a shared environment. “I think it’s more of a random event,” Vuono said. “What happens to get scoured? What happened to stick to smoke particles? What happened to be collected and sequenced on our filters that we put in the air?"

The research teams are working on these questions. Pyroaerobiology necessitates a collaboration of people with many different scientific backgrounds. Kobziar is the fire ecologist, and she now works with people who study smoke science, atmospheric chemistry, microbiologists, environmental microbiologists, people who are interested in public health and infectious disease experts, and ecologists. Together, they are continuing to sample smoke from different kinds of fires, and sampling from higher up into the smoke plumes using drones to fly in and collect samples.

 “More and more people are becoming interested in this topic," Kobziar said. "They are starting to invest some of their excellent brainpower in addressing all the questions that we have."

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