Can Microbes Help Revive Megafire Dead Zones?

A UC Riverside researcher sampling Holy Fire burn scar soil for analysis. Credit: Sydney Glassman/UCR

In a possible “calm before the storm,” California’s 2022 wildfire season was mild, as only about 363,000 acres burned across the state. For perspective, 2.5 million acres burned in 2021, down from a record-breaking 4.3 million acres in 2020. Climate change is not only causing an increase in frequency of fires, but is also upping their size and severity.

“Things can recover but it takes time, and whether or not the land recovers after super-frequent megafires is another story. Can recovery time keep pace with megafires? We don’t know yet,” said Sydney Glassman, an assistant professor of microbiology and plant pathology at the University of California Riverside.

What we do know, thanks to Glassman’s research, is that specific microbes may be able to help bring the land back faster.

The Holy Fire burned more than 23,000 acres across Orange and Riverside counties in 2018. Wanting to understand how the blaze affected bacteria and fungi over time, Glassman led a team of researchers into the burn scar. They sampled soil at 17, 25, 34, 67, 95, 131, 187, 286, and 376 days after the southern California wildfire. At the same time, they collected samples from nearby, unburned soil.

According to the study, published in Molecular Ecology, the fire severely reduced bacterial biomass by 47%, bacterial richness by 46%, fungal biomass by 86%, and fungal richness by 68%. And while the overall mass of microbes did not recover that first year, that’s not to say nothing lived on the charred earth.

In fact, it wasn’t just one type of bacteria or fungi that survived and thrived. Rather, it was a parade of microbes that took turns dominating the burned soil. The study findings show the burned bacterial and fungal communities turned over up to six times within the first 12 months post-fire.

“There were interesting, distinct shifts in the microbes over time. As one species went down, another came up,” said Glassman.

For example, in the early days, Glassman and colleagues unsurprisingly found microbes with high tolerance for fire and high heat. Later, fast-growing organisms with a lot of spores—able to take advantage of space with little microbial competition—made their presence known. Toward the end of the year, organisms able to consume charcoal and other post-fire debris high in nitrogen were found in abundance.

Specifically, the study shows turnover was driven by “fire-loving” pyrophilous microbes, many of which have been previously found in forests worldwide. Fungal secondary succession was initiated by the Basidiomycete yeast Geminibasidium, which traded off against the filamentous Ascomycetes Pyronema, Aspergillus and Penicillium. For bacteria, the Proteobacteria Massilia dominated all year, but the Firmicute Bacillus and Proteobacteria Noviherbaspirillum increased in abundance over time.

Additionally, certain microbes—called methanotrophs—regulate the breakdown of methane, a greenhouse gas. The research team noticed that genes involved in methane metabolism doubled in post-fire microbes.

“This exciting finding suggests post-fire microbes can ‘eat’ methane to gain carbon and energy. [This] can potentially help us reduce greenhouses gases,” said first author of the study Fabiola Pulido-Chavez, UCR plant pathology PhD candidate.

Interestingly, the researchers say the rebound they saw in the soil bears some resemblance to the human body’s response to a major stress. People suffer an illness and take antibiotics. The medicine destroys bacteria in a person’s gut, and new organisms begin to show up that either weren’t there before or did not previously have a large presence. Eventually, a person’s gut bacteria might return to something like its pre-infection state, but there is no guarantee.

“We are trying to understand what gets the land back to where it was before the disturbance, which in this case was an enormous fire,” Glassman said. “A lot of what we’re studying could be transferrable to a human microbiome setting.”

One question that remains is whether the adaptations plants and microbes have developed in response to wildfires will adapt again to megafires or recurrent fires. In the past, where there might have been a period of several decades before a plot of land burned more than once, it is now increasingly common for the same soil to burn again in fewer than 10 years.

What does the climate change-induced increase in wildfire size, severity and frequency do for natural burn recovery?