One morning in September 2020, I, like many other residents of California and Oregon, woke up to an orange sky outside of my window. The eerie color and nature of the sky were due to the biggest wildfires that have ever engulfed California. While extensive work has been done on the effects of wildfires on terrestrial ecosystems, relatively few studies have focused on the effects of wildfires on soil microbiota or the microbial nature of the wildfires themselves.
The Effect of Wildfire on Soil Microbiota
Controlled burn at Joseph W. Jones Ecological Research Center at Ichauway in Newton, Ga. in 2016.
A wildfire can cause surface temperatures to reach above 1000°C, which is approximately the temperature of lava erupting from a Hawaiian volcano. These extreme temperatures, in addition to changing the physical properties and chemical structure of the soil, also adversely affect fungi and bacteria that inhabit its topmost layer. In general, survival favors microbial communities found in deeper soils, and bacteria have better heat tolerance than fungi. In 2019, Whitman et al. investigated the effects of wildfire on microbial communities in the northwestern Canadian boreal forests and found that the community composition of burned area significantly differs from that of unburned areas, with increased abundances of bacterial species in the genera Massilia and Arthrobacter, and fungi in the genera Penicillium and Fusicladium.
Mikita-Barbato et al. studied microbial community structure in organic horizon soils (the soil layer above the top layer of the soil with mostly organic matter, such as decomposing leaves, twigs, moss and lichens on the surface) of the New Jersey Pinelands over a period of 2 years post fire. They found that the composition of bacteria, fungi and archaea changed more dramatically during the first year and slowly during the second year, but still differed significantly from unburned soil. Another study analyzed the effects of different intensities of fire (low, moderate and high) on bacterial community composition in the topsoil (0-10 cm) and the sub-soil (10-20 cm) 6 months post fire. The authors observed several differences in bacterial communities inhabiting the topsoil and the sub-soil and these differences correlated with differences in pH, total nitrogen and ammonium nitrogen between the 2 soil types. Together, these findings suggest that the recovery of the physicochemical properties and microbial communities in soil may be very slow after a wildfire, forecasting long-term consequences for the soil ecosystem.
Regenerating dry sclerophyll forest in Brisbane Water National Park, Australia 76 days post controlled burn conducted May 2013.
Another underexplored area of research is the microbiology of wildfires themselves. The aerosol nature of wildfire smoke can transport ash, dust and harmful chemicals over long distances. Extensive research has been done on the adverse effects of particulate matter in wildfire smoke on human cardiovascular and respiratory health. Recent evidence suggests that wildfire smoke can also aerosolize and transport living things, including bacteria, fungi and their metabolic products. This field of “pyroaeromicrobiology,” which lies at the intersection of fire ecology, atmospheric sciences and microbiology, has the potential to uncover the role of wildfires in spreading pathogenic and beneficial microbes in the ecosphere.
Like the fire itself, the temperature of wildfire smoke can reach above the tolerances of many life forms. However, variability in the kind of fuels, availability of oxygen and other meteorological factors can create a wide array of biological niches that may favor different populations of microbes in space and time. There is evidence that microbes directly hitchhike on the particulate matter found in wildfire smoke. Pyrogenic carbon produced by wildfire can be a temporary substrate for aerosolized soil microbes, and the obscuring of ultraviolet radiation by particulate matter boosts the viability of bioaerosols. Wildfires generate convective air currents that carry the hot fire straight up while cooling air descends to create a cyclical pattern heat column. Such dynamic movements further enhance the concentration of bioaerosols like fungal spores in the smoke. Studies also suggest a positive correlation between indicators of burnt biomass and fungal presence in particulate matter. In fact, the U.S. Centers for Disease Control and Prevention (CDC) places wildland firefighters at a higher risk for coccidiodomycosis (also known as valley fever), an infection caused by a fungus that is aerosolized during soil disturbance.
In October 2020, Moore et al. demonstrated that samples of smoke emitted from controlled burns in regions of North Florida contained 5-fold higher concentrations of microbes compared to ambient air, equivalent to ~7.2 × 109 cells per m2 of burned area. Approximately 80% of these cells were viable, despite their exposure to wildfire temperatures. The authors also conducted controlled combustion experiments in the laboratory and found a higher abundance of bioaerosols and culturable bacteria emitted from burning dead vegetation compared to fresh shrubs. The authors were able to culture ~6% of microbes and found that many bacteria were associated with taxa commonly found in soil and on plant tissues and root nodules.
Both the effect of wildfires on microbial ecology and the study of microbes in wildfire smoke are nascent fields. Most of the studies have been conducted during controlled burns (fires intentionally set by a team of fire experts to restore a healthy ecosystem at a particular place) or during fires generated under lab conditions. Study of naturally occurring wildfires still poses a significant challenge, as their unpredictability makes it difficult to obtain samples prior to ignition, nor can studies be replicated in exactly the same way to generate statistically significant data. The study of microbial composition from different parts of the world with unique climate and soil conditions will be necessary to advance the field of fire ecology and generate predictive models of smoke transport and effects. Such tools will be valuable to mitigate future wildfire challenges posed by changing climate conditions, to restore forests to their healthy states and also to prevent infections caused by fire-generated bioaerosols.
