With the increase of megafires across the world, Australia alone has seen over 24 million hectares burnt during the 2019-2020 bushfire season. In 2016, 534 square kilometres (53,400 hectares) of redwood tanoak forest burned in the Soberanos mega-fire event in the Big Sur region of California, including two sites already part of undergoing fire-ecology studies. The biologists involved in this research saw the opportunity to investigate how soil microbe communities respond to megafire events.
So what exactly are microbes and how do they help?
A soil microbe is any microscopic organism that is present in the environment, including types of bacteria, fungi, protozoa (single-celled eukaryotic organisms), and nematodes (unsegmented roundworms). Together, they play a huge role in the cycling of micro and macro nutrients in soils, that are crucial for maintaining plant and animal life. Fungi in particular have a strong symbiotic relationship with plants, where the majority of plants rely on networks of mychorrizal fungae to make nitrogen more readily obtainable. While we know in general these microbial communities are crucial for restoring ecosystems after bushfires, what we don’t know is how exactly fires impact the microbes themselves.
“It’s not likely plants can recover from megafires without beneficial fungi that supply roots with nutrients, or bacteria that transform extra carbon and nitrogen in post-fire soil,” said lead author and UCR mycologist Sydney Glassman, University of Calfornia mycologist and leader of the study. “Understanding the microbes is key to any restoration effort.”
Taking soils samples at the two burnt sites, and one from an ecologically similar but unburnt site, the team from University of California (USA) and the University of Tokyo (Japan) set out to qualify and quantify the diversity of microbes before and after the mega-fire event, with a focus on bacteria and fungus in particular.
How did microbes respond to the megafire?
DNA was extracted from the soil, then fungal and bacterial samples were targeted and amplified using PCR. The number of species was approximated using the amount of DNA measured in each microbe grouping, called operational taxonomy units (OTU), while species diversity was estimated by sequencing the DNA to see how many different taxonomic groups were present.
In burnt area samples, the number of species had dropped, where fungi OTU decreased by 70% and bacteria OTU as much as 52% in the burnt areas compared to the unburnt.
The composition of the community also drastically changed after the mega-fire, where before the fire, the dominant fungi belonged to Basidiomycota (62%), Mucoromycota (25%) and Ascomycota (10%). After the mega-fire event, the Ascomyocota took over (65%), Basidiomycota fell by 35%, and Mucoromycota was completely wiped out. Within the remaining Basiomycete species, Basidioascus yeasts massively increased, which have the ability to degrade wood components, including the lignin inside the plant cell walls.
For bacteria, before the fire the communities were made up of Proteobacteria (84.4%) and Acidobacteria (15.6%) dominated. After the fire, Proteobacteria numbers were devastated, where Firmicutes became the front-runner with 82%, while Acidobacteria benefited slightly (up to 18%). Penicillium was one of the microbes that prospered, possibly taking advantage of the remains of fallen creatures, while some species are also able to eat charcoal.
The full set of their results can be found in Molecular Ecology.
“One of the reasons there is so little understanding of fungi is that there are so few mycologists who study them,” says Glassman, “But they really do have important impacts, especially in the aftermath of major fires which are only increasing in frequency and severity both here and across the globe.”