Underground fires shape the structure of microbial communities and select for thermophilic bacteria through a temperature gradient.

A detailed diversity analysis of the prokaryotic and fungal communities in soil impacted by an underground fire located in the Trans-Mexican volcanic belt, Mexico, is described. Microbial diversity data obtained from soils at different depths and temperatures (27 °C, 42 °C, 50 ºC and 54 ºC) were analyzed, and Firmicutes increased in abundance as the temperature augmented, and Proteobacteria mainly decreased in abundance at high temperatures compared to unaffected soils. The fungal phylum Ascomycota was the most abundant, with no significant changes. A clear reduction in the richness of both prokaryotic and eukaryotic operational taxonomic units (OTUs) was observed in the affected soils. At the genus level, Bacillus species were the most abundant among bacteria, while Aspergillus, Penicillium, and Mortierella were dominant fungal genera at higher temperatures. Interestingly, the physicochemical parameters of the affected soils modified organic matter, which was indirectly correlated with the presence of some microbial taxa. Likewise, we obtained 308 soil bacterial isolates from both control and affected soils. Among these, the taxa from the phyla Actinobacteria and Firmicutes demonstrated the highest thermotolerance in the affected soils. Our findings shed light on the impact of underground fires on the structure of microbial communities, favoring an abundance of thermotolerant microbes.