Response and adaptation of verrucomicrobial methanotrophs to heat and acidity

Acidophilic microorganisms thrive in environments where the external pH is orders of magnitude lower than their intracellular pH. Verrucomicrobial methanotrophs of the family Methylacidiphilaceae, including Methylacidiphilum and Methylacidimicrobium, inhabit extremely acidic geothermal environments and can grow at a pH < 1.0 and temperatures up to 65 °C. We analyzed and compared their membrane fatty acid compositions at pH 3.0 across strains with different temperature optima. Thermophilic Methylacidiphilum strains almost exclusively contain saturated fatty acids, while the mesophilic Methylacidimicrobium strains we studied incorporate 16-47% unsaturated fatty acids. Notably, the thermophile Methylacidiphilum fumariolicum SolV increases unsaturated fatty acid content in response to a 10 °C temperature decrease but not to a decrease in pH from 3.0 to 1.7. Genomic analysis revealed a conserved fatty acid biosynthesis pathway. Despite constitutive expression of predicted pH homeostasis genes, SolV did not upregulate them upon changing the pH from 3.0 to 1.7. However, genes involved in methane oxidation were strongly upregulated, suggesting a potential metabolic adaptation to extreme acidity.