Microbial cell membrane properties and intracellular metabolism regulate individual level microbial responses to acid stress

Community- and individual-level soil microbial traits greatly depend on environmental conditions, especially soil pH. Given substantial spatio-temporal variations in soil pH, microorganisms exhibit acid tolerance to some extent, although the underlying mechanisms of microbial tolerance capacities remain understudied. In this study, ten microbial species with different functions (plant growth promoting, pathogen biocontrol, and nutrient cycling functions) were cultured under different acid treatments to investigate the acid tolerance capacity and the underlying physiological mechanisms for acid tolerance of these microbial species. Microbial species exhibited substantially different acid tolerance capacities, making them survival at pH from 3.5 to 5.0. Cellular physiological assays further indicated that three aspects could contribute to the acid tolerance capacity of soil microorganisms in the present study, i.e., shield effect by regulating the fluidity of cell membrane to prevent H+ from entering into cells, neutralizing effect by promoting the internal metabolic activities to produce metabolic chemicals such as amino acids and urea that could consume more intracellular H+, and pumping effect by regulating H+-ATPase activity to extrude H+ out of cells, although it cannot be denied that other pathways may also function. However, these pathways did not make the same contribution to the acid tolerance capacity of different microbial species, suggesting that the regulators and underlying mechanisms depended greatly on microbial species. Further studies that combine omics technologies to discern tradeoff among microbial traits such as resource allocation, stress tolerance, etc., would be helpful to clarify the molecular mechanisms underlying microbial tolerance to the environmental stressors such as soil acidification.