The adaptation mechanisms of Acidithiobacillus caldus CCTCC M 2018054 to extreme acid stress: Bioleaching performance, physiology, and transcriptomics

To understand the acid-resistant mechanism of bioleaching microorganism Acidithiobacillus caldus CCTCC M 2018054, its physiology and metabolic changes at the transcriptional level under extreme acid stress were systemically studied. Scanning electron microscopy (SEM), Fourier transform infrared reflection (FTIR) and Xray diffraction (XRD) showed that with an increase in acidity, the absorption peak of sulfur oxidation-related functional groups such as S-O decreased significantly, and a dense sulfur passivation film appeared on the surface of the ore. Confocal laser scanning microscopy (CLSM) revealed that coverage scale of extracellular polymeric substance (EPS) and biofilm fluctuated accordingly along with the increasing acid stress (pH-stat 1.5, 1.2 0.9 and 0.6) during the bioleaching process. In response to acid stress, the increased levels of intracellular glutamic acid, alanine, cysteine, and proline contributed to the maintenance of intracellular pH homeostasis via decarboxylation and alkaline neutralization. Higher unsaturated fatty acid content was closely related to membrane fluidity. Up to 490 and 447 differentially expressed genes (DEGs) were identified at pH 1.5 vs pH 1.2 and pH 1.2 vs pH 0.9, respectively, and 177 common DEGs were associated with two-component system (TCS) regulation, transporter regulation, energy metabolism, and stress response. The upregulation of kdpB helped cells defend against proton invasion, whereas the downregulation of cysB and cbl implied stronger oxidation of sulfur compounds. The transcriptional level of sqr, sor, and soxA was significantly increased and consolidated the energy supply needed for resisting acid stress. Furthermore, eight of the identified DEGs (sor, cbl, ompA, atpF, nuoH, nuoC, sqr, grxB) were verified as being related to the acid stress response process. This study contributes toward expanding the application of these acidophiles in industrial bioleaching.