Importance of Initial Interfacial Steps during Chalcopyrite Bioleaching by a Thermoacidophilic Archaeon

Sessile Lifestyle Offers Protection against Copper Stress in Saccharolobus solfataricus

Some archaea from the genus Sulfolobus are important for bioleaching of copper, where metal resistant microorganisms are required. Biofilm generation is one of the ways microorganisms cope with some stimuli in nature, including heavy metals. The response to external factors, particularly in the biofilm form of life, is still underexplored in archaea. To explore how model thermoacidophilic archaeon Saccharolobus solfataricus faces copper stress during this lifestyle, changes in biofilms were studied using crystal violet staining, confocal fluorescence microscopy, and qPCR approaches. It was found that biofilm formation reached a maximum at 0.5 mM Cu, before starting to decrease at higher metal concentrations. The morphology of biofilms at 0.5 mM Cu was observed to be different, displaying lower thickness, different sugar patterns, and higher amounts of cells compared to standard growing conditions. Furthermore, copA, which is responsive to intracellular Cu concentration, was downregulated in biofilm cells when compared with planktonic cells exposed to the same metal concentration. The latest results suggests that cells in biofilms are less exposed to Cu than those in planktonic culture. In a PolyP-deficient strain, Cu was not able to induce biofilm formation at 0.5 mM. In summary, the findings reported here suggest that the biofilm form of life confers S. solfataricus advantages to face stress caused by Cu.Biofilm formation remains a relatively unexplored topic in archaeal research. Therefore, this knowledge in model organisms such as S. solfataricus, and how they use it to face stress, could be of great importance to engineer organisms with improved capabilities to be applied in biotechnological processes, such as bioleaching of metals.

EpsRAc is a copper-sensing MarR family transcriptional repressor from Acidithiobacillus caldus

The MarR family, as multiple antibiotic resistance regulators, is associated with the resistance of organisms to unfavorable conditions. MarR family extracellular polymeric substances (EPS)-associated transcriptional regulator (EpsR^Ac) was closely associated with copper resistance in Acidithiobacillus caldus (A. caldus). Transcriptional analysis showed high activity of the epsR promoter (PI) in Escherichia coli and differential response to metal ions. The copper content and UV absorption spectrum of the co-purified protein did not increase, but a stoichiometry of 0.667 mol Cu(I) per EpsR^Ac monomer was observed in vitro in copper titration experiments, suggesting that Cu(II) acts with low affinity in binding to the EpsR^Ac protein. Electrophoretic mobility shift assays (EMSA) demonstrated that EpsR^Ac could bind to its own promoter in vitro, and the binding region was the palindrome sequence TGTTCATCGTGTGTGAGCACACA. EpsR^Ac negatively regulated its own gene expression, whereas Cu(II) mitigates this negative effect. EpsR^Ac did not bind to other neighboring gene promoters. Finally, we developed a working model to illustrate the regulatory mechanism of A. caldus in response to extreme copper stress. KEY POINTS: • Identification of a MarR family EPS-associated transcriptional regulator, named EpsR^Ac. • Cu(I) can bind to the EpsR^Ac protein with low affinity. • EpsR^Ac negatively regulates the expression of epsR, and Cu(II) can alleviate this negative regulation.