Figure 1.

General mechanisms for acidophile tolerances and immobilization of metals from the environment. (A) In the upper left, osmotolerance with proteins and transporters. In the upper right, acid tolerance highlighting membrane transport proteins and the principal mechanisms involving proton transport. In the lower left, thermotolerance is displayed, and in the lower right, tolerance to heavy metals such as Pb, Zn, and Hg is illustrated. The main processes of metal immobilization found in acidophilic organisms such as A. ferrooxidans. (B) Clockwise from the top, bioaccumulation involves the internalization of heavy metals into the cell as aquo-ions, followed by bioprecipitation where hydrated metals are precipitated into their respective hydrogen phosphate, sulfide, and carbonate forms. Bioreduction entails the precipitation of soluble metals as solid metals through redox processes. Finally, biosorption processes involve the absorption of soluble metals by the cell in the form of solids, such as phosphates, or in compatible molecules, e.g., amines, organic acids, or hydroxides.