Figure 1.

Potential targets for engineering biomining microorganisms: (A) Acid tolerance. Multiple mechanisms for acid stress tolerance in acidophiles as suggested by Baker-Austin and Dopson [66]: i) Increased influx of potassium into the cell in order to maintain a reversed transmembrane potential, ii) Highly impermeable cell membranes to reduce the influx of protons, iii) Over-production of enzymes/chemicals to bind and sequester protons to maintain pH homeostasis, iv) Increase in active export of protons through transporters, v) Increased synthesis of organic acids to act as uncouplers, vi) Larger proportion of repair systems for DNA and protein repair. (B) Metal tolerance. Multiple transporters for the efflux of metal cations and toxic compounds to assist in the detoxification of the cell [67]. Additionally, the exopolyphosphatase (ppx) enzyme can convert polyphosphates (PolyP) into inorganic phosphate (P_i) that will bind to free metal cations and then be transported out of the cell through the transporters. (C) Osmotolerance. The ability to tolerate high levels of osmotic stress can be achieved through the accumulation of various osmoprotectants, such as ectoine, glycine betaine, trehalose, proline, glutamate, and perisplasmic glucans [73,74,75,76,77,78,79,80,81]. These compounds can either be synthesised in abundance or transported into the cell through transporters when the cell is challenged with osmotic stress. Alternatively, chloride ion channels and pores can be closed to reduce the entry of the ion into the cell [73,74,75,76,77,78,79,80,81]. (D) Thermotolerance. Incorporation of thermostable enzymes and proteins, increase in DNA repair systems, and expression of heat shock proteins as well as the incorporation of modified membrane composition (fatty acids and tetraether lipids) can help to increase the thermostability of cells [82,83].