Abstract

In the extremely acidic conditions found in natural acid rock environments and bioleaching operations, microorganisms have to deal with an abundant supply of (heavy) metals. Iron is required as micronutrient by all acidophiles, and as a primary energy source or alternative electron acceptor by some of them. However iron and other metals can also induce oxidative damage to biomolecules by generating reactive oxygen species (ROS). Acidophilic microorganisms, therefore, are often exposed to highly oxidizing conditions and face the problem of maintaining intracellular metal and redox homeostasis. Although acidophiles fight oxidative stress via a number of ROS scavenging enzymes, they seem to depend mainly on biomolecule repair using a plethora of protein and lipid repair systems. Evidence collected to date indicates that the molecular determinants of this response also serve to protect the cells from other extreme environmental factors such as temperature, pH and metal(loid)s. Acidophiles are well equipped with mechanisms to export, sequester and/or store metals. Reducing the bioavailability of metals also seems to play an important role in their tolerance, via complex formation (with sulfate anions) and binding to extracellular polymeric substances. This chapter summarizes the mechanisms of protection to oxidative stress and metal toxicity in acidophiles, how these are configured to build the robust protection system required to thrive under extreme stress conditions, and how the ability of acidophiles to respond to those conditions may be critical in determining their growth and activity.