Overexpression of the Licanantase Protein in Acidithiobacillus ferrooxidans Leads to Enhanced Bioleaching of Copper Minerals

With growing demands for metals and diminishing global reserves of high-grade ores, it is becoming increasingly important to develop suitable metal extraction methods for processing low-grade materials. Bioleaching is an energy- and environmentally attractive process for the processing of copper and other sulfidic ores. Acidithiobacillus ferrooxidans, a commonly observed member of the acidophilic bioleaching consortia, derives energy from the oxidation of iron and reduced sulfur compounds. The licanantase protein is a major component of secretome of sulfur-grown Acidithiobacillus spp. Here we report the effect of the genetic overexpression of the endogenous licanantase in A. ferrooxidans on the bioleaching of three different copper sulfide minerals: chalcocite, covellite, and chalcopyrite. Significantly improved cell attachment occurred with increased extracellular polymeric substance secretion, and enhanced biofilm formation was observed for the engineered cells with all three minerals as compared to the results with the wild type cells. However, the bioleaching efficiencies of these minerals were affected differently, where improved copper solubilization was observed for the leaching of chalcocite and covellite but no difference was observed for chalcopyrite. We propose that this observation results from the enhanced Fe3+ chelating capability of the engineered licanantase-rich biofilms. The overexpression of licanantase in bioleaching microbes may be an attractive approach for enhancing the bioleaching of some lowgrade copper and other ores.