While multiple theories exist regarding the effect of dissolved oxygen (DO) on the biooxidation of minerals, few studies have been performed the cellular or molecular scale (e.g., genetics) and the mechanism remains unclear. In this paper, the effects of DO concentration on the biooxidation process of refractory sulfide gold ores by Acidithiobacillus ferrooxidans were investigated in the experimental stirred tank bioreactors (STRs). The results indicated that higher biooxidation and cell growth rates were correlated with higher DO concentration. The biooxidation process was restricted at 1.2 ppm DO due to oxygen limitation. Furthermore, the effects of DO on cellular and molecular scale were studied for the first time. The results demonstrated that the oxygen uptake rate (OUR), the Fe2+ oxidation activity and the rus gene expression of A. ferrooxidans all increased with the DO concentration, which might be responsible for the increase of the biooxidation rates with the DO concentration. This study provides insight into the potential impact of molecular-level mechanisms of DO in the biooxidation process of minerals. (C) 2012, The Society for Biotechnology, Japan. All rights reserved.