This review examines why 'Leptospirillum'-like species dominate commercial biooxidation and bioleaching operations rather than Thiobacillus ferrooxidans, which was historically considered the primary mineral-oxidizing bacterium. Using PCR-based molecular methods to analyze bacterial communities in Chilean copper heaps, Australian biooxidation reactors, and South African gold-processing facilities, researchers found 'Leptospirillum ferrooxidans' consistently dominated alongside sulphur-oxidizing bacteria, while T. ferrooxidans was absent or minimal. The authors explain this pattern through kinetic analysis of pyrite oxidation and mineral surface electrochemistry. 'L. ferrooxidans' exhibits superior affinity for ferrous iron, lower inhibition by ferric iron, and maintains activity at higher redox potentials than T. ferrooxidans. Electrochemical studies reveal that at the positive potentials typical of commercial operations—where ferric iron accumulates—'L. ferrooxidans' thrives while the available chemical energy for T. ferrooxidans becomes inadequate. At lower potentials characteristic of natural leaching, T. ferrooxidans initially dominates by directly attacking pyrite surfaces. The shift from T. ferrooxidans to 'Leptospirillum' dominance reflects the electrochemical conditions that develop during continuous ore circulation in industrial settings.

Key findings

• PCR-based molecular analyses of multiple commercial bioleaching and biooxidation plants revealed 'Leptospirillum ferrooxidans' and sulphur-oxidizers dominated bacterial communities, with T. ferrooxidans absent or comprising only 10-17% of cells.
• 'L. ferrooxidans' possesses higher affinity for ferrous iron (Km = 0.25 mM) and greater resistance to ferric iron inhibition (Ki = 42 mM) compared to T. ferrooxidans (Km = 1.34 mM; Ki = 3.1 mM), enabling dominance at high redox potentials.
• Electrochemical analysis demonstrates that at positive potentials (>600-700 mV) typical of industrial operations with accumulated Fe3+, insufficient chemical energy remains available to support T. ferrooxidans growth, favoring 'Leptospirillum' instead.
• The electrochemical potential of the leaching solution determines which iron-oxidizing bacteria dominate: T. ferrooxidans initially dominates at low potentials where direct pyrite attack occurs, but 'Leptospirillum' outcompetes it as Fe3+ accumulates and potentials rise in circulating systems.