The ferrous iron- and sulfur-oxidizing Acidithiomicrobium was the dominant iron-oxidizing bacterium in a mixed culture of moderate thermophiles in a bioreactor with a high concentration of nickel in solution (20 g l(-1)) and a continuous feed of nickel concentrate. This is the first demonstration specifically indicating that this bacterium could be used for industrial processing of a base metal sulfide concentrate. However, its sensitivity to copper determined that it was replaced by Sulfobacillus thermosulfidooxidans in the mixed culture when the feed was a nickel-copper concentrate. The extraction of copper from the nickel-copper concentrate by moderate thermophiles, despite fine grinding of the feed, was relatively poor (50% at 49 degrees C) compared to that achieved with high temperature archaea (92% at 77 degrees C) in single, continuous reactors with 5% w/v concentrate feeds and residence times of 2.7 and 2.5 days respectively. (C) 2016 Elsevier Ltd. All rights reserved.