During large-scale column tests at BHP Billiton’s Johannesburg Technology Centre (JTC) during 2005/6 on a low-grade copper ore, the concentrations of both oxygen and CO2 were continuously monitored in feed and exit gas as well as at various intermediate positions over the height of the column. This paper describes results from a test run at 40 °C fed with an air stream enriched to between 1000 and 2000 ppm CO2. Oxygen consumption very closely tracks iron and copper leaching. CO2 is consumed rapidly from the bottom up, resulting in significant depletion midway through the column, even though an enriched feed was used. Oxidation rates decline in CO2 depleted zones, but were not observed to cease completely. This rate of decline is postulated to be linked to a slowly decaying population unable to regenerate itself. Comparison between O2 and CO2 consumption rates shows a linear correlation beyond a minimum oxidation rate. This minimum rate corresponds to a non-growth maintenance energy requirement, and the slope of the linear correlation to the growth yield. Both are functions of available CO2 in the range 50 to 1000 ppm, with maintenance declining and yield increasing. The findings of this study imply that CO2 supplementation in bioheaps will stimulate microbial growth and CO2 consumption, but not necessarily increase the rate of oxygen uptake and hence leaching. Absence of CO2 is expected to result in gradual population decline, but a degree of oxidation continues on the basis of maintenance. In tall heaps CO2 depletion with height is likely and may therefore result in impaired leaching in the upper zones.