The kinetics of ferrous-iron oxidation by Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans:: effect of cell maintenance

The aim of this work was to determine whether or not the variable maintenance equation proposed by Neijssel and Tempest [Arch. Microbiol. 107 (1976) 215] and modified by Pirt [Arch. Microbiol. 133 (1982) 300] could explain the differences in the experimentally determined and predicted yield and maintenance coefficients of the ferrous-iron oxidising bacteria used in bioleaching operations determined under different growth conditions. The results obtained indicated that the maintenance requirement of energy sufficient cultures is a combination of the variable and constant maintenance energy requirements. For this reason, modelling the bioenergetics of energy sufficient cultures assuming a constant maintenance energy requirement results in a significant overestimation of the maximum bacterial yield. However, the variable maintenance equation proposed by Pirt [Arch. Microbiol. 133 (1982) 300] may be used to determine the maximum bacterial yields and the maintenance coefficients from both energy limited and energy sufficient experiments. This is because this model is able to account for variations in the growth rate as a result of changes in the maintenance requirement due to the micro-organisms being limited by more than one factor. It is thus suggested that the variable maintenance equation be used instead of the constant maintenance equation when quantifying the bioenergetics of micro-organisms unless the energy source is known to be the limiting substrate. (C) 2003 Elsevier Science B.V. All rights reserved.