Quantification of biomass and estimation of cell numbers are essential tasks in the calculation of specific bioleaching rates and thus are important for process characterisation and optimisation. As a fast and convenient alternative to the laborious and expensive technique of qPCR, we present here a fluorometric approach which allows the sensitive and reliable quantification of the iron-oxidising Acidithiobacillus ferrooxidans. Following two different methods of sample preparation, total fluorescence of PicoGreen-stained cells could be measured with good reproducibility by means of a microplate reader. Calibration of these data with results from a parallel cell enumeration by epi-fluorescence microscopy allowed the reliable estimation of cell densities as low as 1.0 x 10(5) cells/mL. In order to determine the potential respiration activity of the investigated cells we provide an oxygen-specific optode-based approach. Due to its fast response, the lack of oxygen consumption, and the insensitivity towards mineral deposits the optode was shown to be an attractive alternative to the frequently used Clark electrode. Moreover, respiration rate and total oxygen consumption could easily be followed of iron-oxidising (and bioleaching) cultures of At ferrooxidans over days and weeks using the manometric OxiTopC system. The observation of transitional plateaus during the pressure decrease of a gas phase from a ZnS-leaching culture of At. ferrooxidans indicated the occurrence of different oxygen-consuming processes. (C) 2015 Elsevier Ltd. All rights reserved.