A novel microwell-based analytical technique for studying ferrous iron biooxidation activity

• Fe oxidation activity of a mixed bioleaching culture was monitored in a microwell plate apparatus.
• Comparable growth and iron oxidation trends to shake flasks were observed.
• A colorimetric method to determine trends in the extent of oxidation was used.
• Microwell plates are well-suited for high throughput research in heap leach biomining.

The broad range of complexities in bioleaching includes the use of mixed microbial communities with diversity of species and strains with different windows of operating conditions. Empirical approaches to characterise these currently use cumbersome experimental systems; hence the need to develop a high throughput research tool, analogous to the techniques used in high throughput pharmaceutical research. In this study, a microwell research tool was evaluated as a growth and measurement tool for mixed autotrophic bioleaching cultures. The tool was assessed by comparing its performance to conventional shake flask apparatus. Mixed mesophilic cultures of predominantly Leptospirillum ferriphilum and Acidithiobacillus ferrooxidans were used. Growth and ferrous iron oxidation kinetics were quantified and assessed. Microwell plates performed similarly to conventional shake flasks with respect to growth and iron oxidation kinetics. The microwell plate apparatus was also used as a measurement system in combination with a microwell plate reader (measuring absorbance change at 428 nm over time). Progressive colour change of growth experiments correlated to ferrous iron oxidation within a defined operating window. We conclude that, using this measurement as a proxy for trends iron oxidation, the microwell research tool is well-suited for high throughput scoping studies to map operating windows for different cultures, in both an unadapted and adapted context. This was confirmed through an activity test utilising fluoride as an inhibitor. Where absorbance measurements at 428 nm are used to track oxidation progress, the research tool has limitations with respect to pH (<2.0) and total iron concentration (<8.0 g l−1).