Modelling growth of, and removal of Zn and Hg by a wild microalgal consortium

Microorganisms isolated from sites contaminated with heavy metals usually possess a higher removal capacity than strains from regular cultures. Heavy metal-containing soil samples from an industrial dumpsite in Northern Portugal were accordingly collected; following enrichment under metal stress, a consortium of wild microalgae was obtained. Their ability to grow in the presence of, and their capacity to recover heavy metals was comprehensively studied; the datasets thus generated were fitted to by a combined model of biomass growth and metal uptake, derived from first principles. After exposure to 15 and 25 mg/L Zn²⁺ for 6 days, the microalgal consortium reached similar, or higher cell density than the control; however, under 50 and 65 mg/L Zn²⁺, 71% to 84% inhibition was observed. Growth in the presence of Hg²⁺ was significantly inhibited, even at a concentration as low as 25 μg/L, and 90% inhibition was observed above 100 μg/L. The maximum amount of Zn²⁺ removed was 21.3 mg/L, upon exposure to 25 mg/L for 6 day, whereas the maximum removal of Hg²⁺ was 335 μg/L, upon 6 day in the presence of 350 μg/L. The aforementioned mechanistic model was built upon Monod assumptions (including heavy metal inhibition), coupled with Leudeking-Piret relationships between the rates of biomass growth and metal removal. The overall fits were good under all experimental conditions tested, thus conveying a useful tool for rational optimisation of microalga-mediated bioremediation.