T. obliquus cultivation under heterotrophic conditions: determination of growth parameters

Microalgae are a promising feedstock for the future of the industrial chemistry. Among microalgae, Tetradesmus obliquus (generally known as Scenedesmus obliquus) is one of the most studied species due to its robustness and its good performances for production of lipids, proteins and carbohydrates. However, the high production costs still limit large part of the possible applications. The exploitation of the heterotrophic metabolism for microalgae cultivation may potentially improve the biomass production sustainability, especially if organic substrates are obtained from wastewaters. However, this latter strategy has technical limits given by the issue of contamination by bacteria, which have growth rates ~ 10 folds higher than microalgae. The estimation of the microalgae growth parameters could allow optimization of the feeding strategy by which wastewaters are supplied to microalgae culture, and consequently, bacteria contamination minimization. In this work, an axenic culture of T. obliquus was isolated by a sonication pre-treatment followed by an incubation in solid media containing ampicillin (0.1 mg mL-1), penicillin (67 U mL-1) and streptomycin (0.07 mg mL-1). The axenic strain was then cultivated in heterotrophic condition under N-replete and N-starvation. Experimental data of biomass production and nitrogen consumption were well predicted by the Droop model. By means of the model fitting with experimental data, the optimal values for maximum theoretical growth rate (μ), maximum nitrogen quota (qmax), minimum nitrogen quota (qmin) and maximum nitrogen uptake rate (ρmax) were estimated. By estimating, in the future, the growth parameters also for other substrates and for contaminant microorganisms, a more effective wastewater supply strategy can be developed.