Mixotrophic and Heterotrophic Metabolism in Brewery Wastewater by Chlorella Vulgaris: Effect on Growth, FAME Profile and Biodiesel Properties

Abstract

Microalgae show potential as renewable and environmentally friendly fuel resources. Wastewaters (WWs) can be utilized as growth media, reducing the associated cultivation costs. Therefore, the aim of this study was to investigate how organic-rich WWs affect the growth and fatty acid methyl esters (FAME) profile of Chlorella vulgaris. This particular strain exhibits high biomass productivity and can thrive in a wide range of WWs. It also has the ability to shift its metabolism from autotrophic to hetero/mixotrophic. Glycerol can serve as a means to direct metabolism towards lipids production. Consequently, C. vulgaris was cultivated in brewery wastewater (BWW) containing varying concentrations of glycerol under both metabolic conditions. When C. vulgaris was cultivated in a mixotrophic environment, it achieved a notably higher biomass yield compared to heterotrophic cultivation. The highest biomass yield, reaching 1.33 g L-1, was achieved utilizing 2 mL of glycerol in BWW, surpassing the control with 1.08 g L-1. However, when a two-phase metabolism was applied, comprising the ten days of mixotrophy followed by the final five days in heterotrophy (MHB), the biomass yield was slightly lower than that obtained under continuous mixotrophic conditions. Nevertheless, it was still double the biomass obtained in a strictly heterotrophic environment. The FAME profile analysis revealed that, within the considered trophic conditions, the highest content of saturated fatty acids (SFA), monounsaturated fatty acids (MUFA), and polyunsaturated fatty acids (PUFA) was observed during mixotrophic cultivation with 10 mL of glycerol, mixotrophic cultivation with 4 mL of glycerol, and MHB with 4 mL of glycerol (35.36%wt, 46.89%wt, and 31.60%wt, respectively). An initial examination of the saturated and unsaturated components of the FAME suggests that lipids extracted from C. vulgaris biomass cultivated mixotrophically and heterorophically in BWW could potentially serve as a valuable feedstock for biodiesel production.