Deciphering metabolic alterations in algae cultivated in spent media as means for enhancing algal biorefinery sustainability

Abstract

The recycling of unfiltered spent media during cultivation of Chlorella vulgaris was studied using metabolomics in an effort to enhance water and nutrient sustainability and reduce operating costs in algal biorefineries. Cultivation in spent media resulted in reduced biomass and lipid productivity by 14% and 19%, respectively, compared to fresh media. The decrease was related to a detected lower nutrient uptake. Nevertheless, carbohydrate content (28% of dry cell weight) and α-linolenic acid content (27 % of fatty acids) were higher in spent media cultures than in fresh media. Metabolomics analysis of intracellular metabolites revealed downregulation of nitrogen assimilation, tricarboxylic acid cycle, structural lipids, and energy metabolism, but upregulation of stress mitigation and carbohydrate synthesis. No growth was supported by spent media during a second cultivation cycle and was likely due to the identified extracellular accumulation of humic acid and free fatty acids that acted as growth auto-inhibitors.