Efficient Utilization of Monosaccharides from Agri-Food Byproducts Supports Chlorella Vulgaris Biomass Production under Mixotrophic Conditions

Microalgae are promising resources for the sustainable production of biofuels, feed, and high-value chemicals. Several strains can grow heterotrophically or mixotrophically on multiple organic substrates even if the high cost associated to their use can hinder scalability and economical sustainability of the overall process. The use of agri-food waste biomass hydrolysates might make the cultivation procedure more sustainable, while at the same time valorising underutilized by-products. In this study, Chlorella vulgaris biomass production and sugar utilization was investigated during mixotrophic cultivation on hydrolysates of two inexpensive and widely available recalcitrant agri-food waste biomasses: barley straw (BS) and citrus processing waste (CPW). CPW hydrolysate supported enhanced biomass production, compared to BS digestate, likely because of the presence, besides glucose, of significant amounts of galactose, which is rapidly metabolized by the algae. Notably, when pure monosaccharides were provided as sole organic carbon, growth stopped before complete sugar consumption. Arrested growth in presence of pure monosaccharides correlated with a drastic drop in extracellular pH, which appears to depend on both carbon and nitrogen sources. Our results show that mixotrophic cultivation of C. vulgaris on BS or CPW hydrolysates results in more efficient conversion of organic carbon into biomass, compared to growth on pure sugars, indicating that these agri-food by-products can be utilized as valid feedstocks for sustainable algal biomass production.