Carrot pomace alone supports heterotrophic growth and lipid production of Auxenochlorella protothecoides

Heterotrophic cultivation of microalgae is vastly studied for the potential it offers as a scalable, space- and time-efficient way to produce biodiesel. From the perspective of economic feasibility, it is understood that renewable, waste-based sources of nutrients must be used to support the cultivation process. In this study, carrot pomace, a waste by-product of the juice industry, was utilized to fully replace synthetic growth medium in the heterotrophic cultivation of Auxenochlorella protothecoides. The carrot pomace underwent minimal pre-treatment to avoid incurring excessive costs. With carrot medium (CM) alone, 4.9 g/L total lipids (9.9 g lipid/kg wet carrot pomace), and with a 5 g/L addition of glucose to the CM, 5.9 g/L (11.8 g lipid/kg wet carrot pomace) could be produced, the latter of which was equivalent to the lipid output achieved with the optimized synthetic growth medium (control). The 5 g/L glucose addition improved the triacylglycerol (TAG) content of the lipids to 78.4%, which was equivalent to the control (76.4%). Growth medium prepared using carrot pomace alone as the sole source of carbon and nutrients was demonstrated to successfully support the growth and lipid production of A. protothecoides. The addition of 5 g/L glucose to the CM improved the lipid output by 19% and the TAG content by 18% and might be preferred for more efficient processing — at only a fraction of the cost of a completely synthetic growth medium, containing 30 g/L glucose in addition to other minerals and trace elements.