Heterotrophic and mixotrophic cultivation of microalgae for biodiesel production in agricultural wastewaters and associated challenges—a critical review

Many studies have demonstrated that heterotrophic and mixotrophic growth for various microalgae species yields greater biomass and lipid content as compared to photoautotrophic cultivation. This review explores the possibility of leveraging the natural ability of the microorganisms to metabolize carbon heterotrophically and mixotrophically in agricultural wastewaters. This has the potential advantage of improving the overall economics for the production of biodiesel and value-added biomolecules from microalgae, mitigating an existing waste stream and minimizing water requirements. However, there are a number of challenges and gaps in scientific knowledge that suggest a need for ongoing research in the area. In this review, specific focus is dedicated to the metabolic mechanisms, reported performances, and practical challenges that contribute to the uncertainty of employing agricultural wastewaters for heterotrophic and mixotrophic microalgae cultures.