Potentialities of dark fermentation effluents as substrates for microalgae growth: A review

•Coupling dark fermentation and microalgae production in a biorefinery concept.•The broad range of fermentation metabolites production is explained.•The use of fermentation metabolites as substrates for microalgae is discussed.•The challenges and prospects of this promising coupling are outlined. In recent years, coupling bacterial dark fermentation (DF) and heterotrophic cultivation of microalgae (HCM) has been pointed out as a promising sustainable approach for producing both gaseous and liquid biofuels. Complex organic waste and effluents that are not susceptible to be directly degraded by microalgae are first converted into volatile fatty acids (VFAs) and hydrogen by DF. In this work, the feasibility of using DF effluents to sustain HCM has been thoroughly reviewed and evaluated. Promising perspectives in terms of microalgae biomass and lipids production are proposed and can be extended as guidelines to promote HCM whatever the organic waste used. Abiotic and biotic factors from DF effluents that promote or inhibit microalgae growth are discussed as well as the use of unsterile DF effluents. Overall, the microalgae growth is favored on effluents containing high acetate concentration (>3gL−1), with a high acetate:butyrate ratio (>2.5), and when pH is strictly controlled. At a low acetate:butyrate ratio (10gL−1), a low substrate:microalgae ratio and the presence of light appear to enhance microalgae growth. Butyrate content appears to be a key factor when coupling DF/HCM since high butyrate concentration inhibits the microalgae growth.