Integration of agro wastes as an alternative substrate for microalgae cultivation: A review

The identification of alternative substrates is essential to reduce cultivation complexity and production costs in biotechnological processes. In the context of bioenergy applications, microalgae have emerged as promising feedstocks; however, the biochemical composition of the biomass is strongly influenced by the cultivation substrate. Consequently, ongoing research efforts focus on optimizing cultivation strategies to improve both biomass yield and product quality. A notable knowledge gap remains regarding the use of agro-waste as an alternative substrate for microalgae cultivation. Within a circular economy framework, agro-waste represents a valuable resource for enhancing biomass productivity and sustainability. Nevertheless, its practical application is challenged by seasonal availability, variability in agricultural practices and processing methods, and the influence of abiotic and biotic factors that can alter chemical composition and limit substrate consistency. This review examines the generation, availability, and compositional characteristics of major agro-wastes streams and evaluates their application in biotechnological processes aimed at bioenergy production. Particular emphasis is placed on microalgae cultivation-either as a standalone process or integrated with other biological treatments-using agro-waste-derived substrates. Reported data indicate that substrates such as vegetable peels, wine lees digestate, and piggery manure digestate can significantly enhance microalgal biomass production, increasing yields from 0.96 g/L up to 3.16 g/L, while also promoting lipid accumulation exceeding 20%. The strong influence of substrate composition on microalgae lipid profiles is highlighted, particularly the reduction of linolenic acid content. The potential integration of agro-waste-derived microalgal biomass into biorefinery platforms, including biodiesel production compliant with the European standard EN14214, is discussed.