Insect Frass as a Sustainable Medium for Microalgae Growth: Bridging Waste Valorization and Eco-Physiological Innovation

The intensive use of chemical fertilizers has boosted agricultural yields but caused severe environmental concerns, including soil degradation, water pollution, and greenhouse gas emissions. Sustainable alternatives are therefore urgently needed. Insect frass, a nutrient-rich by-product of insect farming, and microalgae, with their ability to produce phytohormones and improve soil quality, have both been proposed as promising biofertilizers. While their individual applications are well documented, little attention has been given to their combined use. This review provides an updated synthesis of current knowledge on insect frass composition, the agronomic value of microalgae, and the first experimental evidence on the use of frass as a nutrient source for microalgal cultivation. Benefits and challenges are discussed, including nutrient variability, microbial safety, heavy metal accumulation, and production costs. The integrated perspective offered here highlights the potential of frass–microalgae systems to support circular bioeconomy models and reduce reliance on chemical fertilizers.