Abstract
Improving resource efficiency and strengthening feed system resilience require circular strategies that convert nutrient-rich sidestreams into new inputs. This study assessed whether by-products from insect production and grass processing can be valorised as growth media for freshwater and marine microalgae, contributing to increased nutrient circularity. Insect frass extract (IFE) and grass processing effluent (GPE) were evaluated as alternatives to conventional growth media. Small-scale screening experiments revealed that IFE supported growth across all species tested when supplemented with iron, whereas GPE showed limited suitability. Tolerance to increasing IFE levels was species- and light intensity dependent, with growth rates generally decreasing at IFE levels corresponding to ammonium concentrations of 25–50 mg N/L. Increased light intensities partly alleviated the inhibitory effects of elevated IFE concentrations. Medium-scale cultivation of selected species (Chlorella vulgaris and Phaeodactylum tricornutum) confirmed that IFE can sustain growth when supplemented with iron, and exhibited growth rates, photosynthetic parameters and total lipid contents comparable to controls. Notably, cultivation of P. tricornutum in IFE resulted in an almost 40 % increase in eicosapentaenoic acid. Scale-up challenges like elevated bacterial growth driven by organic carbon availability, reduced light penetration associated with the optical properties of the IFE, and inhibitory effects at high IFE concentrations are highlighted. A screening life cycle assessment indicated only moderately (17 %) higher greenhouse gas emissions for the IFE-based medium compared to a conventional growth medium. Overall, IFE represents a promising nutrient source for microalgae cultivation, particularly in contexts where access to critical nutrients such as phosphorus is limited.