From waste to products: microalgal cultivation in insect frass to obtain valuable biomass

Abstract

Insect frass, a residual by-product of insect farming, is increasingly considered for circular-bioeconomy applications. Here we quantitatively evaluated Acheta domesticus frass as the sole nutrient source for Chlorella vulgaris, Nannochloropsis salina and Phaeodactylum tricornutum, using frass loadings equivalent to control nitrogen supplies (4.12 g L⁻¹ for freshwater species; 250 mg L⁻¹ for marine species) under both autoclaved and non-autoclaved conditions. P. tricornutum did not survive acclimation. In non-autoclaved frass, C. vulgaris reached a μmax of 0.62 d⁻¹ and final cell density of 1.1 × 10⁷ cells·mL⁻¹, similar to control, while N. salina attained 6.5 × 10⁶ cells·mL⁻¹ but with growth rate reduced by ∼50 %. Autoclaving reduced maximum biomass yields by 30–40 % relative to non-autoclaved cultures, although growth rates remained similar. Growth in frass shifted algal biochemical composition toward lower protein content, whereas lipid fractions remained similar to controls. Fatty acid profiles showed increased saturated fatty acids (16:0, 18:0) and reduced levels of 18:2n-6 and 18:3n-3, with an overall decline in PUFA content. In summary, insect frass supported algal growth but yielded lipid characteristics more suitable for biodiesel than for PUFA-rich feed.