About lipid metabolism in Hermetia illucens (L. 1758): on the origin of fatty acids in prepupae

Abstract

Although increasingly targeted in animal nutrition, black soldier fly larvae or prepupae (BSF, Hermetia illucens L. 1758) require the characterization and modulation of their fatty acid profile to become fully integrated within the feed sector. This improvement will only be possible by the understanding of underlaying biochemical pathways of fatty acid synthesis in BSF. In this study, we hypothesized a labelling of de novo synthesized fatty acids in BSF by the incorporation of deuterated water (D₂O) in their feed. Three batches of fifty larvae were reared on two diets with different polyunsaturated fatty acid profiles moistened with 40% of H₂O or D₂O: chicken feed or 40% of chicken feed and 60% of flax cake. Although the occurrence of D₂O in insect feed increased the larval development time and decreased prepupal weight, it was possible to track the biosynthesis of fatty acids through deuterium labelling. Some fatty acids (decanoic, lauric or myristic acid) were exclusively present in their deuterated form while others (palmitic, palmitoleic or oleic acid) were found in two forms (deuterated or not) indicating that BSF can partially produce these fatty acids via biosynthesis pathways and not only by bioaccumulation from the diet. These results suggest the importance of carbohydrates as a source of acetyl-CoA in the constitution of the BSF fatty acid profile but also the potential importance of specific enzymes (e.g. thioesterase II or Δ12 fat2 desaturase) in BSF fatty acid metabolism. Finally, nearly no deuterated polyunsaturated fatty acids were found in BSF fed with deuterium confirming that BSF is not able to produce these types of fatty acids. Despite the high levels of linolenic acid in flax-enriched diets, BSF will simply bioaccumulate around 13% of this fatty acid and will metabolize approximately two-thirds of it into saturated fatty acids as lauric or myristic acid.

Figure 1

Fatty acid profiles of black soldier fly prepupae fed on differentiated PUFAs diets with H₂O or D₂O. (a) fatty acid profile of the diet (CF = 100% of chicken feed), of the black soldier fly prepupae produced on this diet with water (BSF-CF) and produced with deuterated water (BSF-CFD). CLA = Conjugated linoleic acid. (b) fatty acid profile of the second diet (FL = 40% of chicken feed and 60% of flax cake), of the black soldier fly prepupae produced on this diet with water (BSF-FL) and with deuterated water (BSF-FLD). The total of selected fatty acids represents the sum of the two forms (deuterated or not) of this fatty acid. Since the presence of deuterium has an impact on the kinetics of metabolic reactions, both annotations (i.e. “under processing?” and “will be metabolize?”) are related to hypotheses explaining differences in BSF prepupae fatty acid profiles reared on free or containing-deuterium feed. CLA = Conjugated linoleic acid; Z and E letter describe molecule double bonds stereochemistry.

Figure 2

Fatty acid metabolism of black soldier fly prepupae. Whole green arrows = elongation process; whole blue arrow = carboxylation process, red dotted arrows = catabolism process, black dotted arrows = desaturation process, pink dashed arrows = isomerization process. ACC Acetyl-CoA carboxylase, β-ox β-oxidation, CLA Conjugated linoleic acid, ELOVL Fatty acid elongase, FAS fatty acid synthase, Δ-X isom Isomerase, Δ-X Desaturase.