Lipid content and carbon assimilation in Collembola: implications for the use of compound-specific carbon isotope analysis in animal dietary studies

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Abstract
In an effort to understand the relationships between both the lipid content and δ13C values of Collembola and their diet, isotopically labelled (C3 and C4) bakers’ yeasts were cultured and fed to two Collembolan species, Folsomia candida and Proisotoma minuta. The fatty acid composition of Collembola generally reflected that of the diet with the addition of the polyunsaturated components 18:2(n-6), 20:4(n-6) and 20:5(n-3), which appeared to be biosynthesised by the Collembola. Whilst ergosterol was the only sterol detected in the yeast diets, only cholesterol was detected in Collembola, and although the δ13C values of diet and consumer sterols differed by >2‰, the δ13C values indicated that cholesterol was derived entirely from dietary sterol. The bulk δ13C values of Collembola were similar to those of the diets, but fatty acid δ13C values did not necessarily reflect those of the dietary fatty acids, indicating significant de novo biosynthesis of fatty acids within Collembola. Switching the Collembola from C3 to C4 yeast enabled the determination of the rates of incorporation of dietary carbon into Collembolan lipids, and showed that half-lives of the incorporation of dietary carbon varied between 1.5 and 5.8 days at 20°C. Cholesterol exhibited the slowest rate of incorporation in both species, while bulk carbon in F. candida possessed an intermediate rate. These results demonstrate that an understanding of the sources of isotopic fractionation and the role of biochemistry in regulating the δ13C values of individual compounds is important in the application of compound-specific isotopic analysis to the study of animal trophic activities.