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

In an effort to understand the relationships between both the lipid content and delta(13)C values of Collembola and their diet, isotopically labelled (C-3 and C-4) 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 delta(13)C values of diet and consumer sterols differed by >2parts per thousand, the delta(13)C values indicated that cholesterol was derived entirely from dietary sterol. The bulk delta(13)C values of Collembola were similar to those of the diets, but fatty acid delta(13)C 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 C-3 to C-4 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 20degreesC. 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 delta(13)C values of individual compounds is important in the application of compound-specific isotopic analysis to the study of animal trophic activities.