Carbon stable isotope fractionation and trophic transfer of fatty acids in fungal based soil food chains

Stable isotope analysis has been used as a powerful tool in food web studies in terrestrial ecosystems. In addition the occurrence and abundance of fatty acids may serve as indicator for feeding strategies of soil animals. Here we combine both approaches and investigate the fatty acid composition, delta13C values of bulk tissues and individual fatty acids in soil organisms. The fungi Chaetomium globosum and Cladosporium cladosporioides were isotopically labelled by fructose derived from either C3 or C4 plants, and the fungal-feeding nematode Aphelenchoides sp. was reared on C. globosum. Fungi and nematodes were used as diet for the Collembolan Protaphorura fimata. The sugar source was fractionated differently by fungal lipid metabolism in a species-specific manner that points to a sensitivity of physiological processing to the non-random distribution of 13C/12C isotopes in the molecule. As a general trend stearic acid (18:0) was depleted in 13C compared to the precursor palmitic acid (16:0), whereas its desaturation to oleic acid (18:1 omega9) favoured the 13C-rich substrate. Fatty acid profiles of P. fimata varied due to food source, indicating incorporation of dietary fatty acids into Collembolan tissue. Individuals feeding on fungi had lower amounts in C20 fatty acids, with monoenoic C20 forms not present. This pattern likely separates primary consumers (fungivores) from predators (nematode feeders). The isotopic discrimination in 13C for bulk Collembola ranged between -2.6 and 1.4 per thousand and was dependent on fungal species and C3/C4 system, suggesting differences at metabolic branch points and/or isotope discrimination of enzymes. Comparison of delta13C values in individual fatty acids between consumer and diet generally showed depletion (i.e. de novo synthesis) or no changes (i.e. dietary routing), but the fractionation was not uniform and affected by the type of ingested food. Fatty acid carbon isotopes were more variable than those of bulk tissues, likely due to both the distrimination by enzymes and the different lipid origin (i.e. neutral or polar fraction).