Fatty acids of fungi and nematodes—possible biomarkers in the soil food chain?

Abstract

The fatty acid composition of 16 different soil fungi (ascomycetes, basidiomycetes, mitosporic fungi) and a fungal-feeding nematode Aphelenchoides sp. reared on seven fungal species was investigated. Additionally, fatty acid profiles of Aphelenchoides sp. and A. saprophilus grown on the same fungal food source,Agrocybe gibberosa, were compared. Thirteen predominant fatty acids were detected in the fungi. Most of them occurred in each of the 16 species, but relative quantities of individual fatty acids differed, in particular those of unsaturated ones. Most fungal species could be differentiated from each other on this basis. Our study revealed convergence, but also, differences, in the fatty acid composition of systematically related fungi, i.e. a taxonomic or phylogenetic relationship was not necessarily accompanied by similarity in fatty acid profiles. Nematodes comprised a wider fatty acid spectrum than fungi, with 17 predominant fatty acids, and a higher amount of long-chain, polyunsaturated acids than their fungal diet. Fungal host tissue may have supplied palmitic, oleic and linoleic acid present, whereas most of the long-chain unsaturated fatty acids were synthesized by the nematodes. Unsaturated fatty acids mainly belonged to the ω6 and 9 family, indicating a carboxyl-directed desaturation as a major metabolic pathway. The fungal host significantly affected the fatty acid profile of the nematodes. However, we could not assign individual fatty acids as biomarkers reflecting the dietary source, likely due to the considerable convergence within tested fungal species. Of the basidiomycetes analyzed Laccaria laccata, was distinctly separated from the others and this difference could also be detected in the nematodes showing the influence of food type. We conclude that the lipid composition of nematodes is controlled by both the nematode and its diet and that monitoring fatty acid patterns of soil animals may therefore provide a way to detect trophic interactions in belowground food webs.

Introduction

Analysis of lipid composition is an important tool for taxonomic and phylogenetic classification of microorganisms (Tunlid and White, 1992). In particular phospholipids (PLFAs) found in the cell membranes of all living organisms have proved to be of great value in describing microbial community structure. PLFA composition varies widely between different groups of organisms, and fatty acid pattern therefore reflects species composition and relative species abundance of microbial communities (Bååth et al., 1998). Profiles of total cellular fatty acids (Haack et al., 1994, Zelles et al., 1994) or of PLFAs (Tunlid and White, 1992, Kieft et al., 1994) have been used to estimate microbial biomass and to provide insight into the diversity and nutritional status of microorganisms.

Some PLFAs have been identified as useful biomarkers for certain groups of bacteria. Branched chain fatty acids are characteristic for Gram-positive bacteria, while Gram-negative bacteria contain β-hydroxy fatty acids in their lipopolysaccharides (White et al., 1996, Zelles, 1997). The PLFA 18:2 ω6 is regarded as a bioindicator for fungal biomass in soils (Frostegård and Bååth, 1996). The potential of profiling total cellular lipids as a taxonomic tool for fungi has received attention in some recent studies (Viljoen et al., 1986, Jabaji-Hare, 1988, Sancholle and Dalpé, 1993, Bentivenga and Morton, 1994, Graham et al., 1995, Stahl and Klug, 1996). These have demonstrated that fatty acid comparison is a robust measure of similarity below the family level.

The relative percentages of total lipids, neutral lipids and polar lipids of free-living and plant-parasitic nematodes have been investigated (e.g. Fletcher and Krusberg, 1973, Womersley et al., 1982, Badhwar et al., 1995, Abu Hatab et al., 1998, Abu Hatab and Gaugler, 1999), but only a few studies give fatty acid profiles and those which do are predominantly for animal or plant parasites (Badhwar et al., 1995, Abu Hatab and Gaugler, 1997, Holz et al., 1999). Fatty acid patterns of free-living nematodes include reports for bacterial-feeding Turbatrix aceti, Panagrellus redividus and Caenorhabditis elegans (Sivapalan and Jenkins, 1966, Fletcher and Krusberg, 1973, Chitwood and Krusberg, 1981) and plant parasitic species of Aphelenchoides and Ditylenchus (Krusberg, 1967), but to the best of our knowledge fatty acid patterns of free-living fungal feeders have not been described.

Generally, the fatty acid composition of an organism is determined by the particular type of biosynthetic pathway of the given species. However, fatty acid profiles vary due to life cycle or metabolic activity (Tunlid and White, 1990) and are influenced by stress factors, like toxicity, exposure to solvents or starvation (White et al., 1996, Najdek, 1997). Besides this, fatty acid composition is affected by the carbon source of the diet of organisms (Dunlap and Perry, 1967, Tunlid and White, 1990) and there is evidence for trophic transfer of fatty acids between microorganisms and grazing microfauna (Ederington et al., 1995). Influence of the host tissue or the culture medium on the lipid composition of nematodes have been reported for entomopathogenic (Abu Hatab and Gaugler, 1997, Abu Hatab and Gaugler, 1999, Abu Hatab et al., 1998) and plant parasitic species (Krusberg 1967).

We have analyzed the cellular fatty acid profiles of 16 soil fungi selected from different taxonomic groups (ascomycetes, basidiomycetes, mitosporic fungi). This comparative analysis provided further insight in the distribution of fatty acids among species and the usage of fatty acid profiling as a taxonomical tool. We also determined the lipid composition of two free-living fungal feeding nematodes of the genus Aphelenchoides and the trophic transfer of fatty acids between the fungi and their grazers. Aphelenchoides specimens were reared with seven different fungi as a food source and the fatty acid profiles from fungi and nematodes were compared. Our aim was to study (i) if the fatty acid content of the nematodes is influenced by their diet and (ii) if fatty acids characteristic for fungi are assimilated by the nematodes and could serve as biomarkers within the soil food web.