Melanie Pollierer: Saprotrophic and ectomycorrhizal fungi and their consumers can be separated using amino acid fingerprints

Melanie Pollierer
Melanie Pollierer

Dr. Melanie Pollierer, Researcher at University of Göttingen, summarizes her latest publication “Isotope analyses of amino acids in fungi and fungal feeding Diptera larvae allow differentiating ectomycorrhizal and saprotrophic fungi-based food chains”, where she and her colleagues show a novel methodology to identify different fungal guilds along food chains. She highlights the relevance of their findings and shares her experience as a young researcher in Germany.

About the paper

Fruiting bodies of ectomycorrhizal and saprotrophic fungi similar to those analysed along with gnat larvae living and feeding inside. Photo credits: Jörg Müller, Jörg Hailer.
Fruiting bodies of ectomycorrhizal and saprotrophic fungi similar to those analysed along with gnat larvae living and feeding inside. Photo credits: Jörg Müller, Jörg Hailer.

Our paper integrates two major fungal functional groups, saprotrophic and ectomycorrhizal fungi, into soil animal food webs of temperate forests. To be able to separate the two fungal functional groups and to identify the trophic position of fungi and consumers, we applied the state-of-the-art method of dual compound-specific analysis of nitrogen and carbon in amino acids.

It has been a long-standing riddle to separate the contribution of saprotrophic vs. ectomycorrhizal fungi to the nutrition of soil animals in forest ecosystems. These fungal functional types differ fundamentally in the way they acquire carbon; while the former get all energy and nutrients from decomposing organic matter, the latter are tightly linked to the roots of their host plants (trees) for acquiring recently synthesized photosynthetic carbon products. Therefore, the two functional groups are linked to different energy channels originating from aboveground plant compartments, i.e. via leaf litter, or from belowground via roots. Although it has been shown that root-derived carbon is of major importance for soil animal food webs, it remains unclear to what extent animals acquire this carbon by feeding on ectomycorrhizal fungi.

Compound-specific analysis of nitrogen and carbon in amino acids is a powerful new tool to uncover trophic positions and basal resources within food webs. A fingerprinting approach, using δ13C signatures of essential amino acids, has been used to separate bacterial, plant, and fungal contributions to the diet of soil animals. Here, we aimed at going one step further and investigated whether this fingerprinting approach also allows to distinguish between saprotrophic and ectomycorrhizal fungi, and to trace the channelling of carbon from these fugal functional types into animal consumers, in our case fungal feeding gnat larvae. Indeed, fingerprinting clearly separated saprotrophic and ectomycorrhizal fungi, and gnat larvae at least in part were assigned to their correct food source. Further, the use of δ15N values of so-called ‘trophic’ and ‘source’ amino acids to calculate trophic position provided a more realistic classification of the trophic position of fungi and their consumers in the food web compared to other methods such as bulk stable isotope analysis.

In summary, the methodology used can be applied to separate the contribution of saprotrophic vs. ectomycorrhizal fungi to the diet of consumers in soil food webs, and therefore to clarify along which pathways nutrients and energy are channelled to higher trophic levels. This will refine the understanding of decomposer food webs and therefore of nutrient cycling in soil.

About the research

A – Gnat larva (Mycetophilidae) which feeds inside fungal fruiting bodies and B – Springtails on dead leaves, important fungal feeders in forest soil food webs. Photo credits Stefan Scheu, Sarah Bluhm.
A – Gnat larva (Mycetophilidae) which feeds inside fungal fruiting bodies and B – Springtails on dead leaves, important fungal feeders in forest soil food webs. Photo credits Stefan Scheu, Sarah Bluhm.

To be able to integrate the contribution of saprotrophic vs. ectomycorrhizal fungi when investigating soil animal food webs opens exciting new perspectives – what is the contribution of these fungal functional groups to energy fluxes within soil food webs and how does it change, e.g. with different management practices, environmental conditions, and vegetation types?

Energy fluxes and nutrient cycling crucially depend on food webs below the ground. Changes in these fluxes significantly impact nutrient availability to plants, but also carbon storage in soils. This is of particular interest because little is known on how human impact affects these processes and how management practices can be adjusted to foster belowground decomposer communities.

As a next step the proposed methodology needs to be applied to ‘real’ soil animal communities for exploring how the contribution of different fungal functional groups changes, e.g. with management practices and tree species composition. How resistant are soil animal communities to transformations of forest ecosystems? Does this differ, e.g. between temperate and tropical forest ecosystems? Are soil animals specialized and feed on certain fungal functional types, or can they adapt their food spectrum by switching from consumption of saprotrophic to ectomycorrhizal fungi and vice versa? How does this affect the whole soil animal community? Analyses can be done at species level, but also at community level, with species pooled per location. Energy fluxes via different fungal channels can then be calculated using mixing models, providing information on species- or community-level responses to environmental changes.

About The Author

I have been interested in nature, in particular in animals, all my life. I already claimed that I wanted to be a naturalist in kindergarten, when asked what I wanted to become later on.

I followed up on this idea when I started to study biology. After one year of studies I took a little detour, working in different jobs, but I finally realized that becoming a biologist was really what I wanted to do, so I finished my studies.

For my diploma thesis, I was lucky to join a great project, investigating the impact of belowground carbon inputs to soil animal food webs in a whole-forest labelling experiment. The resulting publication is the one I am most proud of, first because I wrote it directly after my diploma thesis, when I was not financed yet, and second because it showed the major importance of belowground inputs for soil animal food webs and until now it is my most cited publication. From there the direction was set and I started my PhD in the ‘Biodiversity Exploratories’, a long-term research platform featuring three major areas across Germany, with grass and woodland managed at different intensities. My main topic were soil animal food webs, and how they change with tree species and management intensity. During my career, also the methods I used evolved. I started with using bulk stable isotope analyses, went on in using marker fatty acids, compound-specific fatty acid analyses, and now finally compound-specific amino acid analyses, always with the goal to gain deeper insights into the hard-to-observe food web of cryptic soil animals. After having had my own researcher position for four years, now I am back to the ‘Biodiversity Exploratories’ as a post-doc, and I am currently using the newly developed fingerprints of fungal functional groups to investigate real forest soil animal food webs.

I like many aspects of being an ecologist, the diversity of work is great, from field work, to lab work, data analysis, and finally writing. I really like writing and finding good expressions. I spent one year in Canada during schooldays, and my high school English teacher there aroused my interest in good writing. Another great thing is to go to conferences, meet interesting people and broaden the horizon by travelling to other countries. A drawback of being a scientist in Germany is that, except for few lucky cases, your position is for a long time non-permanent and there is always uncertainty about how things will develop.

Besides being an ecologist, I am also a mom of two sons, 8 and 10 years old, and I enjoy hiking in nature and gardening. Finding the balance between the different parts of my life is not always easy, but I would not want to miss any of them.

Read the paper here!

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