Growth and metabolism of interacting ectomycorrhizal and saprotrophic fungi: effects on mycelial to hyphal scale

Functions and productivity of fungal communities in soil are affected by interspecies interactions and competition for resources, which, in turn, affects biogeochemical cycles and fluxes of CO₂ from soil. Specifically, saprotrophic and ectomycorrhizal (ECM) fungi compete for limiting nutrients with great effects on overall decomposition rates. Macroscale observations are inconsistent: decomposition can be suppressed (‘Gadgil effect’), typically, in nutrient poor ECM dominated forests, or exacerbated (‘priming’) by ECM fungi foraging for nitrogen in organic matter or introducing labile carbon to soil. Gaining deeper insight into the mechanisms affecting mycelial and hyphal scale processes among the competing fungi could increase understanding of the reasons for these inconsistencies and better predict the direction of overall decomposition rates.

We investigated changes in growth and anabolic metabolism of an ECM fungus (Suillus luteus) interacting with a saprotrophic fungus (Gymnopus confluens) cultured in different concentrations and types of carbon and nitrogen sources on agar plates and inside microfluidic soil chips. The metabolism analysis was performed using stable-isotope labelling (SIP) combined with Raman microspectroscopy in the chips. At the mycelium scale, S. luteus grown in co-culture plates formed denser mycelium and demonstrated increased competitiveness under changing nitrogen concentrations. G. confluens increased its elongation rates and dominated under changing carbon conditions. Supplied with equal amounts of glucose and complex carbon (carboxymethylcellulose, CMC), G. confluens facilitated the growth of S. luteus, which exhibited increased density and elongation rates in co-cultures. Microstructures of the soil chips further affected the growth of S. luteus: while its growth rates in terms of elongation were typically smaller in agar plates, they increased and surpassed those of G. confluens in the chips in all nutrient treatments. This demonstrates the impact of both the nutritional and the physical environment on the outcome of fungal interactions. Furthermore, similar as in the agar plates, C. confluens facilitated growth of S. luteus in chips supplied with both glucose and CMC. To investigate this further, we are setting up an experiment, where deuterium labelled glucose source (glucose-d7) is used and traced as it is incorporated into the fungal hyphae growing in the chips using Raman microspectroscopy. Enrichment of fungal biomass with deuterium is detected via appearance of C-D functional group related spectral bands in the Raman spectra. We expect that this will allow us to determine whether the presence of CMC decreases competitiveness of G. confluens vs S. luetus as it directs its metabolism towards CMC degradation, and whether the carbon released in the process is uptaken by S. luteus as well.