Growth tradeoffs in decomposer fungi

Research Group

Introduction

Hypothesis

Methods

Results

Conclusions

Acknowledgement

Research Group

Miami University Department of Biology

Faith Moore, Sam Schultheis, Jenn Butt, Melany Fisk

Introduction

Fungi play a vital role in terrestrial ecosystems by decomposing organic matter and recycling nutrients.
Their ability to extend hyphae across soil and litter allows them to access and redistribute nutrients in patchy environments.
Most past research has focused on the effects of single nutrients, especially nitrogen (N), on fungal growth.
However, fungi require a balance of nutrients—particularly nitrogen and phosphorus (P)—for key biological processes like protein synthesis (N) and DNA/RNA production (P).
Nutrient imbalance (low N relative to P, or vice versa) is common in soils and may influence fungal foraging strategies.
Faster but less dense hyphal growth may be a strategy to explore new areas when one nutrient is scarce.
This project investigates fungal foraging response to imbalanced N and P availability in a controlled lab experiment.

Hypothesis

Fungi will grow faster but less densely under nutrient-imbalanced conditions compared to balanced ones as a strategy to forage for the limiting nutrient.

Methods

Fungal Species: Multiple root decomposing fungal species were cultured from forest soils in New Hampshire; 55 species selected to represent a range of growth rates.

Experimental Design:
Full factorial design testing two nitrogen (N) levels × two phosphorus (P) levels.

2 balanced treatments: low N and P (LNLP) and high N and P (HNHP);

2 unbalanced treatments: low N and high P (LNHP), and high N and low P (HNLP)

Nutrient levels and N P ratios based on published averages (Carmenzind et al. 2021)

Fungal growth: Growth in length (extension) and mass were measured in 25 mL pipettes containing agar with the different nutrient treatments (Figure 1).

Density (hyphal mass/agar volume) was quantified in two equal segments of the fungal growth in a tube.

Foraging refers to lower density in the leading edge end (segment 2) compared to the inoculated end (segment 1; Figure 1).

Analysis: treatment effects were tested in a mixed effects model with N, P, and their interaction as fixed effects and fungal species as a random effect.

Figure 1. Cultures growing in tubes (A); segmented tubes for measuring mass, density, and foraging index.

Figure 2: Fungal Growth In the race tubes

Results

Figure 3. Treatment responses by fungal extension (A), density in the first and second segments of the culture (B), and foraging index (C), in slow and fast growth groups. A foraging index of 0.5 indicates (C: horizontal line) indicates equal mass distribution between the two equal length segments of the culture. Less mass in the extending end (second segment) than in the inoculated end (first segment) gives a foraging index > 0.5 and is referred to as hyphal “foraging”Extension was greater in low N or low P treatments than in high N and P together (Figure 3A; N x P x growth type interaction, p = 0.016). Hyphal density was greater in slow- than fast-growing fungi (Figure 2; p = 0.003 for main effects of growth type) and also was lower in treatments with low N or low P than high N and P (p = 0.04 for the NxP interaction). Hyphal density of slow-growing fungi was greatest in the first segment (inoculated end of culture) and also in the low N and low P treatments (Figure 3B), consistent with a foraging response under unbalanced nutrition. A greater foraging response was found in treatments with either low N or low P, compared with high N and P together, especially in slow-growers (p = 0.0002 for the N x P interaction; Figure 3C).

Conclusions

We show a tradeoff between growth rates and density across these root decomposer fungal species.

We also found support for our hypothesis that fungi respond to unbalanced nutrition with hyphal foraging:

Unbalanced nutrition promoted hyphal extension, especially in faster growing species
Unbalanced nutrition also promoted extension of less dense hyphal growth, especially in response to low N and in slower growing species.

Acknowledgement

We thank the Hughes program, Miami University, and the USDA Forest Service Bartlett and Hubbard Brook Experimental Forests for support for this project. This work is a contribution to the Hubbard Brook Ecosystem Study and is a product of the MELNHE (Multiple Element Limitation in Northern Hardwood Ecosystems) project funded by the NSF Long-Term Ecological Research Program and the USDA National Institute of Food and Agriculture.

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