Fungi rather than bacteria drive early mass loss from fungal necromass regardless of particle size

Abstract

Microbial necromass is increasingly recognized as an important fast-cycling component of the long-term carbon present in soils. To better understand how fungi and bacteria individually contribute to the decomposition of fungal necromass, three particle sizes (>500, 250-500, and <250 μm) of Hyaloscypha bicolor necromass were incubated in laboratory microcosms inoculated with individual strains of two fungi and two bacteria. Decomposition was assessed after 15 and 28 days via necromass loss, microbial respiration, and changes in necromass pH, water content, and chemistry. To examine how fungal-bacterial interactions impact microbial growth on necromass, single and paired cultures of bacteria and fungi were grown in microplates containing necromass-infused media. Microbial growth was measured after 5 days through quantitative PCR. Regardless of particle size, necromass colonized by fungi had higher mass loss and respiration than both bacteria and uninoculated controls. Fungal colonization increased necromass pH, water content, and altered chemistry, while necromass colonized by bacteria remained mostly unaltered. Bacteria grew significantly more when co-cultured with a fungus, while fungal growth was not significantly affected by bacteria. Collectively, our results suggest that fungi act as key early decomposers of fungal necromass and that bacteria may require the presence of fungi to actively participate in necromass decomposition.

FIGURE 1

Fungal necromass loss (%) with different particle size by microbial strain. (A) necromass loss (%) at 14 days post‐inoculation (dpi) (solid circle) and 28 (hollow circle) dpi. Control panel shows the percentage of mass loss in uninoculated necromass. Error bars show ±1 standard error of the mean. Necromass particle sizes are shown for >500 μm (B), 250–500 μm (C), and <250 μm (D).

FIGURE 2

Respiration by microbial strain. (A) CO₂ loss (μg C h⁻¹) over time measured at 1, 5, 8, 15‐, and 28‐days post‐inoculation. (B) Cumulative CO₂ loss by microbial strain. (C) Correlation between necromass loss (percentage) and cumulative CO₂ (mg C) after 28 days by microbial strain. Lines in C shows the linear relationship (y = mx + b) between the variables for each microbial strain. At each time point, headspace CO₂ concentration measurements were made after a 24‐h incubation period. Error bars show ±1 standard error of the mean (circles).

FIGURE 3

Microbial strain‐induced changes in necromass (expressed as delta “Δ”). Panel (A) necromass mass loss (%), (B) gravimetric water content (GWC) (%), and (C) pH. Values are shown by microbial strain at 15 days post‐inoculation (dpi) (solid circle) and 28 (hollow circle) dpi. Values are compared against the uninoculated control (0 line) at each time point (15 and 28 days). Error bars represent 95% confidence intervals.

FIGURE 4

Absorbance values from Fourier‐transform infrared spectroscopy (FTIR) analyses on necromass colonized by each microbial strain. Panel (A) FTIR spectra showing the position of each peak; (B) absorbance values per peak showing peak position, the associated functional group (when available), and the chemical bond type on the faceted strip. Values are shown by strain at 15 days post‐inoculation (dpi) (solid circle) and 28 (hollow circle) dpi for the 11 peaks identified. Values are compared against the uninoculated control (0 line) at each time point (15 and 28 days). Error bars represent 95% confidence intervals.

FIGURE 5

Microbial strain rRNA copy numbers in co‐culture (expressed as delta “Δ,” relative to when growing alone). Each panel shows the growth of each microbial strain (name on the strip of each panel) when growing with another strain (colours shown in the figure legend). Values, based on qPCR, were generated after a 5‐day incubation period. Error bars show the standard error. Values are compared against their values grown in isolation (0 line) at 5 days post‐inoculations. Error bars represent 95% confidence intervals.