Differentiating the Temperature Response of Soil Fungi and Bacteria

Abstract

The decomposition of soil organic matter (SOM) may be accelerated under increasing temperatures from climate change, which could create feedback loops that result in the loss of soil carbon (C) stocks. Fungi and bacteria regulate the decomposition of SOM, each having their own unique adaptations that allow them to fulfill different roles in the decomposition process. However, fungi and bacteria may respond to temperature differently which could complicate soil C cycling dynamics. To date, few studies have attempted to differentiate the temperature response of fungal and bacterial respiration.

The aim of this research was to differentiate the temperature optima (Topt) and inflection point temperatures (Tinf) of fungal and bacterial respiration. The Topt is the temperature at which respiration is maximal whereas the Tinf is the point at which respiration is most sensitive to temperature.

Soils were collected from different locations along a geothermal temperature gradient (that had average temperatures between ~13-39 °C). The selective inhibition method was used to differentiate the temperature response of fungal and bacterial respiration by treating soils with streptomycin and cycloheximide, respectively. Soils were also treated with and without glucose to determine the non-substrate limited respiration response of fungal and bacterial respiration. Treated soils were incubated in a temperature block that allowed the incubation of soils at temperatures ranging between ~5-52 °C for five hours. Headspace samples were taken from each treatment tube and injected into an infrared gas analyser (IRGA) to measure carbon dioxide (CO2) concentrations and respiration rates. Macromolecular rate theory (MMRT) was used to model the temperature response of the respiration curves to derive the Topt and Tinf of fungal and bacterial respiration, allowing the comparison of the temperature responses of fungal and bacterial respiration.

An additional aim of this research was to measure changes in fungal and bacterial biomass along the geothermal temperature gradient using phospholipid fatty acid (PLFA) analysis. Soil samples were collected from the same sampling locations along the geothermal temperature gradient as those used in the selective inhibition experiments. A modified Bligh and Dyer method was used to extract phospholipids from soil samples, which were used as biomarkers to differentiate the biomass of different fungal and bacterial groups.

Results from selective inhibition experiments showed that respiration rates decreased from streptomycin and cycloheximide treatment, allowing an estimation of fungal and bacterial contributions to total soil respiration, respectively. Fungal contributions to soil respiration (~55%) were greater than bacterial contributions (~21%), suggesting that fungi contribute more to soil C decomposition than bacteria. Furthermore, the Topt of fungal respiration was found to decrease with environmental temperature at a rate of -0.396 °C °C-1 whereas the Topt of bacteria and the microbial community was not correlated with environmental temperature. Additionally, the Topt and Tinf of fungal respiration (38.6 °C and 22.1 °C, respectively) was not significantly different from the Topt and Tinf of microbial respiration (37.0 °C and 21.7 °C, respectively) but was significantly greater than the Topt and Tinf of bacterial respiration (30.4 °C and 19.0 °C, respectively). Results from PLFA analysis showed that gram-positive bacteria and general bacteria dominated total microbial biomass and biomass from identified fungal and bacterial groups peaked at ~19-21 °C. Fungal-to-bacterial (F:B) ratios decreased with increasing environmental temperature, indicating a proportional decrease in fungal biomass relative to bacteria at higher temperatures. These results suggest that fungi were excluded with increasing environmental temperature as they were unable to adapt their metabolism and growth as environmental temperatures increased.

The selective inhibition method presented a range of limitations that may have constrained interpretation of the results. First, inhibitors were not equally effective across the range of incubation temperatures. Inhibition from cycloheximide showed a clear temperature optimum where inhibition was greatest within ~45-50 °C. In contrast, inhibition from streptomycin declined at temperatures above ~30 °C. Secondly, percent inhibition of microbial respiration from streptomycin increased as total bacterial biomass increased, providing evidence that streptomycin was effective in targeting bacteria. In contrast, percent inhibition of microbial respiration from cycloheximide was not correlated with fungal biomass, indicating that cycloheximide may not have been effective in selectively targeting fungi.

Overall, these results indicate that fungi may be more at risk from increasing temperatures under climate change, though it is unknown as to how this could translate to soil C stocks. This was likely the first study that attempted to differentiate the temperature response of fungi and bacteria by selectively inhibiting fungi and bacteria and incubating them across a range of temperatures. However, caution should be taken when interpreting results as a range of methodological constraints were uncovered regarding the veracity of the selective inhibition method. While previous studies have reported some limitations of the selective inhibition method, these issues have likely been under-reported as researchers may not publish inconclusive or negative results. Therefore, this study also highlights the importance of critically assessing methods that may be ineffectual.