Effects of temperature on microbial transformation of organic matter – comparing stories told by purified enzyme assays, chemostat experiments and soils

Abstract:

Temperature (T) is a major determinant of microbial decomposition of soil organic matter (SOM). Quantifying T responses of microbial C fluxes is crucial to improve predictions of SOM dynamics and atmospheric CO₂ concentrations, but interpretation of experimental data is complicated by many properties inherent to soils. Comparing such data with complementary, reductionist experiments can help to identify basic mechanisms and interpret soil measurements.

We quantified T effects on activity levels (i.e., rates of substrate cleavage) of microbial extracellular enzymes β-glucosidase (BGase) and β-N-acetyl glucosaminidase (NAGase), and on rates of CO₂ efflux in soil incubations. We compare the results to those derived from purified enzyme assays, and to measurements of microbial respiration rates in continuous-flow chemostat culture in which a population of the soil bacterium Pseudomonas fluorescens was grown on medium with similar C:N ratio as the incubated SOM (10:1).

Activity levels of both BGase and NAGase decreased by 80% between 25 and 5 °C. These T responses were higher than predictions from intrinsic (i.e., maximum) T responses in purified assays of BGase (minus 50%) and NAGase (minus 67%). This suggests that factors like physical access to substrate or reduced microbial production of enzymes constrained substrate decomposition rates in the soils relatively more at low than at high T.

In chemostats, (mass-)specific bacterial respiration rate at T 14.5 °C was 50% of the rate observed at 26.5 °C; in contrast, CO₂ efflux from the soil incubations decreased by only ~25% from 25 to 15 °C. The reason for this discrepancy can be manifold, including changes in microbial community composition, but results from ongoing measurements of microbial biomass in the soil samples will allow a closer comparison of these respiration rate responses.

Our efforts highlight the significance of experimenting across scales and complexity for a better understanding of SOM dynamics.