Temperature-mediated changes of exoenzyme-substrate reaction rates and their consequences for the carbon to nitrogen flow ratio of liberated resources

Soil microorganisms produce exoenzymes to access resources stored in soil organic matter. Knowledge about the effect of temperature on the rates at which exoenzymes degrade substrates is particularly important for understanding carbon and nitrogen cycling with warming, and possible feedbacks to climate change. Here, we studied reaction rates of purified enzymes and substrates in controlled laboratory conditions at temperatures ranging from 5 °C to 27 °C. We employed three enzyme-substrate pairings representative of reactions common to soil profiles: β-glucosidase and β-d-cellobioside (BGase/BG), β-N-acetyl glucosaminidase and N-acetyl-β-d-glucosaminide (NAGase/NAG), and peroxidase and 3,4-Dihydroxy-l-phenylalanine (peroxidase/l-Dopa). Across the entire temperature range studied, BGase showed the highest specific activity (Vmax 27 °C = 1338 μmol h−1 mgBGase−1), followed by NAGase (Vmax 25 °C = 260 μmol h−1 mgNAGase−1) and peroxidase (Vmax 25 °C = 36 μmol h−1 mgperoxidase−1). From 7.5 °C to 25 °C, the specific activities of BGase, NAGase and peroxidase increased by 103%, 111% and 835%, respectively. The activation energy (Ea) required for a reaction to proceed thus was highest for peroxidase/l-Dopa (99.8 kJ moll-Dopa−1), followed by NAGase/NAG (41.3 kJ molNAG−1) and BGase/BG (31.4 kJ molBG−1). We use a simple model, parameterized with empirical data from these reactions in three different ways, to illustrate how the flow of carbon relative to nitrogen can change with temperature as these resources are liberated from their organic precursors. The results highlight the importance of relative temperature sensitivities among reactions and the substrates' carbon to nitrogen ratio as key determinants of temperature-mediated changes in relative availabilities of carbon and nitrogen to microorganisms.