Sensitivity of enzyme kinetics to temperature across five biomes in southern California

Background/Question/Methods

The return of organic carbon to the atmosphere through terrestrial decomposition is mediated by the breakdown of complex organic polymers by extracellular enzymes produced and excreted by microbial decomposer communities. Elucidating how future climate change will affect this flux therefore requires an understanding of extracellular enzyme kinetics and the sensitivity of these enzymes’ reaction rate (V_max) and substrate affinity (K_m) to environmental changes. To determine how enzyme kinetics vary with climate, we sampled litter from five sites in southern California, spanning a gradient of 4.0-24.5º C in mean annual temperature (MAT) and 129-630 mm mean annual precipitation. Litter was ground and assayed for potential activity of seven extracellular enzyme classes using fluorescently labeled substrates. Assays were performed at eight different substrate concentrations to calculate K_m for each enzyme class, and at six different temperatures to determine the temperature sensitivity of both V_max and K_m. Temperature sensitivities of V_max and K_m were then tested for significant relationships with the MAT of the litter’s native site. We hypothesized that enzyme V_max and K_m would exhibit greater sensitivity to temperature in colder sites, and reduced sensitivity to temperature in warmer sites. 

Results/Conclusions

Potential extracellular enzyme activities varied significantly by site for all seven enzyme classes (p<0.0001). Six of the seven enzymes exhibited their highest V_max in the grassland, followed by both the desert and savannah, exhibiting their lowest V_max in both the oak pine forest and subalpine forest. Leucine aminopeptidase was unique in that its V_max was highest in the desert instead of the grassland. Enzyme K­_m values did not exhibit consistent trends across sites. As hypothesized, V­_max was in general most sensitive to temperature in the colder forest sites, but we found marginally significant or significant negative relationships between V_max temperature sensitivity and site MAT across all five sites for only three enzyme classes -  cellobiohydrolase (p<0.056), β-glucosidase (p<0.085), and N-acetyl-glucosaminidase (p<0.038). A negative relationship between K_m temperature sensitivity and site MAT was only significant for N-acetyl-glucosaminidase (p<0.032). Our results indicate that as climate becomes warmer in the American Southwest, the extracellular enzyme activity of microbial decomposer communities will likely increase more in colder biomes as a function of higher intrinsic temperature sensitivity of enzyme V_max parameters. However, for chitinases such as N-acetyl-glucosaminidase, this increased temperature sensitivity of V_max in colder biomes will be offset by increased temperature sensitivity of K_m.