Sensitivity of enzyme kinetics to temperature across five biomes in southern California
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 (Vmax) and substrate affinity (Km) 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 Km for each enzyme class, and at six different temperatures to determine the temperature sensitivity of both Vmax and Km. Temperature sensitivities of Vmax and Km were then tested for significant relationships with the MAT of the litter’s native site. We hypothesized that enzyme Vmax and Km would exhibit greater sensitivity to temperature in colder sites, and reduced sensitivity to temperature in warmer sites.
Potential extracellular enzyme activities varied significantly by site for all seven enzyme classes (p<0.0001). Six of the seven enzymes exhibited their highest Vmax in the grassland, followed by both the desert and savannah, exhibiting their lowest Vmax in both the oak pine forest and subalpine forest. Leucine aminopeptidase was unique in that its Vmax was highest in the desert instead of the grassland. Enzyme Km values did not exhibit consistent trends across sites. As hypothesized, Vmax was in general most sensitive to temperature in the colder forest sites, but we found marginally significant or significant negative relationships between Vmax 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 Km 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 Vmax parameters. However, for chitinases such as N-acetyl-glucosaminidase, this increased temperature sensitivity of Vmax in colder biomes will be offset by increased temperature sensitivity of Km.