Temperature sensitivities of extracellular enzyme V and K across thermal environments.

The magnitude and direction of carbon cycle feedbacks under climate warming remain uncertain due to insufficient knowledge about the temperature sensitivities of soil microbial processes. Enzymatic rates could increase at higher temperatures, but this response could change over time if soil microbes adapt to warming. We used the Arrhenius relationship, biochemical transition state theory, and thermal physiology theory to predict the responses of extracellular enzyme V and K to temperature. Based on these concepts, we hypothesized that V and K would correlate positively with each other and show positive temperature sensitivities. For enzymes from warmer environments, we expected to find lower V , K , and K temperature sensitivity but higher V temperature sensitivity. We tested these hypotheses with isolates of the filamentous fungus Neurospora discreta collected from around the globe and with decomposing leaf litter from a warming experiment in Alaskan boreal forest. For Neurospora extracellular enzymes, V Q ranged from 1.48 to 2.25, and K Q ranged from 0.71 to 2.80. In agreement with theory, V and K were positively correlated for some enzymes, and V declined under experimental warming in Alaskan litter. However, the temperature sensitivities of V and K did not vary as expected with warming. We also found no relationship between temperature sensitivity of V or K and mean annual temperature of the isolation site for Neurospora strains. Declining V in the Alaskan warming treatment implies a short-term negative feedback to climate change, but the Neurospora results suggest that climate-driven changes in plant inputs and soil properties are important controls on enzyme kinetics in the long term. Our empirical data on enzyme V , K , and temperature sensitivities should be useful for parameterizing existing biogeochemical models, but they reveal a need to develop new theory on thermal adaptation mechanisms.