Differential temperature sensitivity of intracellular metabolic processes and extracellular soil enzyme activities
Authors: Adekanmbi Adetunji Alex , Dale Laurence , Shaw Liz , Sizmur Tom 
Published: June 2023

Abstract. Predictions concerning the feedback of soil heterotrophic
respiration to a warming climate often do not differentiate between the
extracellular and intracellular steps involved in soil organic matter
decomposition. This study examined the temperature sensitivities of
intracellular metabolic processes and extracellular soil enzyme activities
and how they are influenced by previous temperatures. We pre-incubated soils
at 5, 15, or 26 ∘C to acclimatize the
microbial communities to different thermal regimes for 60 d before
measuring potential activities of β-glucosidase and chitinase
(extracellular enzymes), glucose-induced respiration (intracellular
metabolic processes), and basal respiration at a range of assay temperatures
(5, 15, 26, 37, and 45 ∘C). A higher pre-incubation temperature
decreased the soil pH and C/N ratio and decreased β-glucosidase potential
activity and respiration but not chitinase potential activity. It is likely
that this legacy effect on β-glucosidase and respiration is an
indirect effect of substrate depletion rather than physiological
acclimatation or genetic adaptation. Pre-incubation temperature effects on
temperature sensitivity were subtle and restricted to extracellular
activities, perhaps because of the short (60 d) duration of the
pre-incubation at temperatures that were below the initial optimum
(∼ 30 ∘C) for the mesophilic soil community.
However, we found that the intracellular and extracellular steps differ in
their temperature sensitivity, and this observation differs depending on the
range of temperature used for Q10 estimates of temperature sensitivity.
Between 5 and 15 ∘C intracellular and extracellular processes show equal temperature sensitivity, but between 15 and 26 ∘C intracellular metabolic processes were
more temperature sensitive than extracellular enzyme activity, and between 26 and 37 ∘C extracellular enzyme activity was more
temperature sensitive than intracellular metabolic processes. This result
implies that depolymerization of higher molecular weight carbon is more
sensitive to temperature changes at higher temperatures (e.g. higher
temperatures on extremely warm days), but the respiration of the generated
monomers is more sensitive to temperature changes at moderate temperatures
(e.g. mean daily maximum soil temperature). However, studies using multiple
soil types and a greater range of pre-incubation temperatures are required
to generalize our results. Nevertheless, since climate change predictions
currently indicate that there will be a greater frequency and severity of
hot summers and heatwaves, it is possible that global warming may reduce the
importance of extracellular depolymerization relative to intracellular
metabolic processes as the rate-limiting step of soil organic matter
mineralization. We conclude that extracellular and intracellular steps are
not equally sensitive to changes in soil temperature and that the previous
temperature a soil is exposed to may influence the potential activity, but
not temperature sensitivity, of extracellular and intracellular processes.