Interactive effects of temperature and exo-enzyme age on substrate decay vary between C- and N-acquiring enzymes (Invited)
Abstract
Decomposition of soil organic matter (SOM) is catalyzed by a vast array of exo-enzymes (EEs) produced by microorganisms and secreted into the soil matrix. The catalytic rate, i.e. the specific activity of an enzyme, typically declines with time after release, reducing the potential yield of assimilable resources from the enzyme. The rates at which EEs catalyze SOM decay is also influenced by the temperature at which the reactions proceed. The catalytic rates of functionally distinct enzymes (like carbon (C) or nitrogen (N) acquiring enzymes) show different responses to warming, leading to temperature-induced changes in relative liberation rates of C and N, and thus, in their relative availability for microbial acquisition. This effect might be amplified or mitigated by age-related decreases in catalytic rates, depending on how temperature affects the aging process of each enzyme. Although independent effects of temperature and age on enzyme kinetics have been established, how EE age interacts with temperature to influence the release of assimilable C and N is unknown. Because C vs. N limitation can influence microbial C economies, any interactive effects between temperature and EE age on substrate decay may be important determinants of microbial CO2 losses in a warming climate. Furthermore, quantifying effects of temperature on EE catalytic decline can provide insight into patterns of microbial decomposition strategies in changing temperature regimes. We determined specific activity rates of C-acquiring (β-glucosidase, BGase) and N-acquiring (N-acetyl-glucosaminidase, NAGase) enzymes immediately after EE reconstitution and at multiple time points thereafter (up to 2.5 d for NAGase, and up to 22 d for BGase), at multiple temperatures (5, 15, and 25°C), using fluorescently labeled substrates. We assume the moment of EE reconstitution represents the time of microbial EE exudation into the soil solution. Temperature influenced the catalytic decline of NAGase and BGase differently. Rates of age-induced catalytic decline of NAGase increased with temperature. At 25°C, rates declined to ~40% of their initial levels after 1 d; at 5°C, ~1.5 d had elapsed before rates had declined to that same fraction of the lower, initial levels. During the first 3 d, BGase activity at 25°C declined linearly with age. Thereafter, BGase activity at 25°C remained constant at ~50% of the initial rate. At 5°C, BGase activity was lower than at 25°C and insensitive to EE age. Though warming temperatures increase reaction rates for EEs and their respective substrates, our data suggest that NAGase does not retain its initial catalytic rate as it ages in temperatures as cool as 5°C, and that BGase catalytic rates decline with age only at relatively warm temperatures. In turn, this suggests that the C:N flow ratio - the ratio of assimilable C to assimilable N - varies as a function of the interaction between time since EE production and temperature. Our work articulates some of the complexities of predicting microbial resource needs and availability of assimilable resources, particularly given the relatively lasting value of microbial investment in BGase production at cool temperatures compared to NAGase.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.B22D..01B
- Keywords:
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- 0428 BIOGEOSCIENCES Carbon cycling;
- 0414 BIOGEOSCIENCES Biogeochemical cycles;
- processes;
- and modeling;
- 0469 BIOGEOSCIENCES Nitrogen cycling;
- 1615 GLOBAL CHANGE Biogeochemical cycles;
- processes;
- and modeling