title: Carbon Availability Modifies Temperature Responses of Heterotrophic Microbial Respiration, Carbon Uptake Affinity, and Stable Carbon Isotope Discrimination.
abstract: Microbial transformations of organic carbon (OC) generate a large flux of CO2into the atmosphere and influence the C balance of terrestrial and aquatic ecosystems. Yet, inherent heterogeneity in natural environments precludes direct quantification of multiple microbial C fluxes that underlie CO2production. Here we used a continuous flow bioreactor coupled with a stable C isotope analyzer to determine the effects of temperature and C availability (cellobiose concentration) on C fluxes and13C discrimination of a microbial population growing at steady-state in a homogeneous, well-mixed environment. We estimated C uptake affinity and C use efficiency (CUE) to characterize the physiological responses of microbes to changing environmental conditions. Temperature increased biomass-C specific respiration rate and C uptake affinity at lower C availability, but did not influence those parameters at higher C availability. CUE decreased non-linearly with increasing temperature. The non-linear, negative relationship between CUE and temperature was more pronounced under lower C availability than under relatively high C availability. We observed stable isotope fractionation between C substrate and microbial biomass C (7~12‰ depletion), and between microbial biomass and respired CO2(4~10‰ depletion). Microbial discrimination against13C-containing cellobiose during C uptake was influenced by temperature and C availability, while discrimination during respiration was only influenced by C availability. Shifts in C uptake affinity with temperature and C availability may have modified uptake-induced13C fractionation. By stressing the importance of C availability on temperature responses of microbial C fluxes, C uptake affinity, CUE, and isotopic fractionation, this study contributes to a fundamental understanding of C flow through microbes. This will help guide parameterization of microbial responses to varying temperature and C availability within Earth-system models.
authors.affiliation: Department of Ecology and Evolutionary Biology, Kansas Biological Survey, University of Kansas, Lawrence KS, USA.
authors.forename: Kyungjin
authors.initials: K
authors.lastname: Min
authors.affiliation: Department of Ecology and Evolutionary Biology, Kansas Biological Survey, University of Kansas, Lawrence KS, USA.
authors.forename: Christoph A
authors.initials: CA
authors.lastname: Lehmeier
authors.affiliation: Department of Ecology and Evolutionary Biology, Kansas Biological Survey, University of Kansas, Lawrence KS, USA.
authors.forename: Ford Ballantyne
authors.initials: FB
authors.lastname: Iv
authors.affiliation: Department of Ecology and Evolutionary Biology, Kansas Biological Survey, University of Kansas, Lawrence KS, USA.
authors.forename: Sharon A
authors.initials: SA
authors.lastname: Billings
keywords: carbon use efficiency; chemostat; isotope; microbial growth; nutrient; resource; respiration; stoichiometry