Temperature-mediated changes in microbial carbon use efficiency and 13C discrimination.

Understanding how carbon dioxide (CO₂) flux from soils feeds back to climate warming depends in part on our ability to quantify the efficiency with which microorganisms convert soil organic carbon (C) into either biomass or CO₂. Quantifying ecosystem-level respiratory CO₂ losses often also requires assumptions about stable C isotope fractionations associated with the microbial transformation of soil organic substrates. However, the diversity of organic substrates' δ13C and the challenges of measuring microbial C use efficiency (CUE) in soils fundamentally limit our ability to project soil, and thus ecosystem, C budgets in a warming climate. Here, we quantify the effect of temperature on C fluxes during metabolic transformations of cellobiose, a common microbial substrate, by a cosmopolitan soil microorganism growing at a constant rate. Specific respiration rate increased by 250% between 13 and 26.5 °C, decreasing CUE from 77 to 56%. Specific respiration rate was positively correlated with an increase in respiratory 13C discrimination from 4.4 to 6.7 ‰ across the same temperature range. This first demonstration of a direct link between temperature, microbial CUE and associated isotope fluxes provides a critical step towards understanding δ13C of respired CO₂ at multiple scales, and towards a framework for predicting future soil C fluxes. [ABSTRACT FROM AUTHOR]