Abstract
Abstract. Understanding how carbon dioxide (CO2) flux from ecosystems feeds back to climate warming depends in part on our ability to quantify the efficiency with which microorganisms convert 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 organic substrates. However, the diversity of organic substrates' δ13C and the challenges of measuring microbial C use efficiency (CUE) in their natural environment fundamentally limit our ability to project 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 microorganism growing at a constant rate. Biomass C specific respiration rate increased by 250 % between 13 and 26.5 °C, decreasing CUE from 77 to 56 %. Biomass C 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 ecosystem C fluxes.
Highlights
Because Earth’s organic carbon (C) cycle is a key regulator of climate, a central goal of biogeochemistry is to understand biosphere–atmosphere C exchange
Because specific growth rates were similar across the experimental temperatures (137 mg g−1 h−1, ±8 (1 SD); or 13.7 % h−1 in relative terms; Fig. 3b), the decline in C use efficiency (CUE) was due to the 2.5-fold increase of SRR with temperature, which rose from 45 mg g−1 h−1 at 13 ◦C to 113 mg g−1 h−1 at 26.5 ◦C (Fig. 3b)
Our observations clearly show a decline in microbial CUE with increasing temperature when C substrate is plentiful and demonstrate the mechanism driving it – an increase in SRR
Summary
Because Earth’s organic carbon (C) cycle is a key regulator of climate, a central goal of biogeochemistry is to understand biosphere–atmosphere C exchange. Though we have a reasonably comprehensive understanding of how environmental conditions influence CO2 uptake by photosynthetic organisms, our understanding of how respiratory CO2 fluxes respond to environmental conditions significantly lags behind. This is especially true for respiratory CO2 derived from heterotrophs, which may account for more than half of respiratory C losses from soils and aquatic systems (Kucera and Kirkham, 1971; Hanson et al, 2000; Cotner and Biddanda, 2002; Subke et al, 2006). The influence of temperature on the physiology of heterotrophic microbes must be well understood to project shifts in the global C balance in a warmer climate
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