Climate-Ecosystem Carbon Feedbacks

February 24, 2013

Respiration by plants and microorganisms is primarily responsible for mediating carbon exchanges between the biosphere and atmosphere. Climate warming has the potential to influence the activity of these organisms, altering the exchanges between carbon pools. Traditionally, the respiratory release of CO2 into the atmosphere is thought to be more temperature-sensitive than photosynthesis (carbon fixation), generating a positive climate-ecosystem carbon feedback with the potential to accelerate climate warming by up to 1.4 times. Long-term warming studies, however, show that terrestrial ecosystem respiration rates consistently return to pre-warming levels within a few years of warming. Theoretical models partially attribute this ephemeral respiration response to biological ‘thermal acclimation’. That is, physiological ‘down-regulation’ of warm-adapted species could ameliorate the predicted respiratory losses of soil carbon under climate change scenarios.

Thermal acclimation is well documented in plants, a process that, when accounted for, can substantially increase robustness in global-scale carbon models. Uncertainty remains, however, regarding the capacity for heterotrophic soil microbes – the organisms responsible for 25% of CO2 emissions annually – to acclimate to temperature. Fundamental biochemical and organismal theory, however, suggests that cellular and population-level processes should influence the growth and respiration responses to temperature of all organisms. Such biochemical tradeoffs have been proposed to partially explain the leveling of mass specific respiration rates of microbial communities following prolonged warming. Evidence for this is, however, confounded by the fact that warming can also drive changes in community composition, which may simply favor less metabolically active species.

In a paper recently published in Ecology Letters, YCEI Postdoctoral Fellow, Thomas Crowther, and his postdoctoral advisor, Mark Bradford, explore the temperature sensitivity of individual saprotrophic basidiomycete fungi growing in agar. Their research compared growth and respiration rates of cold-, intermediate- and warm-acclimated individuals at temperatures commonly experienced in temperate woodland ecosystems. In almost all cases they found that the warm-acclimated individuals had lower growth and respiration rates at intermediate temperatures than cold-acclimated isolates. This provided definitive evidence that these widespread dominant heterotrophic fungi acclimate rapidly and efficiently to temperature. The authors conclude that inclusion of such microbial physiological responses to warming is essential to enhance the robustness of global climate-ecosystem carbon feedback models.

Crowther, T. W., Bradford, M. A. (2013) Thermal acclimation in widespread heterotrophic soil microbes. Ecology Letters. doi: 10.1111/ele.12069