The Development of Molecular Approaches for Investigating the Temperature Response of Soil Carbon Fluxes

Soil microbial respiration and methanogenesis are key sources of atmospheric carbon. Conflicting evidence exists as to how these processes acclimate to temperature fluxes. Thermal acclimation of temperate soils might result in downregulation of microbial soil respiration rates and, therefore, has the potential to mitigate atmospheric soil carbon losses predicted due to global warming. The extent of thermal acclimation and the mechanisms behind it (substrate depletion, genetic adaption, physiological changes, or microbial community composition) remain unclear. We investigated the temperature response of soil microbial respiration by warming (37 °C and 45 °C) and cooling (15 °C) a soil sample over a 14-week period. Soil respiration rates acclimated to temperature after 8 weeks and metagenomic analysis of the acclimated soil microbiomes revealed an increase in the relative abundance of Proteobacteria, Actinobacteria and Chlamydiae phyla. Additionally, the investigation of the physiological temperature response of an important environmental methanogen led to novel insights into methanogenic acclimation. Methanosarcina barkeri incubated at optimum (37 °C), cool (15 °C) and warm (45 °C) temperatures for 4 weeks displayed thermal acclimation of methanogenesis at the end of the incubation period. Transcriptomic analysis of acclimated cultures identified potential molecular signatures of methanogenic acclimation, primarily the apparent preference for utilizing the methyl-reducing methanogenic pathway within acclimated cultures. Understanding the acclimation responses of soil carbon fluxes will increase the predictive power of climate change models and enable more accurate predictions of the carbon-sequestration potential of natural carbon offsetting projects. This study demonstrates the importance of including the temperature responses of soil microbial communities and individual soil organisms in predicting the loss of carbon from soils due to climate change