Bioenergetic controls on microbial activity

Soils store the largest pool of carbon in the terrestrial biosphere and contain more than double the amount of carbon in the atmosphere. Yet, soil processes remain a partly missing piece in our understanding and predictive capability of the soil’s response to climate change and feedback thereof on climate. The biogeochemical cycling of carbon in soils is tightly coupled to the metabolic activity of microorganisms: heterotrophic microorganisms use soil organic matter (SOM) as an energy source by oxidizing organic carbon and transferring the released electrons onto terminal electron acceptors. Under conditions where the microbial energy investment exceeds energy gain, SOM may accumulate due to bioenergetic constraints on microbial activity. Electron transfer reactions and associated energy transformations can therefore be considered the universal currency driving the biogeochemical cycling of carbon.

In this project, we are exploring how bioenergetic constraints on microbial activity affect the transformation and persistence of SOM. In a first step, we aim to quantitatively link carbon mass fluxes to energy fluxes in closed soil incubations under oxic and anoxic conditions. To assess energy fluxes, we are currently testing the application of various methods to characterize the energy content of organic matter (bomb calorimetry, differential scanning calorimetry, photoelectrochemistry) and the heat release during microbial activity (isothermal microcalorimetry), which- together with theoretical concepts and tabulated thermodynamic data- will allow us to determine thermodynamic state variables for the reactions occurring in our incubations. By combining these assessments with the analysis of carbon fluxes resulting from microbial utilization of isotopically labelled substrates, we aim to link carbon mass fluxes to energy fluxes. In a second step, we plan to use the described methodologies to assess bioenergetic constraints on microbial activity in soil systems. We hypothesize that microorganisms preferentially utilize energy-poor organic compounds that require low energy input for oxidation under anoxic conditions, resulting in the accumulation of relatively more energy-rich organic compounds. We will test our hypotheses in a series of laboratory incubation experiments and mesocosm experiments.