Expanding Understanding: Investigating the Information Value of Calorespirometric Ratio in Dynamic Processes of Soil Microbial Growth Using Calorimetry

Abstract

The preservation of soil services and functions is sustained by the catalytic activity of microbial communities. These communities use energy and carbon – especially for microbial growth – from the transformation of organic matter (OM).  A fraction of this OM is assimilated as carbon source for growth (anabolism). Another fraction is oxidized and the resulting electrons are transferred to various terminal acceptors to derive the energy for growth (catabolism). The distribution of carbon and energy into anabolism and catabolism determines carbon use efficiency (CUE) and energy use efficiency (EUE). Accurate quantification of the relationships between carbon and energy fluxes relies on key parameters like the metabolic heat (Qm),  calorespirometric ratio (CR), carbon dioxide evolution rate (CER), the apparent specific growth rate (μapp), and the degree of anaerobicity (ηA). However, determining these parameters faces challenges at technical (sample size and instrument sensitivity) and experimental (thermal disturbance, sample aeration) levels impacting the precise quantification of energy and carbon flux relationships. To address these challenges under controlled conditions, we examined microbial turnover processes in a model arable soil amended with a readily metabolizable substrate (glucose). We utilized three commercial isothermal microcalorimeters (IMC)  with volume-related thermal detection limits (LODV) ranging from 0.05 to 1 mW L-1. Comparison between three IMCs were conducted to figure out the influence of LODV on measuring accuracy. Calorimetric experiments (half ampoules were closed and half were aerated for 5 minutes on selected days) were compared to explore the effect of oxygen limitation and thermal perturbation on the calorimetric signal. CER was monitored by measuring the additional heat resulting from CO2 absorption in NaOH solution used as CO2 trap. The range of errors associated with calorimetrically derived μapp, Qm, and CR was determined experimentally and compared with the requirements for quantifying CUE and ηA from a theoretical perspective. Significant differences in Qm and µapp were observed between IMCs which have the lowest and highest LODV. IMC with the lowest LODv provided the most accurate results. Opening ampoules for gas exchange did not significantly impact Qm. However, regular ampoule opening during calorimetrically derived CER measurements led to notable measurement errors for CER due to strong thermal perturbation of the signal. If established models are used to calculate CUE and ηA from CR, unrealistically high values are obtained and the accuracy of CR do not fit to the requirements. There are two ways to cope with this problem. On the one hand, new thermodynamic balance models need to be developed that dispense with the error-prone CR value. On the other hand, new calorespirometric methods must be developed to determine the CR value more reliably. Initial results for both approaches will be presented.