Linking substrate identity to the formation of mineral-associated organic matter in soils: A model systems approach

Abstract:

The processing of low molecular weight (LMW) compounds in soil solution by microbial metabolism is recognized as an important driver of the formation of mineral-associated organic matter (MAOM) and is increasingly being depicted in finer resolution within soil carbon and nitrogen models. We seek to provide empirical datasets of key modeling parameters (substrate uptake rates and substrate use efficiencies) using targeted and untargeted exometabolomic approaches (¹H-NMR, HR-LCMS) coupled with stable isotope probing (¹³C, ¹⁵N). We have conducted this work using soil microorganisms isolated from a forest soil (Penicillium spinulosum, Paraburkholderia solitsugae, and Ralstonia pickettii), representing a gradient in growth rate (0.046, 0.287, and 0.316 hr^-1, respectively). Initially, we conducted ecophysiological studies of these isolates in liquid culture, temporally tracking the uptake of 35 targeted LMW compounds at realistic starting concentrations of 25 µM each in the mixture. We measured overall carbon use efficiency and used ¹³C-labeling to monitor the individual substrate use efficiencies for 5 compounds of interest (glucose, acetate, formate, glycine, and valine) along a gradient of energy content (nominal oxidation state of carbon, NOSC = ^-1 - 2). We found the fastest growing organism, R. pickettii, to have a much lower CUE (0.23) than the other bacterium, P. solitsugae (0.49), or the fungus, P. spinulosum (0.49). Maximum substrate uptake rates scaled with isolate growth rate, ranging from an average of 0.06 – 3.2 mmol/hr/gCDW. No clear relationship between NOSC and uptake preference or individual, substrate use efficiency was observed, though all isolates used formate (NOSC = 2) with the lowest efficiency. Current work is focused on using sorption isotherms to characterize the affinity of biomass produced for the model clay fractions, illite and goethite. Finally, a fully factorial experiment is in progress to grow these isolates in a model sand-clay mixture to track MAOM formation from added LMW substrates and assess their stability to perturbations. Our aim is to use this model systems approach to provide molecular-scale observations that can be used to constrain mechanistic modeling efforts while continuing to introduce more complexity and link to larger scale fluxes.