The oxidation of organic carbon substrates by the soil microbiome and their subsequent accumulation as organic matter or emission as CO2(g) depends strongly on the interaction between the organic molecules and mineral surfaces. Clays and oxide minerals can stabilize organic compounds through sorption reactions or promote their oxidation to smaller or less reducing compounds. Manganese oxides, the strongest solid-phase oxidants in natural systems, can oxidize organic molecules through successive decarboxylation steps, potentially leading to the release of CO2. While manganese oxides may alter pool of available carbon sources, the extent to which they contribute to soil carbon cycling and the molecular-scale mechanisms at play are understudied. Here, we investigated the fate of glucose, a simple and readily available carbon substrate, in mineral (δ-MnO2), bacterial biomass (Pseudomonas putida KT2240) and composite mineral-biomass suspensions. Kinetic experiments were conducted over 6-days and a range of analytical methods were used to monitor changes in carbon and manganese chemistry and to compile mass and electron balances. Under abiotic conditions, formic acid was the main product of glucose oxidation and its concentration matched the measured concentrations of Mn(II) and solid-phase Mn(III). No CO2 was measured and the dissolved organic carbon (TOC) concentration remained constant. Instead, glucose oxidation by P. putida KT2240 at pH 7 led to the complete loss of TOC from solution and concomitant production of CO2. The decrease in TOC and increase in CO2, however, was attenuated in the mineral-bacteria composites. In addition, formic acid was detected but consumed over time. These results suggest that manganese oxides may compete with heterotrophic organisms for organic carbon substrates, but also may led to the abiotic production of small organic acids that may fuel other bacterial metabolisms.

The fate of bioavailable carbon sources in presence of manganese oxides

ACS Spring Meeting 2021

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