CO2 Production in Sterile Solutions of D-galacturonic acid, Sodium Glutamate, and L-aspartic Acid: Evidence for Low Temperature Abiotic Decarboxylation

Abstract:

It is imperative that we incorporate an accurate understanding of peatland carbon (C) mineralization dynamics into global climate models given the immensity of global peatland C stores (equivalent to ~134% of atmospheric C stores based on upper-end estimates). Because of the higher warming potential of CH₄ relative to CO₂, accurate estimation of CO₂:CH₄ emission ratios is crucial to improving peatland-climate models. Historically, it has been assumed that cellulose-derived glucose is the dominant substrate for C mineralization in peatlands. Since glucose has an oxidation state of 0, CO₂ has an oxidation state of +4, and CH₄ has an oxidation state of -4, simple electron balance leads us to predict a 1:1 CO₂:CH₄ production ratio. However, in Sphagnum dominated peat bogs these ratios often greatly exceed 1.

Prior studies indicate that galacturonic acid (GalA), an important metabolite in Sphagnum dominated peatlands, could contribute to the apparent excess CO₂ by undergoing non-enzymatic browning reactions that are postulated to involve eliminative decarboxylation. Addition of amine-bearing compounds has been shown to increase the extent of browning in GalA solutions by presumably inciting a Maillard reaction sequence, which is also postulated to cause CO₂ production. The scope of such investigations has thus-far been limited to the analysis of thermally-treated solutions, which limits our ability to discern the extent of their occurrence in low-temperature peatland environments. We tested the ability for abiotic GalA and GalA+amine solutions to produce CO₂ at low temperatures using a series of sterile (ultra-filtered) incubations inoculated with various combination of GalA, amine-bearing compounds (glu + asp), and formaldehyde. We observed significant CO₂ production in both GalA and GalA+amine and solutions. Production in GalA+amine solutions exceeded that of GalA solutions by a factor of ~5.6. Production in control and amine-only solutions was negligible. Preliminary sterility assessments indicate that all CO₂ production was abiotic, though further assessments are necessary (and underway). These findings support the premise that abiotic CO₂ production can occur in GalA and GalA+amine solutions at low temperatures, indicating this reaction could be occurring in Sphagnum dominated peatlands.