Redox mediators modify end product distribution in biomass fermentations by mixed ruminal microbes in vitro

Abstract

The fermentation system of mixed ruminal bacteria is capable of generating large amounts of short-chain volatile fatty acids (VFA) via the carboxylate platform in vitro. These VFAs are subject to elongation to larger, more energy-dense products through reverse β-oxidation, and the resulting products are useful as precursors for liquid fuels production. This study examined the effect of several redox mediators (neutral red, methyl viologen, safranin O, tannic acid) as alternative electron carriers for mixed ruminal bacteria during the fermentation of biomass (ground switchgrass not subjected to other pretreatments) and their potential to enhance elongation of end-products to medium-chain VFAs with no additional run-time. Neutral red (1 mM) in particular facilitated chain elongation, increasing average VFA chain length from 2.42 to 2.97 carbon atoms per molecule, while simultaneously inhibiting methane accumulation by over half yet maintaining total C in end products. The ability of redox dyes to act as alternative electron carriers suggests that ruminal fermentation is inherently manipulable toward retaining a higher fraction of substrate energy in the form of VFA.

Redirection of Metabolic Hydrogen by Inhibiting Methanogenesis in the Rumen Simulation Technique (RUSITEC)

“…An increase in the average carbon chain length of end products linked to greater [2H] availability has been reported previously with the use of redox mediators as methanogenesis inhibitors (neutral red, methyl viologen, safranin O, and tannic acid; Nerdahl and Weimer, 2015 ). Similarly, with NOP and AQ, part of [2H] spared from methanogenesis was diverted toward the production of more energy-dense and reduced straight-chain VFA.…”

Potential roles of nitrate and live yeast culture in suppressing methane emission and influencing ruminal fermentation, digestibility, and milk production in lactating Jersey cows

“…Those authors discussed that LYC should lower redox potential, which might therefore increase flux toward more reduced end products, whereas NO 3 − should increase redox. Thus, LYC might have shifted carbon flux toward more reduced (and elongated) end products such as caproate (not measured) or branched-chain VFA (Nerdahl and Weimer, 2015) or by increasing chain length of longchain fatty acids in membranes (not measured) unless it enhanced digestibility to support more microbial growth (Ungerfeld and Kohn, 2006). The NO 3 − and LYC interacted (P < 0.01) in acetate, propionate, and acetate: propionate.…”

Association of aqueous hydrogen concentration with methane production in continuous cultures modulated to vary pH and solids passage rate

“…The rate of H 2 escape into headspace probably was not decreased by lower culture pH; if this were true, H 2 (aq) measurements-from which H 2 is equilibrated for a standard sample pH across all treatments-would have increased detected quantities of H 2 (aq). More likely, the lower H 2 (aq) in LpH could be due to H 2 diversion away from escape resulting from reducing equivalent disposal into increasing chain length of fermentation end-products, such as valerate (as noted in Table 3), with more negative redox potential of bacteria if methanogenesis is disrupted (Nerdahl and Weimer, 2015).…”