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

Authors: NERDAHL, MICHAEL - University Of Wisconsin; Weimer, Paul

Submitted to: Applied Microbiology and Biotechnology Express (AMB Express)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2015
Publication Date: 8/3/2015
Publication URL: http://handle.nal.usda.gov/10113/62659
Citation: Nerdahl, M., Weimer, P.J. 2015. Redox mediators modify end product distribution in biomass fermentations by mixed ruminal microbes in vitro. Applied Microbiology and Biotechnology Express (AMB Express). DOI: 1186/s13568-015-0130-7.

Interpretive Summary: The microorganisms in the rumen of dairy cattle and other ruminant animals are responsible for converting feeds into volatile fatty acids (VFA) that the animals use for energy and for milk and meat production. Most of the VFA produced in the rumen contain two or three carbon atoms, with only a small amount of VFA containing more carbon atoms. In addition, the rumen fermentation results in substantial losses of carbon and energy as methane. There are nutritional and environmental benefits to increasing the carbon number of VFA in the rumen and in certain industrial fermentations that employ rumen bacteria, but the potential to do so is limited by an incomplete understanding of what controls the ratios of the different VFA products. We showed that adding certain “redox dyes” at low concentrations can substantially decrease methane production and stimulate the production of VFA containing four and five carbon atoms in rumen fermentations conducted in laboratory culture vessels. Although redox dyes themselves are not practical feed additives, the data show that manipulation of VFA carbon number in the rumen is possible by controlling certain reactions in the fermentation. These results can be used to develop alternative strategies that can be used to decrease methane production and improve VFA carbon number in practice.

Technical 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 beta-oxidation. This study examined the effect of several redox mediators (neutral red, methyl viologen, safranin T, tannic acid) as alternative electron carriers for the 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 and keeping total end-product C quantity consistent. The ability of redox dyes to act as alternative electron carriers suggested that ruminal fermentation is inherently manipulable toward reducing methanogenesis and retaining a higher fraction of substrate energy in the form of VFA.