Abstract
Enteric fermentation from ruminants is a primary source of anthropogenic methane emission. This study aims to add another approach for methane mitigation by manipulation of the rumen microbiome. Effects of choline supplementation on methane formation were quantified in vitro using the Rumen Simulation Technique. Supplementing 200 mM of choline chloride or choline bicarbonate reduced methane emissions by 97–100% after 15 days. Associated with the reduction of methane formation, metabolomics analysis revealed high post-treatment concentrations of ethanol, which likely served as a major hydrogen sink. Metagenome sequencing showed that the methanogen community was almost entirely lost, and choline-utilizing bacteria that can produce either lactate, ethanol or formate as hydrogen sinks were enriched. The taxa most strongly associated with methane mitigation were Megasphaera elsdenii and Denitrobacterium detoxificans, both capable of consuming lactate, which is an intermediate product and hydrogen sink. Accordingly, choline metabolism promoted the capability of bacteria to utilize alternative hydrogen sinks leading to a decline of hydrogen as a substrate for methane formation. However, fermentation of fibre and total organic matter could not be fully maintained with choline supplementation, while amino acid deamination and ethanolamine catabolism produced excessive ammonia, which would reduce feed efficiency and adversely affect live animal performance.
Highlights
With MMC being the second most dominant[5]
The MMC rely on a simplified methanogenesis pathway that utilizes methylated compounds, such as methylamines to generate energy, requiring only one mole of H 2 per mole of C H46
Possible reasons for failure of MMC to dominate the rumen methanogen niche might be that MMC divert less energy towards ATP production from methanogenesis than other methanogens, or that their growth is limited by the insufficient availability of methylated substrates, such as mono-(MMA), di-(DMA) and trimethylamines (TMAs)
Summary
With MMC being the second most dominant[5]. The MMC rely on a simplified methanogenesis pathway that utilizes methylated compounds, such as methylamines to generate energy, requiring only one mole of H 2 per mole of C H46. Methylotrophic methanogenesis would out-compete hydrogenotrophic methanogenesis at a low H 2 concentration, as MMC would consume H 2 and reduce dissolved H 2 to a level that does not meet the thermodynamic conditions required to produce ATP by hydrogenotrophic methanogens[8] Despite these thermodynamic and H2 threshold advantages, and temporary H2 limiting situations in certain periods of the feeding cycle, Methanobacteriales remains the dominant methanogen order in the rumen. The present study aimed to exploit this limitation and to use it to drive changes in the rumen microbiome towards a simplified rumen methanogen population with MMC as the dominant order by providing an abundant supply of either methylated substrate or their precursors as selection pressure. In depth metagenomics and metabolomics approaches were applied for the identification of the cholinemediated mechanisms affecting the ruminal microbiome associated with methanogenesis the use of alternative H2 utilization pathways
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