Potential and existing mechanisms of enteric methane production in ruminants

Abstract

Enteric methane (CH 4) emissions in ruminants have attracted considerable attention due to their impact on greenhouse gases and the contribution of agricultural practices to global warming. Over the last two decades, a number of approaches have been adopted to mitigate CH 4 emissions. However, the mechanisms of methanogenesis have still not been fully defined. According to the genome sequences of M. ruminantium in the rumen and of M. AbM4 in the abomasum, the pathways of carbon dioxide (CO 2) reduction and formate oxidation to CH 4 have now been authenticated in ruminants. Furthermore, in the light of species or genera description of methanogens, the precursors of methanogenesis discovered in the rumen and research advances in related subjects, pathways of acetate dissimilation via Methanosarcina and Methanosaeta as well as metabolism of methanol to CH 4 might be present in the rumen, although neither process has yet been experimentally demonstrated in the rumen. Herein the research advances in methanogenesic mechanisms including existing and potential mechanisms are reviewed in detail. In addition, further research efforts to understand the methanogenesis mechanism should focus on isolation and identification of more specific methanogens, and their genome sequences. Such increased knowledge will provide benefits in terms of improved dietary energy utilization and a reduced contribution of enteric CH 4 emissions to total global greenhouse gas emissions from the ruminant production system.

The Role of Chitosan as a Possible Agent for Enteric Methane Mitigation in Ruminants

“…Emissions of enteric CH 4 vary according to dry matter intake, growth rate, housing, live weight, level of production, ration composition, and rumen fermentation pattern [9,10]. Ration and type of feed affect the availability of metabolic hydrogen for CH 4 synthesis in the rumen; feeding concentrate and grains (low fiber diets) reduce the population of methanogens (range 10 7 to 10 9 g −1 ) while more propionic acid is synthesized, which utilizes H 2 , whereas pasture-fed (methanogens 10 9 to 10 10 g −1 ) ruminants yield higher concentrations of acetic acid in rumen liquor, resulting in a higher availability of metabolic H 2 [11]. However, high-grain levels in rations can decrease rumen pH and result in health problems in ruminants such as acidosis [5,10,11].…”

“…Ration and type of feed affect the availability of metabolic hydrogen for CH 4 synthesis in the rumen; feeding concentrate and grains (low fiber diets) reduce the population of methanogens (range 10 7 to 10 9 g −1 ) while more propionic acid is synthesized, which utilizes H 2 , whereas pasture-fed (methanogens 10 9 to 10 10 g −1 ) ruminants yield higher concentrations of acetic acid in rumen liquor, resulting in a higher availability of metabolic H 2 [11]. However, high-grain levels in rations can decrease rumen pH and result in health problems in ruminants such as acidosis [5,10,11]. During the last decade, methane mitigation strategies have been the subject of intensive study by several research groups [12].…”

Metabolic Modeling Elucidates the Transactions in the Rumen Microbiome and the Shifts Upon Virome Interactions

“…The degradation of plant cellulose, biosynthesis of branched chain amino acid and short-chain fatty acid (SCFA) and production of hydrogen were predicted by R. flavefaciens model and were in agreement with experimental observations (Helaszek and White, 1991; Flint et al, 2008; Zheng et al, 2014). Protein digestion by P. ruminicola and methane production from carbon di-oxide and hydrogen consumption by M. gottschalkii were also validated (Wallace et al, 1997; Qiao et al, 2014; Henderson et al, 2015; Seedorf et al, 2015).…”

“…Each of the three metabolic models were checked for the capacity to produce biomass and metabolites they were known to produce (Helaszek and White, 1991; Wallace et al, 1997; Flint et al, 2008; Qiao et al, 2014; Zheng et al, 2014; Henderson et al, 2015; Seedorf et al, 2015). To ensure these metabolic functionalities, the reactions missing in metabolic pathways were systematically identified and added manually from biochemical databases (Kanehisa and Goto, 2000; Apweiler et al, 2004; Henry et al, 2010) after an extensive search for each of the missing enzyme activities in related organisms such as Ruminococcus albus, Bacteroides thetaiotaomicron , and Methanobrevibacter smithii for R. flavefaciens, P. ruminicola , and M. gottschalkii , respectively.…”

Bioenergy and Biorefinery: Feedstock, Biotechnological Conversion, and Products

“…Methane, on the other hand, is produced by a group of microbes referred to as methanogens. Although it can be produced from H 2 + CO 2 , some limited number of the methanogenic genera are known to utilize organic acids such as acetic acid as a substrate for methane production . An additional interest in the controlled production of these biogases is the vital role it can play in waste treatment.…”