Rumen methanogens and mitigation of methane emission by anti-methanogenic compounds and substances

Abstract

Methanogenic archaea reside primarily in the rumen and the lower segments of the intestines of ruminants, where they utilize the reducing equivalents derived from rumen fermentation to reduce carbon dioxide, formic acid, or methylamines to methane (CH4). Research on methanogens in the rumen has attracted great interest in the last decade because CH4 emission from ruminants contributes to global greenhouse gas emission and represents a loss of feed energy. Some DNA-based phylogenetic studies have depicted a diverse and dynamic community of methanogens in the rumen. In the past decade, researchers have focused on elucidating the underpinning that determines and affects the diversity, composition, structure, and dynamics of methanogen community of the rumen. Concurrently, many researchers have attempted to develop and evaluate interventions to mitigate enteric CH4 emission. Although much work has been done using plant secondary metabolites, other approaches such as using nitrate and 3-nitrooxy propanol have also yielded promising results. Most of these antimethanogenic compounds or substances often show inconsistent results among studies and also lead to adverse effects on feed intake and digestion and other aspects of rumen fermentation when fed at doses high enough to achieve effective mitigation. This review provides a brief overview of the rumen methanogens and then an appraisal of most of the antimethanogenic compounds and substances that have been evaluated both in vitro and in vivo. Knowledge gaps and future research needs are also discussed with a focus on methanogens and methane mitigation.

The effect of total and individual alfalfa saponins on rumen methane production

“…Most methanogens present in the rumen belong to the Methanobacteriaceae family (89% and 99% of methanogens present in the rumen fluid and protozoa fractions, respectively), and the washout of protozoa resulted in a 27% decrease in methanogen population . For this reason, it is also worth mentioning the protozoa‐associated methanogens (PAM); these are methanogens that have become endosymbionts of protozoa, which produce hydrogen by malate oxidization . This relationship is thought to grant mutual benefits to both.…”

“…This relationship is thought to grant mutual benefits to both. About 16% of rumen protozoa contain methanogens in their cells . This low percentage can be explained by the unstable nature of the relationship; it may suggest that protozoa do not only play the role of hydrogen donors for methanogens.…”

“…Although a reduction was obtained in the protozoa population, no change in methane production was observed . As mentioned by Patra et al ., research has shown that saponins are likely to have little direct effect on methanogens, although it is known that they inhibit protozoa in the rumen, reducing both hydrogen production and the population of PAM. The effect of saponins on protozoa is attributed to their ability to form complexes with cholesterol in the cell membrane, altering its permeability and provoking cell lysis .…”

“…have become endosymbionts of protozoa, which produce hydrogen by malate oxidization. 18 This relationship is thought to grant mutual benefits to both. About 16% of rumen protozoa contain methanogens in their cells.…”

Inter- and intra-domain functional redundancy in the rumen microbiome during plant biomass degradation

“…Despite these major advances, a holistic understanding of the rumen microbiome is still lacking, including answers to rather simple questions such as “who is doing what and when during feed degradation?”. Such a fundamental understanding of the rumen ecosystem, as it was proposed by Hungate already in the early 1960s [11], can help to specifically manipulate the rumen microbiome, to lower CH 4 emissions, without hampering animal productivity, milk and meat quality or being harmful to the animal [14, 26].…”