Carbon dioxide is the main greenhouse gas emitted by human activities. As part of the Earth’s carbon cycle, carbon dioxide is also naturally present in the atmosphere: It is constantly exchanged between the atmosphere, ocean and land surface as it is produced and absorbed by many microorganisms, plants and animals. But human activities affect the carbon cycle by adding more carbon dioxide to the atmosphere and by damaging natural sinks, for example forests which would otherwise remove carbon dioxide from the atmosphere.

Nitrous Oxide, like carbon dioxide, is also naturally present in the atmosphere as it is part of the global nitrogen cycle: Nitrogen circulates between the atmosphere, plants, animals and bacteria breaking it down in the oceans and soils. Even though it has a variety of natural sources, human activities such as wastewater management or nitrogen fertilization increase the amount of nitrous oxide in the atmosphere. These molecules stay in the atmosphere for about 120 years but can be removed by certain types of bacteria or even destroyed by chemical reactions or ultraviolet radiation.

In contrast to other greenhouse gases, fluorinated gases do not have natural sources and only arise from human activities. F-gases are emitted through different industrial processes and many of them have a very high global warming potential in comparison to other greenhouse gases. Therefore, even very small atmospheric concentrations can have great effects on global temperatures. Additionally they have very long atmospheric lifetimes, lasting up to thousands of years. F-gases are removed from the atmosphere only when they get destroyed by sunlight in the far upper atmosphere. In general, fluorinated gases are the most potent and longest lasting sort of greenhouse gases emitted by humans.

Methane is the second most important greenhouse gas. It is emitted by several natural sources such as wetlands but also by different human activities like leakage from natural gas and livestock farming. Methane can be removed from the atmosphere by natural processes in soil and chemical reactions in the atmosphere. As its lifetime in the atmosphere is, compared to other greenhouse gases, quite short (~12 years), targeting methane emissions as a means of reducing the overall greenhouse gas emissions in a relatively short period of time is a reasonable approach to reduce global warming.
Globally around 60% of all methane emissions originate from human activites like industry, agriculture and waste management.

Even though the total methane emissions decreased by around 10% between 1990 and 2012 the emissions associated with agricultural activities did not. This is mostly due to the intensification of livestock farming. Methane is set free in the course of digestion in ruminant animals like cattle and emitted into the atmosphere. Therefore, by increasing livestock production methane emissions are increased as well.

Methane is formed in a process called methanogenesis which, for example, takes place in the rumen of cattle. However, the methane is not further utilized by any of the microorganisms living in the animal’s digestive tract nor by the animal itself, so it gets exhaled to the atmosphere and thus contributes to global warming. Read here (LINK) what has been done in the past against these emissions, or here (LINK) how exactly methanogenesis takes place.

Origin of Methane

Methane from biological sources can originate in various different environments like peat bogs, deep-sea hydrothermal vents and the digestive tracts of animals. Despite this variation there are certain common aspects: All biologically formed methane originates from methanogenic archaea and all these methanogens are strictly anaerobic organisms [1]. However, there is not only one mode of methanogenesis. Some methanogens perform hydrogenotrophic methanogenesis which mostly relies on the reduction of carbon dioxide while hydrogen usually acts as the electron donor. The methylotrophic methanogens, in contrast, are additionally able to use a variety of other compounds for methane generation, for example certain methyl compounds. Lastly, in acetoclastic methanogenesis acetate is used as a substrate for methane formation [2,3]. Hydrogenotrophic methanogens comprise, for example, the order Methanobacteriales and species like Methanobrevibacter ruminantium. Methanosarcinales is the most important order of the methylotrophic methanogens with species like Methanosarcina barkeri. Representatives of both orders and both modes of methanogenesis can be found in the rumen of cattle, one of the most important biological methane sources [4]. In the hydrogenotrophic pathway carbon dioxide is initially reduced with electrons taken from the oxidation of hydrogen or alternative electron donors. The resulting formyl group is then subsequently reduced. Involved in this process are several unique cofactors to which the groups of the different oxidation states - some of them cyclic - are being bound until methane is released. The reduction reactions are coupled to the export of sodium ions across the cell membrane. The methylotrophic pathway, in which C1 compounds like methanol or methylamines can be utilized, joins the hydrogenotrophic pathway at a late stage. A total of four C1 molecules is needed here, one of which is oxidized to provide electrons for the reduction of the other three. In the acetoclastic pathway the electrons required for reduction are taken from the oxidation of the carbonyl carbon of acetate [2,3].

Digestion in Ruminants

Like all ruminants, cattle are herbivores, which means that they mainly feed on plants – the most demonstrative example for this might be a cow grazing on a meadow. For the vegetable components to be properly utilized, however, the cow needs a stomach completely different from the kind you find in other mammals like, for example, humans. Indeed, the stomach of ruminants consists of four components: rumen, reticulum and omasum are succeeded by the abomasum, and those four compartments work closely together in the digesting process. Thanks to this, the grazing cow is able to utilize cellulose in order to gain energy, which is not possible for most other mammals.

When a cow ingests food, the latter is swallowed without being chewed at first, passes through the reticulum and enters the rumen. Here, fermentation is carried out by microorganisms, which can take 20 to 50 hours. Next, small balls are formed from the processed food in the reticulum by muscle contractions, and these balls are regurgitated, chewed and mixed with saliva. The resulting food mush is then swallowed once again; this time, though, it does not enter the rumen after passing the reticulum, but is channelled into the omasum. Thus, the reticulum works as a kind of filter separating the unprocessed food of the first swallowing from the processed mush of the second.

The importance of the rumen in digestion arises from the numerous microorganisms living in it. In fact, each gram of rumen contents contains about 1010 to 1011 bacteria belonging to 300-400 different species, resulting in a great variety of metabolic profiles. Most of these organisms are obligate anaerobes, meaning that they can only survive and grow in the absence of oxygen. However, a certain amount of oxygen entering the rumen with every swallow can hardly be prevented; thus, there are also a few facultative anaerobes present capable of consuming oxygen.

Among the billions of microorganisms in the rumen there are different groups specialized on degrading compounds like cellulose, starch, or lactate. The sugars set free by breaking down these substances are subsequently fermented. The size and structure of the rumen ensures that the food is properly mixed and that the microorganisms can work on it for a sufficiently long time. During this process different kinds of gases originate. The so-called volatile fatty acids (VFAs) like acetic, propionic, and butyric acid are particularly important for the animal, as they pass through the rumen wall into the bloodstream, serving as a main source of energy. Unfortunately, alongside the VFAs greenhouse gases like carbon dioxide and methane are produced and released into the atmosphere by eructation.

All rumen microbes are well adapted to the conditions prevailing in this special ecosystem. Its temperature averages at about 39°C, but is also dependent on external influences.The pH value is relatively constant, ranging between about 6.5 and 7.0, although acids are permanently produced – this constancy is due to bicarbonate contained in and swallowed with the saliva and to ammonium set free in the course of the deamination of amino acids. Under certain nutritional conditions, though, stronger deviations of the pH value may occur due to the rapid proliferation of bacteria like Streptococcus bovis which produce lactic acid.

In newborn ruminants the microflora and -fauna of the rumen is not yet properly developed. However, through contact with the mother animal or aerosols a rapid colonization occurs and a microbial population emerges, ensuring the functionality of the rumen. In the view of this it is imaginable to influence the fermentation processes in the rumen by dispensing microbial probiotics to the calf directing the composition of the population in a certain way. Furthermore, there are different approaches to particularly influence methane production inside the rumen by dispensing so-called direct-fed microbials (DFM); these are, however, as yet restricted to making use of alternative metabolic pathways competing with methanogenesis.

1. Hristov AN, Oh J, Firkins JL, Dijkstra J, Kebreab E, Waghorn G, Makkar HP, Adesogan AT, Yang W, Lee C, Gerber PJ, Henderson B, Tricarico JM (2013) Special topics – Mitigation of methane and nitrous oxide emissions from animal operations: I. A review of enteric methane mitigation options. J. Anim. Sci. 91(11):5045-69
2. Jeyanathan J, Martin C, Morgavi DP (2014) The use of direct-fed microbials for mitigation of ruminant methane emissions: a review. Animal 8(2):250-61
3. Madigan MT, Martinko JM, Stahl DA, Clark DP (2012) Brock Biology of Microorganisms. 13th ed. San Francisco: Pearson Education. pp. 762-6
4. Weimer PJ (1992) Cellulose Degradation by Ruminal Microorganisms. Crit Rev Biotechnol 12(3):189-223