The Rumen: An Extraordinary Microbial Ecosystem

The rumen has the task of digesting complex plant compounds, transforming them into simpler ones that can be assimilated by the animal. It is for this reason that a symbiosis is established between the microbial ecosystem and the ruminant.

Ruminants use compounds in their diet that other species, such as humans, are unable to assimilate. This phenomenon is due to a large variety of microorganisms that have the task of digesting these nutrients anaerobically (without oxygen) in the rumen, through a process called fermentation.

All this is necessary for the survival of many species of considerable importance for agriculture, such as, for example, cows. In this article we explain what rumen is and what are some of the microorganisms involved in this fascinating process.

The rumen

Ruminants (cattle, goats, cervids and sheep) have a rather complex digestive system, made up of four cavities:

• The lattice.
• The rumen.
• The omasum.
• The abomasum.

Within the rumen, thousands of microscopic organisms produce enzymes that contribute to the digestion of plant fibers and cellular material. For this reason we can say that the rumen is a microbial ecosystem, in which bacteria make up 60% of the population of microorganisms.

Rumen process

We need to know that the rumen communicates with the mouth through the esophagus. These are the steps followed by food until digestion:

1. First, animals ingest plant foods. These contain cellulose, starch, pectins and other elements that ruminants are unable to digest directly because they do not have the necessary enzymes.
2. Then the food passes from the mouth to the rumen, where the microorganisms transform these complex molecules into simpler ones (low molecular weight fatty acids), carbon dioxide and methane.
3. Once these molecules have been broken down into others that the animal’s intestine can absorb, the food returns to the mouth, where it is chewed again and swallowed.
4. Finally, the semi-digested food reaches the reticulum, then the omasum and the abomasum, which serves as the main stomach, because this is where the digestive process takes place.

Fermentation

These microbial communities produce some enzymes that have essential functions for the breakdown of carbohydrates (cellulose, starch and sugars), as well as nitrogen compounds and lipids. This breakdown is accomplished through a process called “fermentation”.

The fermentation process is essential for the production of energy (in the form of ATP), for the growth of microorganisms and for the production of essential molecules for the animal, such as glucose. They are also very important in compounds that contain nitrogen, essential for the synthesis of proteins.

In this way, the digestive system of these animals is able to obtain energy without having to resort to external elements such as vitamin B or essential amino acids, because it is precisely the microorganisms that produce it within them.

Symbiotic relationship in an anaerobic environment

It must be emphasized that, as we can see, the rumen is an example of mutualistic symbiosis: ruminants provide microbes with an adequate environment for their growth and activity. In return, the microorganisms offer the host nutrients that come from the food, which could not be digested otherwise.

This rumen ecosystem is made up of a large variety of microorganisms, which establish a symbiotic relationship in an oxygen-free environment.

This microbiota is made up of bacteria, archaea, protozoa and fungi. Bacteria are the most susceptible to the physico-chemical properties of the rumen. Those that predominate in this community belong to two  phyla  or taxonomic groups:

• Firmicutes ; especially those belonging to the genera Butyvibrio, Lachnospira, Succiniclasticum  and  Ruminococcus.
• Bacteroidetes : the predominant genus is  Prevotella.

Archaea make up approximately 1% of the microbial mass, while among eukaryotes we encounter protozoa, which occupy a third of the total, and some fungi.

Rumen bacteria

Cellulose, the main component of the plant cell wall, must be digested correctly; for this purpose, cellulolytic bacteria are essential.

In this case, a pH below 5.5 damages the digestion process of the fibers and a temperature of 39 ° C compromises the bacterial adhesion capacity.

Equally important are amylolytic bacteria, due to the presence of starch in the diet of cattle and cows that eat grain.

Lactic acid bacteria metabolize lactic acid and control its accumulation, thus helping to keep the pH within the appropriate values.

Finally, pectin-degrading bacteria also play an essential role, since pectin accounts for 10-20% of the total carbohydrates in the diet of these animals.

Methanogenic archaea

The activity of microorganisms is the main source of greenhouse gases in agriculture. Methane is generated by methanogenic archaea and is obtained as the final product of fermentation. This process is considered an energy loss, because it represents 6-10% of the total energy.

When this gas is expelled into the environment, it contributes to the greenhouse effect. During methanogenesis, the levels of CO2 and hydrogen present in the environment are reduced : this is a necessary phenomenon. 80% of the methane is generated by the fermentation of the fiber (cellulose), while the remaining 20% ​​originates from the decomposition of the dung.

Protozoa

These microbes are involved in the process of decreasing the risk of acidosis following the consumption of foods that have a high concentration of easily digestible sugars.

90% of protozoa belong to the genus  Entodiminiomorphida, whose main function is the hydrolysis and fermentation of cellulose. Affine diplopastron  exhibits amylotic activity, thanks to which it produces maltose and glucose.

Mushrooms

There are cellulolytic fungi that produce certain enzymes capable of hydrolyzing cellulose and xylans. The fungal activity promotes the digestion of the plant cell wall.

They are especially important when ruminants ingest lignified substances. For example,  Neocallimastix frontalis  soluble the lignin of the cell walls so that the bacteria have access to the cellulose without any problem.

The importance of microbes

As we have seen, microorganisms are essential in the degradative metabolism of food ingested by ruminants. This is just another example of the importance of these single-celled organisms in the animal world.

Furthermore, to conclude, it is good to clarify that it is absolutely essential that this microbiota remains in good health, in order to avoid the presence of physiological problems in the animal, such as acidosis.