The rumen microbiota comprises diverse anaerobic microbes that drive fermentation, nutrient conversion, and host metabolic regulation. They include principally bacteria, protozoa, fungi, and archaea.
BACTERIA
Rumen bacteria account for 1010 organism/mL of rumen fluid and several hundred species have been characterized to date. By volume, they comprise up to 50% of the total microbial biomass. Bacteria species are an important source of microbial protein, which supply the ruminant with 75-80% of its metabolizable protein. Bacteria are also important for producing enzymes that digest fiber (cellulose, hemicellulose), starch and sugars.
PROTOZOA
Ciliate protozoa are organisms larger than bacteria and account for 106 organisms / mL of rumen fluid, however they still make up to 50% of the total microbial biomass. They have various activities:
- Cellulolytic and hemicellulolytic can digest plant particles
- Different protozoa have a positive role digesting starch (more slowly than bacteria)
- Others protozoa can consume lactic acid, thereby limiting the risk of acidosis
- Some types of protozoa are able to remove oxygen so they have a stabilizing effect upon anaerobiosis.
However, most of them degrade proteins very efficiently and release ammonia, so they can waste dietary protein. These proteins represent around 25% of the microbial protein available for the animal. Ciliate protozoa produce large amounts of hydrogen, which is a substrate for methanogens. The ciliate species are predators of other rumen microbes. In fact, a single protozoal cell can swallow up to several thousand bacteria in an hour so they play a very important role in rumen microbial population stability.
FUNGI
Rumen fungi comprise up to 8-10% of microbial biomass and are strictly anaerobic. They play an essential role in fiber digestion due to the production of filamentous rhizoids which invade plant tissues, and their efficient enzymatic activities. This physical action to plant cell walls, can facilitate access to more digestible tissues and help release polysaccharides, which are linked to lignin increasing the pool of digestible energy for the rumen microbiota.
ARCHAEA
This group of microorganisms is involved in methane (CH4) production from a limited range of substrates. They are thus called methanogenic Archaea. Archaea is an ancient lineage of microorganisms that have the appearance of bacteria but which are phylogenetically distinct. Archaea are considered as very strict anaerobes and use some of the fermentation products, notably H2 and CO2, to form CH4 which is eructated by the ruminant animal (Atwood et al., 2020).
