Rumen
Authors: Mackie Roderick I , McSweeney Christopher S , Aminov Rustam I 
Published: January 2022

The rumen is a large pregastric fermentation compartment (foregut) which maintains a diverse but concentrated population of anaerobic bacteria, protozoa and fungi that are responsible for a variety of degradative and fermentative reactions. During this process, biodegradable organic matter, mainly plant cell wall polymers, are converted into volatile fatty acids and microbial biomass which supply energy and protein to the host (ruminant) animal. An important reason for the evolution of foregut fermentation is detoxification of phytotoxins (of plant origin) and mycotoxins (of fungal origin). The concept of interspecies hydrogen transfer in which the mutually beneficial unidirectional transfer of hydrogen from a hydrogen‐producing to a hydrogen‐utilising bacteria in a coupled reaction that maintains low partial pressures makes the transfer process thermodynamically feasible is important in ruminal methanogenesis. Currently, modern ‘Omics’ technologies are being applied to the study of rumen microbial ecology, genomics, (meta)genomics and (meta)transcriptomics.

Key Concepts

The ruminant is a specialised model of foregut fermentation. The most obvious feature of ruminants is rumination that enables reduction in particle size and exposure of maximal surface area to microbial attack in the large foregut fermentation tank called the rumen.

In the rumen, biodegradable organic matter, mainly plant cell wall polymers, are converted into volatile fatty acids and microbial biomass which supply energy and protein to the host (ruminant) animal.

The rumen microbial population is characterised by its high population density, wide diversity and complexity of interactions.

The rumen contains representatives of all three domains of life (Bacteria, Archaea and Eukarya). Bacteria are predominant but a variety of ciliate protozoa and anaerobic fungi are widely distributed.

Fermentation of substrates in the rumen yields short‐chain volatile fatty acids (primarily acetic, propionic and butyric acids), carbon dioxide, methane, ammonia and occasionally lactic acid.

Based on cultivation methods, plant cell wall hydrolysis is carried out by specialist bacteria (mainly the genera
Ruminococcus
and
Fibrobacter
), ciliate protozoa and anaerobic fungi. However, cultivation‐independent approaches suggest other groups of uncultivated Firmicutes may also be important in fibre degradation.

Protein is extensively degraded in the rumen and used to resynthesise bacterial protein. Many rumen bacteria preferentially utilise ammonia and 60–80% of bacterial protein is synthesised from this precursor.

Phytotoxins occur widely in many common feeds, including grain, protein supplements and forages. An important reason for the evolution of foregut fermentation is detoxification of phytotoxins and mycotoxins.

In the rumen, methanogenesis from carbon dioxide reduction by hydrogen dominates terminal electron flow. The concept of interspecies hydrogen transfer in which the mutually beneficial unidirectional transfer of hydrogen from a hydrogen‐producing to a hydrogen‐utilising bacteria in a coupled reaction that maintains low partial pressures makes the transfer process thermodynamically feasible.

Methane emissions from enteric fermentation in livestock, mainly ruminants account for the single largest agricultural source of anthropogenic greenhouse gas emissions. These considerations have led to an increase in efforts to identify technologies to mitigate ruminant methane emissions.

The advent of genomics (the mapping and sequencing of genomes and analysis of gene and gene function) has revolutionised the biological sciences. Currently, ‘omics’ technologies are being applied to the study of rumen microbial genomics, (meta)genomics and (meta)transcriptomics.