Rumen

Abstract

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 that 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, which 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, metagenomics and metatranscriptomics.

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.

  • On the basis of 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 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, metagenomics and metatranscriptomics.

Keywords: anaerobic bacteria; ciliate protozoa; chytridiomycete fungi; volatile fatty acids; methane; cellulase; ammonia assimilation; detoxification

Figure 1.

Summary diagram describing the interrelationships between the ruminant forestomach, its resident microbial population and the host animal.

Reproduced from Hungate 1985. © Nature Publishing Group.
Figure 2. Biochemical pathway of hydrogen‐dependent reduction of carbon dioxide to methane. The C unit is sequentially modified, reduced and transferred bound to novel coenzymes, which participate in the reaction cycle. The chemical structures of the unusual methanogenic coenzymes are located adjacent to the reaction steps in which they participate. Chemical structures for these cofactors can be found in Rouvierre and Wolfe and DiMarco et al..
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References

Amaya KR, Kocherginskaya SA, Mackie RI and Cann IK (2005) Biochemical and mutational analysis of glutamine synthetase type III from the rumen anaerobe Ruminococcus albus 8. Journal of Bacteriology 187: 7481–7491.

Beauchemin KA (2011) Reducing Methane Emissions Form Livestock. Technical Factsheets, Agriculture and Agri‐Food Canada. www.agr.gc.ca/eng/science-and-innovation/science-publications-and-resources/

Berg Miller ME, Yeoman CJ, Chia N et al. (2012) Phage–bacteria relationships and CRISPR elements revealed by a metagenomic survey of the rumen microbiome. Environmental Microbiology 14: 207–227.

Brulc JM, Antonopoulos DA, Miller ME et al. (2009) Gene‐centric metagenomics of the fiber‐adherent bovine rumen microbiome reveals forage specific glycoside hydrolases. Proceedings of the National Academy of Sciences of the USA 106: 1948–1953.

DiMarco AA, Bobik TA and Wolfe RS (1990) Unusual coenzymes of methanogenesis. Annual review of Biochemistry 59: 355–394.

Dodd D, Moon YH, Swaminathan K, Mackie RI and Cann IK (2010) Transcriptomic analyses of xylan degradation by Prevotella bryantii and insights into energy acquisition by xylanolytic bacteroidetes. Journal of Biological Chemistry 285: 30261–30273.

Findley SD, Mormile MR, Summer‐Hurley A et al. (2011) Activity‐based metageneomc screening and biochemical characterization of bovine ruminal protozoan glycoside hydrolases. Applied and Environmental Microbiology 77: 8106–8113.

Hess M, Sczybra A, Egan R et al. (2011) Metagenomic discovery of biomass‐degrading genes and genomes from cow rumen. Science 331: 463–467.

Hungate RE (1985) Anaerobic transformations of organic matter. In: Leadbetter ER and Poindexter JJ (eds) Bacteria in Nature, vol. 1, pp 39–95. New York: Plenum.

Kabel MA, Yeoman CJ, Han Y et al. (2011) Biochemical characterization and relative expression levels of multiple carbohydrate esterases by the xylanolytic rumen bacterium Prevotella ruminicola 23 grown on an ester enriched substrate. Applied and Environmental Microbiology 77: 5671–5681.

Kim JN, Cann IK and Mackie RI (2012) Purification, characterization and expression of multiple glutamine synthetases from Prevotella ruminicola 23. Journal of Bacteriology 194: 176–184.

Moon Y‐H, Iakiviak M, Bauer S, Mackie RI and Cann IKO (2011) Biochemical analyses of multiple endoxylanases from the rumen bacterium Ruminococcus albus 8 and their synergistic activities with accessory hemicellulose degrading enzymes. Applied and Environmental Microbiology 77: 5157–5169.

Morgavi DP, Kelly WJ, Janssen PH and Attwood GT (2013) Rumen microbial (meta)genomics and its application to ruminant production. Animal 7(Suppl. 1): 184–201.

Rouvierre PE and Wolfe RS (1988) Novel biochemistry of methanogenesis. Journal of Biological Chemistry 263: 7913–7916.

Further Reading

Ferry JM (1993) Methanogenesis. Ecology, Physiology, Biochemistry and Genetics. New York: Chapman and Hall.

Hobson PN and Stewart CS (1997) The Rumen Microbial Ecosystem, 2nd edn. New York: Chapman and Hall.

Mackie RI and White BA (1997) Gastrointestinal Microbiology: Gastrointestinal Ecosystems and Fermentations, vol. 1. New York: Chapman and Hall.

Mackie RI, White BA and Isaacson RE (1997) Gastrointestinal Microbiology: Gastrointestinal Microbes and Host Interactions, vol. 2. New York: Chapman and Hall.

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Mackie, Roderick I, McSweeney, Christopher S, and Aminov, Rustam I(Sep 2013) Rumen. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000404.pub2]