Question: I recently read a news article about how much of the recent increases in methane emissions have originated from agriculture and not fossil fuel use. Since methane is a potent greenhouse gas this trend had serious consequences for the environment. A fraction of this agricultural methane is produced from the digestion of cellulose by ruminants like cows. I would like to know how ruminants can digest cellulose even though most mammals cannot.
Ruminants, like goats, buffalo, and deer, are able to digest and obtain useable energy from cellulose. This ability is very rare amongst other animals (with the notable exception of termites) because animals generally do not produce sufficient amounts of cellulose hydrolyzing enzyme known as cellulase. Ruminants also produce insufficient amounts of cellulase to break down cellulose, however these animals form a symbiotic relationship with bacteria which produce large amounts of the enzyme. These bacteria, such as Ruminococci, are stored in the rumen, an organ to which cellulose rich foods are sent to be digested into smaller carbohydrates. After exposure to the cellulase rich environment of the rumen, the digested food is regurgitated and again swallowed to enter the ruminant’s digestive system. This predigested mass contains much smaller and easier to digest carbohydrates than cellulose. A byproduct of this process is the production of large amounts of methane in the rumen as the cellulose is digested.
Though the digestion of cellulose by ruminants does not seem like an important biochemical process it has important implications for the biofuels industry. Due to the stable β(1à4) linkages found in cellulose, it is far more difficult to process into fuels like ethanol than polysaccharides like starch with their less stable α(1à4) linkages. There is a vast amount of energy trapped in the excess cellulose produced by agriculture which has the potential to be converted to biofuels. However, this requires that the cellulose first be converted to smaller carbohydrates. Though this is currently possible with technologies like acid hydrolysis and using commercial enzymes with low specific activity but it is a very inefficient process. The use of cellulose digesting bacteria found in ruminants may greatly increase the efficiency of cellulose based biofuel manufacturing while producing energy rich methane gas which, if contained, can be used for energy instead of a pollutant.
- Debanjan D.
- “Methane Emissions Are Spiking, But It Might Be More Cow Than Car.” Climateprogress; 2016 [7/1/2016]
- Zhao J. “Turning Waste Into Food: Cellulose Digestion” Dartmouth Undergraduate Journal of Science. 2011. [7/1/2016]