To optimise the synthetic or metabolic pathway of micro-organisms related to methane synthesis by employing modern molecular biotechnology to obtain genetically modified microorganisms. Then the genetically modified micro-organisms are introduced back into the rumen ecosystem to establish a relatively stable microbiota that can replace or compete with the original pathway of methanogenesis, to reduce methane synthesis in the rumen.
Most methane emissions from ruminants are synthesised by methanogenic archaea in rumen. The methanogens mainly use carbon dioxide and hydrogen to synthesise methane. Protozoa and other microbes involved in cellulose-degrading or glucose-metabolic pathways provide carbon dioxide and hydrogen, and other mono carbon compounds necessary for methanogens. Therefore, the process of methane synthesis is implicated with complex symbiotic relationships of ruminal microbes and improper manipulation may break metabolic homeostasis in rumen. However, the development of modern molecular biotechnology and gene engineering technology provides a great opportunity for the improvement of rumen microbiota to bring about optimal reduction in methane emissions.
With respect to the process of feed degradation and methane synthesis, there are some possible links in realising the methane-mitigating goal with the application of developing genetically modified microorganisms. First, digestibility is one of the important factors influencing methane synthesis in the rumen. Cellulose, semi-cellulose and lignin contents are high in forage and they are difficult to degrade completely, and therefore they are positively associated with methane emissions. Based on mutagenic breeding methods and transgenic technology, high-efficiency exogenous genes could be introduced into microbial genomes, and then express high-efficiency degrading enzymes in rumen. As a consequence, the cellulose decomposition bacteria are strengthened to better degrade refractory carbon structure in forage, thus resulting in high efficient feed digestibility and energy use. Since more energy is obtained from an equal quantity of feed and animal production is improved, methane emission per unit of product could be reduced.
The reaction of carbon dioxide and hydrogen to form methane is a key step to decrease the hydrogen partial pressure in the rumen, so finding new hydrogen competitor or methane oxidative pathway could reduce methane production. For example, acetogens can also utilise hydrogen as substrate and have been found to be dominant in kangaroos’ rumen. If acetogens that can out compete methanogens in hydrogen intake are selected by genetically modified technology and then form stable microflora in rumen, less methane would be produced from ruminants. Methane oxidation may be another possible solution to solve this problem. Methanotrophic bacteria can oxidize methane to carbon dioxide, and they inhabit widely diverse environments. Through genetic modification, bacteria with high methane-oxidative efficiency can be obtained. Once these bacteria are introduced into rumen and form stable microflora, methane will be used to form carbon dioxide without affecting ruminal fermentation.
At present, the researchers worldwide engaging in methane emission mitigation of ruminants mainly focus on nutrition regulation, optimisation of feed formula and application of additives. In comparison, the methane mitigation in ruminants using genetic modification is only just now being investigated. This technology, marked by complexity of operation, excessively high up-front investment and long period of study, requires multi-disciplinary cooperation. All these factors together restrict the development of genetic modification of micro-organisms to reduce methane emissions.
- Improving digestibility, fermentation, energy utilisation efficiency of feed, and animal performance.
- Methanogens and other micro-organisms form symbiotic relationships and benefit mutually (Thiele et al, 1988; Joblin et al, 1989), so introducing genetically modified microbes favours the homeostasis of microbial diversity and complexity of symbiotic relationship in rumen, avoiding side effects onrumen ecosystems.
- Many approaches for reducing methane emissions have been tried, including research on feed preparation, vaccines, and additives (Han et al, 1997; Beauchemin et al, 2005; Yvette et al, 2009; Wright et al, 2004; Machmüller et al, 2001; Animut et al, 2008). However, these approaches lack sustainability and heritability. In comparison, once the genetically modified microbes survive in rumen, they will be carried by ruminants as long as they live and can be inherited by their offspring, without any extra costs to maintain methane mitigation.
- Although chemical inhibitors or antibiotics can reduce methane synthesis, the long-term adoption may cause residues of organic matter or antibiotics in meat and milk and bad health conditions of animals. However, genetic modification of micro-organisms in rumen can eliminate all the adverse effects mentioned above and achieve methane emission reduction on the premise that food security is guaranteed.
In spite of the advantages of genetic modification of rumen micro-organisms in reducing methane emissions in rumen, several problems and technical barriers remain.
- Most of the microorganisms in rumen are hard to isolate or culture. Mutagenic screening and genetic modification require more information on the mechanism and ecological functions of microbial metabolism and are still at a trial stage.
- Relevant reports indicate that technical barriers exist for introducing genetically modified strains into the rumen ecosystem, as well as for establishing a stable microflora and a stable symbiotic relationship (Wallace et al., 1994; Cotta et al., 1997; McSweeney et al., 1994).
Genetic modification of rumen organisms is a systems engineering problem involving nutrition, molecular biology, physiology, genetics, microbiology, biological chemistry, and so on. Though this field has just started, the perspective of methane mitigation in ruminants has been highlighted by this technology. Since the research on genetic modification of rumen micro-organisms is based on the principles of genetics, this modification is, in theory, supposed to be inheritable, which is the greatest advantage of this technology. Once this technology can be put into actual application, ruminants will not only reduce methane emissions but also be capable of passing their ability in methane reduction to their offspring, permanently.
Compared to others, this technology could change the rumen methane problem once and for all, in theory. If this is so, it would remarkably reduce production costs and achieve considerable economic benefits because no more extra expense would be required to maintain long-term methane mitigation.
On the whole, the genetic engineering of rumen micro-organisms to achieve methane reduction in ruminants is still at the initial stage. Currently, the emphasis is still being placed on basic research, and there is a long way to go to realise actual application.
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