Methanogens are bacteria that produce methane gas. As a result of their metabolism they can be used as sources of biogas. This would be a renewable source of energy as opposed to the natural gas extracted from oil deposits, which is a fossil fuel.

Ecology

Methanogens require anaerobic conditions such as the digestive systems of herbivores, marshes or lake bottoms. Many require warm conditions to work best.

They are associated with a source of organic matter (e.g. plant remains or sewage) with heterotrophic bacteria. The heterotrophs break down this organic matter to release compounds such as ethanoic acid (aka actetic acid or vinegar) and hydrogen. The ethanoate ions are a substrate for the methanogens.

Classification

Methanogens belong to the Archaea group of the Prokaryotes*. There are several species.

Examples
Methanococcus jannaschii
Methanobacterium thermoautotrophicum

*strictly speaking, microbiologists no longer refer to bacteria as such. The various groups of the prokaryote kingdom have been split into several further kingdoms (eg Archaea and Eubacteria) based on the genetics and biochemistry of microbes.

Biochemistry

Methanogens are chemoautotrophs.
Methanogens use a number of different ways to produce methane.
Example using ethanoate (acetate) that may be derived from the decomposition of cellulose:

Or using hydrogen and carbon dioxide produced by the decomposers:

Biogas production

Biogas is the gas produced by the decomposition of wastes in farming sewage treatment. It is a bi-product of the cleaning up of waste water. Biogas consists of about 40% CO₂ and 60% CH₄. It could, therefore be used as a source of renewable energy.

Biogas production requires the following:

• a fermenter, which is supplied with an innoculum of bacteria (methanogens and decomposers).
• anaerobic conditions
• an optimum temperature of 35°C
• an optimum pH of 6.5 to 8. This needs to be monitored as the decomposers produce acids and they work faster than the methanogens consume the acids.
• organic waste (biomass) eg sewage, wood pulp

Methanogens and the greenhouse effect

About half of the methane produced by methanogens is used up as an energy source by other bacteria. Most of the other half is lost to the atmosphere (about 600 million tonnes y^--1) where it acts as an important greenhouse gas. Therefore, as more land is converted to rice paddy fields and pasture for grazing animals more methane will be produced. In addition to this, as global warming progresses the permafrost will thaw in the regions covered by tundra. Tundra contains extensive reserves of peat (partially decomposed vegetable matter). As the peat warms and melts, it will provide a source of material for methanogens. So the amount of methane release into the atmosphere will accelerate. This in turn will drive global warming even further.

Sources

Allison Whipple et al  The Microbial Biorealm Methanogen Overview

University of Florida, Civil Engineering, Laboratory AgendaWaste digester design

Dan Amerson Decomposition of Organic Matter