Methanogens: These bacteria are of marshy habitats. They are capable of converting CO₂, methanol and formic acid into methane, hence named methanogens. These methanogens are present in the guts of several ruminant animals such as cows and buffaloes, and they are responsible for the production of methane or biogas from the dung of these animals. These are chemoautotrophs.

Biogas production is a process that involves the decomposition of organic materials in the absence of oxygen to produce a mixture of gases, primarily methane and carbon dioxide. This process occurs in a controlled environment called a biogas digester, where specific groups of bacteria play a crucial role in the production of biogas. Among these groups of bacteria, methanogens are the ones that are exploited in biogas production.

1. Methanogens:
- Methanogens are a group of bacteria that belong to the domain Archaea.
- They are strict anaerobes, meaning they can only survive and function in the absence of oxygen.
- Methanogens are unique because they produce methane as a metabolic byproduct during the degradation of organic matter.
- They play a crucial role in the final stage of biogas production by converting the intermediate products of anaerobic digestion, such as volatile fatty acids and alcohols, into methane gas.
- Methanogens are able to carry out this process due to their unique metabolic pathways and enzymes that can convert carbon dioxide and hydrogen, as well as other organic compounds, into methane.
- Some common examples of methanogens include Methanobacterium, Methanosarcina, and Methanococcus.

2. Methanotrophs:
- Methanotrophs, on the other hand, are bacteria that are capable of utilizing methane as a source of energy and carbon.
- They can oxidize methane and use it as a substrate for their metabolic activities.
- However, methanotrophs are not involved in biogas production but rather play a role in mitigating methane emissions by consuming it from the environment.

3. Organotrophs:
- Organotrophs are bacteria that obtain their energy and carbon from organic compounds.
- While they may be present in the biogas digester, they are not directly involved in the production of biogas.
- Organotrophs can degrade complex organic matter into simpler compounds, which can then be utilized by methanogens to produce methane.

4. Eubacteria:
- Eubacteria is a broad term that refers to bacteria belonging to the domain Bacteria.
- While some eubacteria may be involved in the initial stages of organic matter degradation in a biogas digester, they are not specifically exploited for biogas production.

In summary, methanogens are the bacterial group that is specifically exploited in biogas production. They have unique metabolic capabilities to convert organic compounds into methane gas, which is the primary component of biogas.