the high level of horizontal gene transfer among microbial communities, microbial ecology is also of importance to studies of evolution. Symbiosis Microbes, especially bacteria, often engage in symbiotic relationships (either positive or negative) with other microorganisms or larger organisms. Although physically small, symbiotic relationships amongst microbes are significant in eukaryotic processes and their evolution. The types of symbiotic relationship that microbes participate in include mutualism, commensalism, parasitism, and amensalism, and these relationships affect the ecosystem in many ways. Mutualism Mutualism in microbial ecology is a relationship between microbial species and humans that allow for both sides to benefit. One such example would be syntrophy, also known as cross-feeding, of which 'Methanobacterium omelianskii ' is a classical example. This consortium is formed by an ethanol fermenting organism and a methanogen. The ethanol-fermenting organism provides the archaeal partner with the H2, which this methanogen needs in order to grow and produce methane. Syntrophy has been hypothesized to play a significant role in energy- and nutrient-limited environments, such as deep subsurface, where it can help the microbial community with diverse functional properties to survive, grow and produce maximum amount of energy. Anaerobic oxidation of methane (AOM) is carried out by mutualistic consortium of a sulfate-reducing bacterium and an anaerobic methane-oxidizing archaeon. The reaction used by the bacterial partner for the production of H2 is endergonic (and so thermodynamically unfavored) however, when coupled to the reaction used by archaeal partner, the overall reaction becomes exergonic. Thus the two organisms are in a mutualistic relationship which allows them to grow and thrive in an environment, deadly for either species alone. Lichen is an example of a symbiotic organism. Commensalism Commensalism is very common in microbial world, literally meaning "eating from the same table". Metabolic products of one microbial population are used by another microbial population without either gain or harm for the first population. There are many "pairs "of microbial species that perform either oxidation or reduction reaction to the same chemical equation. For example, methanogens produce methane by reducing CO2 to CH4, while methanotrophs oxidize methane back to CO2. Amensalism Amensalism (also commonly known as antagonism) is a type of symbiotic relationship where one species/organism is harmed while the other remains unaffected. One example of such a relationship that takes place in microbial ecology is between the microbial species Lactobacillus casei and Pseudomonas taetrolens. When co-existing in an environment, Pseudomonas taetrolens shows inhibited growth