Commensalism

Commensalism

Meaning of Commensalism

Commensalism is a term employed in ecology to describe a relationship between two living organisms where one benefits and the other is not significantly harmed or helped. It is derived from the English word commensal, meaning the sharing of food, and used of human social interaction. The word derives from the Latin com mensa, meaning sharing a table.

Examples of commensalist relationships involving microorganisms

When there is a need for the production of growth factors commensalism plays an important role. For example, Flavobacterium brevis excretes cysteine which is taken up by Legionella pneumophila. Transformation of insoluble compounds to soluble ones and further into gaseous components is observed in commensalism. For example, methane from sediment benefits methane oxidising population in the overlying water and hydrogen sulphide production in buried sediments which helps the photo-autotrophic sulphur bacteria on the sediment surface. Conversion of organic molecules by one population which become substrates for other population is also commonly seen. For example, fungi release extracellular enzyme which decomposes lignin to form simple glucose moieties which benefit the other microorganisms.

Two organisms tend to remain together to satisfy their metabolic needs. For example, Mycobacterium vaccae, growing on propane as a source of carbon and energy, will gratuitiously oxidise cyclohexane to cyclohexanone which is readily used up by other microorganisms like Pseudomonas. Here Pseudomonas benefit because they are unable to metabolise cyclohexane, whereas the Mycobacterium remains unaffected because it does not assimilate cyclohexanone. Removal or neutralisation of a toxic material is also observed. For example, oxidation of hydrogen sulphide by Beggiatoa detoxifies it, thus benefiting the H2S sensitive aerobic organisms. Some tend to detoxify through immobilisation. For example, Leptothrix reduces manganese concentration thus permitting the growth of other microbes to which higher manganese concentration would be toxic.

Commensalism is more difficult to demonstrate than parasitism and mutualism, for it is easier to show a single instance whereby the host is affected, than it is to prove or disprove that possibility. Oftentimes, a detailed investigation will show that the host indeed has become affected by the relationship. An example of commensalism: cattle egrets foraging in fields among cattle or other livestock. As cattle, horses, and other livestock graze on the field, they cause movements that stir up various insects. As the insects are stirred up, the cattle egrets following the livestock catch and feed upon them. The egrets benefit from this relationship because the livestock have helped them find their meals, while the livestock are typically unaffected by it.

Another example of commensalism: birds following army ant raids on a forest floor. As the army ant colony travels on the forest floor, they stir up various flying insect species. As the insects flee from the army ants, the birds following the ants catch the fleeing insects. In this way, the army ants and the birds are in a commensal relationship because the birds benefit while the army ants are unaffected. The Demodex folliculorum mites living in human eyelash follicles have a similar relationship with their ...