Symbiosis comes from two Greek words that mean with and living. It describes an ecological relationship between two organisms from different species that are sometimes, but not always, beneficial to both parties. 

Symbiosis is an intimate, long-term and specific association between organisms of two or more species. This definition is largely identical to the original concept of symbiosis coined by Anton de Bary in 1879 and can be broadly applied to a multitude of relationships of beneficial, neutral, or harmful nature. Accordingly, a symbiosis beneficial to both partners is called ‘mutualism,’ a relationship harmful to one partner is termed ‘parasitism,’ and ‘commensalism’ is beneficial to one partner and largely neutral for the other. There also exists an alternative definition of symbiosis, frequently employed by European scientists, that exclusively refers to mutualistic relationships. In this chapter, the original broad concept is taken into consideration, since for many associations the effect on the organisms is unknown, and often there are no clear borderlines between mutualism, commensalism, and parasitism/pathogenicity but rather an interactive continuum. Moreover, the mechanisms underlying the establishment and maintenance of different types of associations can be based on very similar molecular mechanisms.

One of the main functions of secondary metabolites is communication. It is therefore not surprising that small molecules can play a crucial role in symbiotic systems, which use highly developed signaling and recognition mechanisms to establish and maintain the interaction. In symbionts, natural products are employed as chemical cues for partner localization, as invasive toxins, as developmental triggers, or as protective agents to eliminate competitors or to increase the host’s fitness. Owing to these highly specialized roles, many of which are unique for symbiosis, selection conditions during the evolution of natural product pathways can differ from those in free-living organisms. This might explain why the chemistry of symbionts is often highly distinct even if closely related nonsymbiotic species exist. Thus, research on symbiotic systems not only reveals unique mechanisms in chemical ecology and metabolic evolution but is also rewarding in terms of natural product discovery.

What is symbiosis?

The close interactions between two or more species that are usually beneficial to both are called symbiosis. Symbiotic relationships are essential to many organisms and ecosystems providing a balance within their environments. The two species in a symbiotic relationship live together and benefit each specie in some way. However, there are a couple of types of symbiosis that are not beneficial to both species and may harm one of the species.

Types of symbiosis:

Symbiotic relationships are broadly defined by their benefits or costs to the interacting organisms. The major relationships include:

• Mutualism: both organisms benefit.
• Commensalism: one organism benefits while the other is neither helped nor harmed.
• Amensalism: neither organism benefits, but one is harmed.
• Parasitism: the parasitic organism benefits at the expense of the host.
• Competition: neither organism benefits.

1. Mutualism: Some symbiotic relationships benefit both organisms involved. Mutualism is a relationship between two organisms of different species that provides each with needed resources, like shelter, food, or protection from predators. One common example is the relationship between flowers and bees. As the bees collect nectar for food, pollen grains adhere to their bodies and are carried from flower to flower. This allows the flowers to reproduce. This mutualistic relationship propagates the flower species while providing food for the bees.    
2. Commensalism: commensalism, is an interaction that benefits one species while neither helping nor harming the other. An example is a relationship between cattle egrets and grazing herbivores. The herbivores, including cattle, horses, and buffalo, disturb insects as they graze, making the insects easier for birds to catch.
3. Parasitism: In parasitism, one organism benefits from the relationship at the expense of the other. The parasitic organism may live inside another organism’s body (endoparasitism) or on its surface (ectoparasitism). The host species often weakens and sometimes dies, but in most cases, the parasite needs it to stay alive so it can keep feeding on it.

Example: Head lice on small amounts of blood on the scalp. The host human feels itchiness in their hair as the lice bite and move around.

Competition: Many species compete for the same resources in an ecosystem, which is called competition symbiosis. It may seem like this type of relationship is the opposite of symbiosis, but ecosystems depend on a balance of different species being present. 

Example: Both wolves and bears hunt the same prey in their habitats. But if the bears consume all the prey in the area, the wolves will die off, and the habitat will be imbalanced.

Amensalism:  Amensalism refers to an ecological interaction between two species, but in this association among organisms of two different species, one is destroyed or inhibited, and the other remains unaffected.

For example: when cattle trample on grass, the grass is crushed. However, the cattle do not benefit from this action nor are harmed in the process.

Examples of symbiosis:

1. Microbes:  microbes are important for human health. For example, human breast milk contains oligosaccharides, which are short chains of sugar molecules. There is no nutritional value to babies, but the microbes are important for developing a baby’s immune system.
2. Parasitism: Fleas and mosquitoes feed on the blood of other organisms. In the case of a mosquito, it may be harmful to the other organism. Fleas become harmful, for example, to a dog or cat when they act as their host for them.
3. Pets: Dogs, cats and other domesticated animals live together in symbiotic relationships, with both benefitting from their interactions with the humans.
4. Taxoplasma: It is a parasitic protist that may infect a range of animals including mice, rats and people. To reproduce sexually, the protist must infect a cat. The cat is not directly affected, but when a mouse is infected it brings harm. Mice infected with toxoplasma lose their fear of cats, which of course may bring them in close contact with a predator.
5. Cattles and egrets: Cattle do not benefit but are unaffected as egrets eat the insects that have been disturbed as the cattle forage for food.
6. Mutualism: Cleaning symbiosis takes place in the ocean when shrimp and gobies clean fish, receiving nutrients as they remove parasites, dead tissue, and mucous from the hosts.

Conclusion: 

The close interactions between two or more species that are usually beneficial to both are called symbiosis. Symbiotic relationships are essential to many organisms and ecosystems providing a balance within their environments. The categorization of symbiosis types (mutualism, parasitism, commensalism, etc.) is an artificial construct. In reality, biological interactions do not always fit into such discrete categories. Rather, symbioses should be perceived as a continuum of interactions ranging from parasitism to mutualism.