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4.1.4: Symbiosis

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    Some biological interactions are brief, such as predation. In others, species are closely associated for long periods. Such associations are called symbiotic ("living together"). One species always benefits in a symbiosis, but the other may be harmed (parastitism), unaffected (commensalism), or benefited (mutualism). Some experts considered all parasitisms, commmensalisms, and mutualisms to be symbiotic, but others only consider interactions in which species are living together in a close association (such as when one species lives on or in the other) as symbiotic. 

    Termites have a symbiotic relationship with microorganisms called protozoa that live in the insect’s gut (figure \(\PageIndex{a}\)-a). The termite benefits from the ability of the protozoa to digest cellulose, a carbohydrate important in plant structure. However, the protozoa are able to digest cellulose only because of the presence of symbiotic bacteria within their cells that produce the cellulase enzyme. The termite itself cannot do this; without the protozoa, it would not be able to obtain energy from its food (cellulose from the wood it chews and eats). The protozoa benefit by having a protective environment and a constant supply of food from the wood chewing actions of the termite. In turn, the protists benefit from the enzymes provided by their bacterial endosymbionts, while the bacteria benefit from a doubly protective environment and a constant source of nutrients from two hosts.

    Lichen are a symbiotic relationship between a fungus and algae (or a photosynthetic bacteria; Figure \(\PageIndex{a}\)-b). In lichen, the algal cells are fully surrounded by the fungus. The sugars produced by the algae through photosynthesis provide nourishment for both organisms. The physical structure of the fungus protects the algae from the elements and makes certain nutrients in the atmosphere more available to the algae. The algae of lichens can sometimes live independently given the right environment, but many of the fungal partners are unable to live on their own. Note that while both lichen and the example involving termites mutually benefit the species involved, symbioses are not always mutualisms.

    Several light brown termites with segmented bodies and large heads (left). Fuzzy, blue-green lichen covers a tree trunk (right).
    Figure \(\PageIndex{a}\): (a) Termites form a symbiotic relationship with protozoa living in their guts. Their association which allows both organisms to obtain energy from the cellulose the termite consumes. (b) Lichen is a fungus that has symbiotic photosynthetic algae living in close association. (credit a: modification of work by Scott Bauer, USDA; credit b: modification of work by Cory Zanker)

    The remora and the shark is a commensal symbiosis. The top fin of the remora is modified into a sucker with which it forms an attachment to the shark. When the shark feeds, the remora picks up scraps. Additionally, the remora gains transportation and protection. The shark makes no attempt to prey on the remora and is unaffected by it (figure \(\PageIndex{b}\)).

    A light gray remora upside down and attached to the top of a shark in an aquarium
    Figure \(\PageIndex{b}\): A remora attached to the top of a shark in an aquarium. This is an example of a symbiosis because the remora lives in close association with the shark. Image by Brian Snelson (CC-BY)

    There are many additional examples of symbiosis. Coral are small animals that harbor photosynthetic microorganisms called dinoflagellates. (The corals group together and deposit hardened calcium carbonate skeletons.) Mycorrhizae are fungi that surround or live within plant root cells and help plants absorb nutrients from the soil in exchange for sugars. In fact truffles are often found in oak forests because the fungus that produce this spore-forming body establishes mycorrhizae on oak roots. Root nodules are structures on the roots of legume plants that house nitrogen-fixing bacteria (see The Nitrogen Cycle). Epiphytes are plants adapted to live on top of other plants and are common in the tropical rainforest (see Biomes). Humans have a diversity of microorganisms, called the normal flora, that live in and on our bodies. While we often think of bacteria as causing infection (a parasitism), the species of our normal floral exemplify commensalism and mutualism. For example, our gut microbiota aids with digestion and helps us synthesize certain vitamins.

    Attributions

    Modified by Melissa Ha from the following sources:

    This page titled 4.1.4: Symbiosis is shared under a CC BY-NC license and was authored, remixed, and/or curated by Melissa Ha and Rachel Schleiger (ASCCC Open Educational Resources Initiative) .