3.9: Symbiosis

Most of the interactions between species involve food, i.e., competing for the same food supply, eating (predation), and avoiding being eaten (avoiding predation). These interactions are often brief. There are many cases, however, where two species live in close association for long periods. Such associations are called symbiotic ("living together"). In symbiosis, at least one member of the pair benefits from the relationship. The other member may also benefit (mutualism), be relatively unaffected (commensalism), or may be injured (parasitism).

Types of Symbiosis

• Mutualism (both members benefit)
• Commensalism (one benefits, the other is unaffected)
• Parasitism (one benefits, the other is harmed)

Mutualism

Symbiotic relationships in which each species benefits are mutualistic. There are hundreds of examples of mutualism between a heterotroph and an alga.

• Paramecium bursaria is a ciliate that engulfs unicellular green algae into vacuoles within its cell.
  • The paramecium certainly benefits from the food synthesized by the alga. It can be cultured apart from the alga but then must be given extra food.
  • The alga presumably benefits from the carbon dioxide produced by its host as well as the host's ability to transport it to a spot where there is ample light.
• Many other aquatic animals, such as sponges, sea anemones, worms, and clams, also harbor algae within their cells which lets them get food from the photosynthesis of their partner.

Mutualistic relations between plants and fungi are very common. The fungus invades and lives in or among the cortex cells of the secondary roots. The association is called a mycorrhiza. The fungus helps the host plant absorb inorganic nitrogen and phosphorus from the soil. Some mycorrhizal fungi also secrete antibiotics which may help protect their host from invasion by parasitic fungi and bacteria. Many mushrooms are the spore-forming bodies of mycorrhizal fungi. The truffle is often found in oak forests because the fungus that produces it establishes mycorrhiza on oak roots.

Figure \(\PageIndex{1}\): Mutualistic relations between plants and fungi are very common. The fungus helps the host plant absorb water, inorganic nitrogen, and phosphorus from the soil, and the plant provides sugar to the fungus. Nefronus, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Endosymbiosis

Endosymbiosis is a mutualistic relationship between a host and an organism living within its body or cells.

The pea aphid and its endosymbiont

The pea aphid, Acyrthosiphon pisum, is an insect pest that sucks the juices from its host plant. However, plant sap is deficient in several essential amino acids. The pea aphid thrives nonetheless thanks to specialized cells within its body that contain the gamma proteobacterium, Buchnera aphidicola, that can live nowhere else. The genome of this obligate intracellular Gram-negative bacterium encodes a number of enzymes needed to complete the synthesis of the amino acids needed by its host. In return, the aphid's genome encodes enzymes needed by Buchnera to synthesize its lipopolysaccharide cell wall and has lost genes that might otherwise repel infection by Gram-negative bacteria.

Symbiotic nitrogen fixation

One of the most important examples of mutualism in the overall economy of the biosphere is the endosymbiotic relationship between certain nitrogen-fixing bacteria and their legume hosts. A large body of evidence supports the view that intracellular endosymbiotic relationships gave rise to eukaryotes with their mitochondria and chloroplasts.

Cleaning Symbiosis

The drawing shows the Nile crocodile opening its mouth to permit the Egyptian plover to feed on any leeches attached to its gums. Cleaning symbiosis is more common in fish.

Figure \(\PageIndex{2}\): Cleaning symbiosis. Herodotus asserted that the trochilus bird, possibly a sandpiper, was able to enter the mouth of the Nile crocodile in what would now be called a cleaning symbiosis. Drawing by Henry Scherren, 1906

Commensalism

Commensalism means "at table together." It is used for symbiotic relationships in which one organism benefits, but the other doesn't seem to be affected, either positively or negatively.

Some examples:

• the remora and the shark. The dorsal fin of the remora (a bony fish) is modified into a sucker with which it forms a temporary attachment to the shark. When the shark feeds, the remora picks up scraps. The shark makes no attempt to prey on the remora.
• Some species of barnacles are found only as commensals on the jaws of whales. And there are other species of barnacles found only as commensals on those barnacles!
• Many of the bacteria live in our large intestine. They feed on food that we cannot digest and do not harm us.

Parasitism

A parasite is an organism that lives on or in the body of another organism (the host) from whose tissues it gets its nourishment, and to whom it does some damage. Animals are parasitized by viruses, bacteria, fungi, protozoans, flatworms (tapeworms and flukes), nematodes, insects (fleas, lice), and arachnids (mites). Plants are parasitized by viruses, bacteria, fungi, nematodes, and a few other plants.

Parasites damage their host in two major ways:

• consuming its tissues, e.g., a mosquito eats blood
• releasing toxins, for example, the bacteria Tetanus bacilli secrete tetanus toxin, which can be deadly.

The relationship between parasite and host varies along a spectrum that extends from "hit and run" parasites that live in or on their host for a brief period and then move on to another, with or without killing the first, to parasites that establish long-term, chronic infections. Both parasite and host must evolve to ensure the survival of both because if the parasite kills its host before it can move on, it destroys its habitat.

The "simplification" of Parasites

During the course of adapting to conditions in their host, parasites often lose structures and functions that were essential for their ancestors (and any free-living relatives). The tapeworm has no eyes, no digestive tract, and only vestiges of nervous, excretory, and muscular systems. While you may call them degenerate, these losses represent a gain in efficiency and improved specialization. What good would these structures be any way in the human intestine? On the other hand, the tapeworm produces hundreds of proglottids: egg-forming machines that improve the likelihood that the tapeworm will leave descendants that reach another host.

The Evolution of Symbiosis

It seems plausible that what begins as a parasitic relationship might over the course of time evolve into a mutualistic one as the two organisms evolve to minimize the damage to the host.

And there is some evidence for this. In 1966, K. W. Jeon discovered a culture of amoebas that had become infected with bacteria (60,00 to 150,000 per cell). The infection slowed their rate of growth and made them much more fragile. But five years later, the amoebas still were infected but now no ill effects could be seen. Most interesting for our question, the amoebas — or at least their nuclei — had become dependent on the bacteria.

• When the nucleus was removed from an infected amoeba and replaced with one from an uninfected strain, the combination worked fine.
• But when the nucleus from an uninfected cell was replaced with one from an infected cell, the combination usually failed to survive.

Evidently, after 5 years, the nuclei had become dependent on a bacterial function (an enzyme produced by the bacteria but no longer by the host). What started as parasitism had evolved into mutualism (the bacteria could not be grown outside their host). But it doesn't always work like that. There are other examples where a mutualistic relationship seems to have evolved into a commensalistic or even parasitic one. Some parasitic fungi seem to have evolved from ancestors living in the mutualistic partnership of a lichen.

Some of the bacteria living in our large intestine supply us with vitamin K, thus evolving from commensalism to mutualism.

Mutually beneficial symbiotic relationships can lead to simplification as well. Some marine annelid worms have completely lost the digestive tract of their relatives (like the common earthworm). One species gets its nourishment from a large population of at least 5 different species of bacteria living underneath its outer skin. The most abundant of these are chemoautotrophs (using chemicals as an energy source) that manufacture food from carbon dioxide using the energy provided by oxidizing inorganic substances (H₂S, H₂) in the sediments in which the worm lives.

The nature of a symbiotic relationship can also change as circumstances change. Some fungi, bacteria, and protozoans that live harmlessly in most of us can cause opportunistic infections — that is become parasitic — in immunodeficient people, e.g., those with AIDS.

Contributors and Attributions

• Symbiosis by John W. Kimball is licensed CC BY 3.0. Original source: https://www.biology-pages.info/.
• Kevin Raskoff (Monterey Peninsula College)