How Symbiosis Works

The traditional definition of symbiosis is a mutually beneficial relationship involving close physical contact between two organisms that aren't the same species. Most biologists still adhere to this definition. Some biologists, however, consider any interspecies relationship involving frequent close contact to be symbiosis, regardless of which of the organisms benefits. This includes commensalism, in which one organism benefits and the other isn't affected much at all, and parasitism, in which one organism benefits and the other is harmed. In this article we're going to focus on mutually beneficial symbiosis.

There are several forms of symbiosis. In some instances, the organisms require the symbiotic relationship in order to survive. This is known as obligate symbiosis. In other cases, the symbiotic relationship gives each organism a greater chance of survival but isn't absolutely necessary. This is known as facultative symbiosis. Symbiotic relationships aren't always symmetrical -- they can be obligate for one organism and facultative for the other.

The "close physical contact" part of the definition is worth looking at more closely. In most cases, it's fairly straightforward -- one organism may make its home directly on another organism's body, or even live inside it. But biologists also consider the biochemical relationship between two organisms. If they're generating and sharing enzymes, proteins, gases or other chemicals then they can also said to be symbiotes.

Endosymbiotes live inside another organism. And by inside, biologists really mean inside -- in between cells or within the body tissues (like the acoel flatworm). Ectosymbiotes live on the body of another organism. (Note that organisms that live within another's digestive tract are considered ectosymbiotes. Apparently living in someone's else's intestines doesn't qualify as a close enough relationship for biologists to call them endosymbiotes.)

Evolution can seem pretty astonishing by itself. The varied and specific adaptations used by many organisms can seem at times to defy logic. Symbiosis only makes it seem more unlikely -- how could two separate species evolve traits that just happen to fit so perfectly together? In fact, many people who question evolution point to symbiosis as "proof" that these couldn't happen naturally.

Natural selection is the key to understanding how symbiosis evolves. In a given population, some organisms will have traits that are more advantageous to successful reproduction than others. Organisms with those traits are therefore more likely to pass them along to succeeding generations, while those without them have a greater chance of dying before they reproduce. Thus, over many generations, the population will tend to look more and more like the individuals with the successful traits.

The success or failure of traits depends on population pressure -- circumstances that make it more difficult for individuals to survive. Traits that allow a creature to take advantage of the other life forms in its environment will be just as successful as the traits that allow it to escape (or eat) them.

Most symbiotic relationships probably started out as facultative. Over many generations, the organisms came to depend more on the symbiosis because natural selection favored those traits and not others. Eventually, the symbiosis became the sole source of the food, shelter, enzyme or whatever else the symbiotes derived from one another.

Another way to look at symbiosis is as evolution's toolbox. Trees need the nutrients found deep within the soil. They could evolve more efficient root systems that would allow them to extract those nutrients themselves -- in fact, many trees have. But this can take a lot of time (tens of thousands of years or more) and might not happen at all. It just so happens that fungi already have this ability. When the two species find themselves in close proximity, it is much faster to evolve a way to incorporate the "tool" already available to the other organism than to reinvent the wheel.

Some biologists are proponents of a theory known as symbiogenesis. This theory, which has fairly widespread acceptance, suggests that symbiosis is actually the key to the origins of complex life on Earth. Symbiogenesis theorists think that increasingly diversified microbes entered into a series of symbiotic relationships, with different microbes performing the tasks vital to microbial existence. These relationships evolved into a tightly integrated network of reciprocal microbes, each acting as a cog in the machine. They eventually evolved a casing enclosing them all. The microbes that made up this "team" became the parts of a cell: mitochondria, nuclei, ribosomes.

Are you a symbiote? Absolutely. Your digestive tract contains trillions of bacteria and other microorganisms. In fact, most of the mass of fecal matter is made up of bacteria. These bacteria serve a number of functions, but they primarily break down things that our digestive system is unable to process by itself. For example, a lot of carbohydrates make their way to the intestines undigested. The bacteria there break the carbs down into various acids that can be absorbed and processed. The result: We get more nutrients and calories from our food. Antibiotics can kill off a lot of these bacteria, reducing our digestive efficiency until they grow back [source: University of Glasgow]. The bacteria, for their part, get a steady supply of food delivered straight to them.

This digestive aid is a great benefit to people with limited access to food resources. They need to get every calorie they can from their food. However, scientists have been studying the contributions of human gut bacteria to widespread obesity in Western nations. Experiments have shown that mice raised in a sterile environment, with no bacteria to aid digestion, remained lean even though they were fed a high-calorie, high-fat diet [source: PNAS]. Manipulating our own symbiotic relationship with gut bacteria could lead to the development of an effective diet pill.

The bacteria in your gut are pretty complex, providing benefits we don't fully understand yet. Some scientists think they might aid our immune system by providing "practice," allowing us to produce antibodies that protect us against more harmful microbes. Indeed, gut bacteria themselves can be very harmful to us if they move out of the digestive tract into the bloodstream. They might also outcompete microbes that would be harmful if they were able to move in and live in our intestines.

Symbiotic relationships aren't rare. Here are some particularly cool examples:

Cecropia Trees and Azteca Ants

Cecropia trees have hollow trunks, and inside they secrete a sugary liquid that's nutritious to ants. Azteca ants colonize the trees, filling the trunk with millions of ants, who receive shelter and food from the tree. The tree is vulnerable to vines, which can grow on it, weigh it down or choke it. Azteca ants patrol the Cecropia and use their jaws to cut away any vines.

Cleaner Fish

There are many examples of fish that helpfully clean the parasitic bacteria and fungi from the bodies (or even from inside the mouths) of other fish. The pilot fish, client fish, cleaner wrasse and seniorita fish all eat parasites from other species, gaining a nice meal. The other fish gain protection from the damage these parasites would cause if left unchecked. They not only halt aggressive behaviors to allow cleaner fish to do their work, they have been known to go out of their way to visit them.

Cleaner relationships exist out of the water as well. Egrets, oxpeckers, plovers and brown-headed cowbirds all spend a good portion of their lives riding around on other animals. The birds pick off insects, ticks and other parasites to eat. Plovers hop into the mouths of basking crocodiles to eat leeches. The animals (zebras, bison, warthogs, domestic cattle) are kept clean of harmful insects. The cleaner birds also act as an alarm system, notifying their host when danger is present.

Honeyguides

A species of bird called the honeyguide prefers to eat beeswax and bee grubs. However, it isn't large enough to break open bee hives. To accomplish this, it finds a nearby mammal, sometimes a human or a badger-like creature called a ratel. It then hops around to get attention and then leads its "assistant" to the hive. The assistant wants the honey, so it breaks open the hive to eat it, exposing the wax and grubs to the honeyguide.

The Rhizosphere

Plants need nitrogen. It's a vital nutrient, important to healthy growth. However, plants lack the mechanism to extract nitrogen from the air. They can get it from the ground if the ground happens to be fertile, but the supply can be easily depleted. In a perfect example of the "evolution's toolbox" aspect of symbiosis, certain plants have found another species that does have the ability to extract (or "fix") nitrogen from the air. Legumes, a plant family that includes potatoes, peanuts and many others, bond with rhyzobia bacteria. The bacteria actually grows in nodules on the legumes' roots. The legume provides the energy necessary for the rhyzobia to break the strong chemical bonds in free nitrogen, and the rhyzobia produce nitrogen for the plant, plus enough to keep the surrounding soil fertile for years.