Organisms rarely live in isolation. Many rely on other creatures as sources of food or nutrients. Photosynthetic plants and microbes provide oxygen that humans need to live. Trees offer shelter to other plants and animals. Some relationships between different organisms, though, are more involved. One organism may depend on another for its survival. Sometimes they need each other. This is called symbiosis.
Often, especially with microbes, one organism lives inside another — the host. When both organisms benefit from the relationship, it is called mutualism. When only one organism benefits, but the other one is not harmed, it is called commensalism. Microbial symbiosis occurs between two microbes. Microbes, however, form associations with other types of organisms, including plants and animals. Bacteria have a long history of symbiotic relationships, and have evolved in conjunction with their hosts. Other microbes, such as fungi and protists, also form symbiotic relationships with other organisms.
Bacteria form symbiotic relationships with many organisms, including humans. One example is the bacteria that live inside the human digestive system. These microbes break down food and produce vitamins that humans need. In return, the bacteria benefit from the stable environment inside the intestines. Bacteria also colonize human skin. The bacteria obtain nutrients from the surface of the skin, while providing people with protection against more dangerous microbes.
Symbiotic bacteria also live in nodules on the roots of bean plants. These bacteria convert nitrogen gas into a form that the plants can use. In return, the plants provide the bacteria with a safe place to live. In some cases, the symbiotic relationship is more strong. One particular roundworm has a bacterium living inside it. The roundworm infects and kills insects, using a toxin produced by the bacterium. This is an example of mutualism, because the roundworm and bacterium need each other to survive.
Fungi and Plants
Fungi and plants form mutually-beneficial relationships called mycorrhizal associations. The fungi increase the absorption of water and nutrients by the plants, and benefit from the compounds produced by the plants during photosynthesis. The fungus also protects the roots from diseases. Some fungi form extensive networks beneath the ground, and have been known to transport nutrients between plants and trees in different locations.
Fungi and plant roots form two different kinds of associations. In one type, the fungus grows outside the roots as a thick mat, or between certain cells in the root. The fungus, however, never enters any of the plant cells. With the other type of association, the fungus actually penetrates the cell walls of the roots. They break through the cell wall, but not the inner plasma membrane. In both types, the fungus extends its filaments outward to collect nutrients and water from the soil, which are in turn passed onto the plant.
Lichens: Fungi and Algae
Lichens are an example of a symbiotic relationship between two microbes, fungi and algae. So far, around 25,000 lichens have been identified. They grow on rocks and tree trunks, with colors ranging from pale whitish green to bright red and orange. The lichens grow in several forms: thin and crusty coverings; small branching strands; or flat, leaf-like structures. They are usually the first plants to grow in the cold and dry habitats that they favor.
In this mutually-beneficial relationship, the fungus forms the body of the lichen — the thallus. This structure attaches to the surface of a rock or tree. The fungal cells absorb water and nutrients from the surrounding environment. Algal cells grow inside the cells of the fungus. The algal cells convert sunlight to chemical energy through photosynthesis. This process benefits the fungus. In return, the algal cells are protected from the environment.
Certain protists and algae form a symbiotic relationship known as living sands. This type of association occurs in tropical and semitropical seas, and appears as green, orange, brown or red deposits containing calcium carbonate. Living sands were used in the construction of the Egyptian pyramids. Many different types of algae combine with their protist hosts. Without the algae, the protists cannot survive very long. Similar to living sands, some protists extract chloroplasts from diatoms, a type of algae. The chloroplasts provide the protists with the ability to convert sunlight to chemical energy through photosynthesis. Eventually, the chloroplasts break down and stop functioning.
An even better-known example of symbiotic protists are the ones that live in the guts of termites. These microbes break down cellulose in the wood particles that termites eat. This enables the termite to obtain nutrition from the wood. Without the help of the protists, the termite would not be able to digest the wood. In this case, the protist is called an endosymbiont, which means it lives inside its host, the termite.
Some scientists believe that early in the history of the planet, different types of microbes joined together to form a new type of organism. At the time, certain bacteria had the ability to convert sunlight to chemical energy, or generate chemical energy from oxygen. These microbes were engulfed by larger bacteria, forming a microbial symbiosis. The host cell protected the smaller microbe inside, while benefitting from the skills of its new partner.
In the beginning, the two bacteria could still function separately. Eventually, the microbes living inside lost the ability to survive on their own, and they became specialized components of the host cells. These structures later became the mitochondria and chloroplasts of eukaryotic cells. Mitochondria generate energy using oxygen, and chloroplasts convert sunlight into chemical energy in plant cells. Supporting this theory is the fact that mitochondria and chloroplasts both have their own DNA, separate than that found in the nucleus of the cell.