Symbiosis

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Symbiosis is simply defined as living together. Scientists use this term to describe intimate relationships between members of different species. By definition there are at least two species in a symbiotic relationship; it is unknown the maximum number of species that a symbiosis can sustain. This number may be very great; fungal partners (mycorrhizae) of plant roots link many photosynthetic plants of different species in one continuous networked symbiosis. Partners may belong to the same kingdom (for example, plants in symbiosis with other plant species) or may include partners from different kingdoms. A lichen symbiosis consists of partners from two or three kingdoms—a fungus, a protist (algae), and often a cyanobacterium (eubacteria). The smaller partner(s) are usually called the symbiont(s) and the larger partner the host. The host's cells, body, body surface, or even its home may be shared with its symbionts.

To what extent must two species live together be considered a symbiosis? A general rule is that the partners must spend a significant amount of time together (part or all of their life cycles). This sustained contact enables a relationship to develop that affects how both species adapt and evolve. The symbiotic relationship is usually classified as belonging to one of three types: mutualism (benefiting both partners), parasitism (one partner, the parasite, benefits at the expense of the host), or commensalism (one partner benefits while the other is unaffected). However, it is too simplistic to place symbioses into such restrictive categories since the environment and ecological interactions with other species may affect the nature of the relationship. Under one set of conditions a relationship may be characterized as mutualistic, while under different conditions it may be parasitic. For example, the relative benefit for plants to host ants as a way to defend them against herbivores depends on the degree of herbivory and must be weighed against the cost of synthesizing the nutritional compounds needed to support resident ants. Both of these are subject to external influences.

Symbiosis provides an important source of evolutionary novelty. Special symbiont capabilities include photosynthesis and the transfer of photo-synthetic products from cyanobacteria and algae to animal and fungal hosts, and the supply of nutrients (nitrogen fixation by bacteria in legumes and

SYMBIOTIC ASSOCIATIONS INVOLVING PLANTS OR PHOTOSYNTHETIC ALGAE PARTNERS
Type of Symbiotic Relationship		Partners	Nature of Interaction
Symbionts Living in Host	Lichen	Symbionts: Algae, cyanobacteria (Rhizobium, actinomycetes, cyanobacteria)	Algae provide photosynthetic sugars and cyanobacteria provide nutrients (nitrogen)
		Host: Fungus	Fungus provides protection against environmental extremes
	Coral (and anemones)	Symbionts: Algae	Algae provide photosynthetic sugars
	Host: Cnidarian (animal)	Animal host provides recycled nutrients (nitrogen and phosphorus) and protection
	Bacteria-plant	Symbionts: Nitrogen-fixing bacteria	Bacteria provide nutrients (nitrogen)
		Host: Plant (legumes, alders, cycads, Azolla ferns)	Plant host provides photosynthetic sugars
Symbionts Living on or in Intimate Contact With Host	Ant-plant mutualisms	Symbionts: Ant colonies	Ants provide defense against herbivores, nutrients from colony wastes, protection
		Host: Plant (e.g., acacia trees)	Plant host provides nutrition (nectar, food bodies) and shelter (hollow thorns)
	Plant-plant	Symbionts: Parasitic plant (mistletoes, Rafflesia)	Gains photosynthetic sugars, water, and other nutrition from host
		Host: Plant	Harmed only
	Mycorrhizae	Symbionts: Fungus (ectomycorrhizae include many basidiomycete fungi; endomycorrhizae include many ascomycete fungi)	Absorption of nutrients and water from soil; transfer of photosynthetic sugars among different plant species
		Host: Plant (many partners)	Plant host provides photosynthetic sugars

by cyanobacteria in cycads) to plant hosts. Other novel capabilities in marine animal symbioses include light production (luminous bacteria in marine fishes and invertebrates) and chemosynthesis by sulfur-reducing bacteria in hydrothermal vent host animals. In exchange for these services, the host provides the symbiont shelter and/or nutrition. These exchanges allow symbiotic relationships to thrive in marginal environments where resources such as energy and nutrients are limiting. Lichens are able to colonize bare rocks because the fungus provides shelter and protection against desiccation, its algal partners provide nutritional energy through photosynthesis, and its cyanobacteria provide nitrogen to the algae and to the fungal host. In the coral reef ecosystem, symbiotic algae called zooxanthellae photosynthesize and provide energy-rich sugars to their host corals. In nutrient-poor and sunlit tropical seawater, this symbiosis forms the base of the food web and supports the high diversity of all coral reef organisms. Other examples of mutualistic symbioses include the relationship between fungi and the roots of higher plants. Mycorrhizae (the fungal symbionts) associate with roots of higher plants and increase the water and nutrient uptake capabilities of plants. In return, they receive photosynthetic products from their host plants. In 1997, Suzanne Simard and her colleagues found that mycorrhizae connect and transport photosynthetic products between plants and trees in different environments. Other symbionts such as parasitic orchids take advantage of this association by connecting to the fungal network and withdrawing nutrients for their own use.

In parasitic symbioses, the parasite must avoid host defenses and obtain nutrients while remaining in or on the host. In so doing, the parasite often loses the ability to live independently. Plants such as dwarf mistletoes and the largest flower in the world, Rafflesia, have lost the ability to photosynthesize; they must derive nourishment from their photosynthetic host plants. These are considered to be obligate symbioses (one or more partners is dependenton another and cannot survive alone). Often these relationships include more than one partner, each with a different role in the symbiosis. For example, insect aphids specialized to suck plant juices from their hosts must rely on intracellular bacteria for essential amino acids not available in plant tissues.

The few examples of commensalistic symbioses are behavioral; one partner taking advantage of the activities of another to obtain food. It is unlikely that the unaffected partner is truly unaffected. If it is really unaffected, commensalism is difficult to define in the context of symbiosis. The definition of symbiosis usually assumes that an interaction is taking place, which means both partners must participate.

Flexibility in the type and amount of nutritional exchanges and in the roles of partners enables the symbiotic relationship to adapt and evolve over time to meet the different needs of the partners. A symbiont, host, or both may lose the ability to live independently because the partner has irrevocably assumed certain critical life functions. This concept is fundamental to the endosymbiotic theory of the origin of eukaryotic cells. Chloroplasts and mitochondria are the remnants of former symbionts that provided novel metabolic functions (photosynthesis and respiration) to their host cells.

Finally, symbiosis plays an important and often overlooked role in ecology. Nitrogen-fixing bacteria and mycorrhizal fungi provide nutrients to primary producers, and symbiotic associations like lichens are usually the first colonizers. Feeding interactions among symbiotic partners may increase the energy efficiency of food chains and promote nutrient recycling. When one thinks about saving species and biodiversity, the emphasis should be placed on understanding and preserving symbiotic relationships. If one partner is lost, all dependent partners will perish. It is rare that any species lives in isolation.

Coevolution; Endosymbiosis; Interactions, Plant-Fungal; Interactions, Plant-Insect; Interactions, Plant-Plant; Interactions, Plant-Vertebrate; Lichen; Mycorrhizae; Parasitic Plants.

Douglas, A. E. Symbiotic Interactions. Oxford, England: Oxford University Press, 1994.

Margulis, Lynn. Symbiosis in Cell Evolution, 2nd ed. San Francisco: Freeman, 1993.

——, and Rene Fester, eds. Symbiosis as a Source of Evolutionary Novelty. Cambridge, MA: M.I.T. Press, 1991.

Paracer, Surinder, and Vernon Ahmadjian. Symbiosis: An Introduction to Biological Associations, 2nd ed. Oxford: Oxford University Press, 2000.

Simard, S. W., D. A. Perry, M. D. Jones, D. D. Myrold, D. M. Durall, and R. Molina. "Net Transfer of Carbon Between Ectomycorrhizal Tree Species in the Field." Nature 388 (1997): 579-82.

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