Symbiosis is a close ecological relationship or association between the individuals of two (or more than two) different species. In symbiosis, at least one member of the pair benefits from the relationship. The other member may be injured (parasitism, relatively unaffected (commensalism), may also benefit (mutualism). In other words, at least one member of the partner gets symbiotic nutrition.
Bacterial endosymbiosis has a recurring importance in the evolution of insects. Approximately 10-20% species of insects depend on bacterial associates for their nutrition and reproductive viability. In nutritional symbiosis, both mutuals contribute to each other organic nutrients, inorganic minerals, or digestive enzymes. Some important examples of nutritional mutualism are: nitrogen fixation, Mycorrhiza (fungus and root association), Syntrophy (mutual production of biochemical substances and nutrients), and Lichen.
Several types of symbiotic nutrition are known, the well studied example of symbiotic nutrition is nitrogen-fixing associations (between various species of bacteria and leguminous plants). Some of the more important associations are listed below. In symbiotic associations the plants is identified as the host and the microbial partner is known as the microsymbiont. The most common form of symbiotic association result in the formation of enlarged, multicellular structures, called nodules, on the root (or occasionally the stem) of the host plant. In the case of legumes, the microsymbiont is bacterium of one of three genera’s: Rhizobium, Bradyrhizobium, or Azorhizobium. Collectively, these organisms are referred to as Rhizobia. Curiously, only one nonleguminous genus, Parasponia (of the family Ulmaceae) is known to form root nodules with a Rhizobia symbiont.
A limited number of non-nodule-forming associations have been studied, such as that between Azolla and the cyanobacteria Anabaena. Azolla is a small aquatic fern that harbors Anabaena in pockets within its leaves. In Southeast Asia, Azolla has proven useful as green manure in the rice paddy fields where it is either applied as manure or co-cultivated along with the rice plants. Because more than 75% of the rice acreage consists of flooded fields, free living cyanobacteria and anaerobic bacteria may also make a significant contribution. These practices have allowed Asian rice farmers to maintain high productivity for centuries without resorting to added chemical fertilizers.
Lichen is a common example of nutritional symbiosis. They are symbiotic organisms made up by the association of green algae or cyanobacteria and filamentous fungi. They co-exist in an obligate and intimate, but ectosymbiotic, association. The body of lichen is mainly made of a close network of fungal mycelium and is responsible for absorption of nutrients, water, minerals, and gases. Phycobiont, which comprise about 10 percent of the thallus, are set in in the compact mass of mycelium and are responsible for nitrogen fixation, production of food, and photosynthesis for both partners.