Symbiotic bacteria

Symbiosis was first defined by Marko de Bary in 1869 in a work entitled "Die Erscheinung der Symbiose"[1] in which he defined the term as "namely, the living together of parasite and host". The definition of symbiosis has evolved to encompass a sustained relationship between two or more different organisms[2] "over a considerable fraction of the life of the host."[3]

Associated with the term "symbiosis" are terms: mutualism, commensalism, parasitism, and amensalism.[4] This may define or limit the type of "living together" of two organisms, be they plant, animal, protist or bacteria they practice.

Some types of cyanobacteria are endosymbiont. The theory of endosymbiosis was first published in 1970 by Lynn Margulis in a book titled "Origin of eukaryotic cells"[5] in which she claimed that eukaryotic cells came to exist by a series of symbiotic engulfment.

Certain plants establish a symbiotic relationship with bacteria, enabling them to produce nodules that facilitate the conversion of atmospheric nitrogen to ammonia. In this connection, cytokinins have been found to play a role in the development of root fixing nodules.[6] It appears that not only must the plant have a need for nitrogen fixing bacteria, but they must also be able to synthesize cytokinins which promote the production of root nodules, required for nitrogen fixation.

Symbiotic bacteria are able to live in or on plant or animal tissue. In digestive systems, symbiotic bacteria help break down foods that contain fiber. They also help produce vitamins. Symbiotic bacteria can live near hydrothermal vents. They usually have a mutual relationship with other bacteria. Some live in tube worms.

There are two major modes of transmission for symbiotic bacteria. The first is horizontal transmission in which microbes are acquired from the environment and either the environment or the host population serves as the inoculum for the symbiosis.[7] An example of horizontal transmission is the deep sea tube worm and its symbiont.[7] The second type of transmission is vertical transmission in which the symbiont is passed down from the parent to the offspring and there is no aposymbiotic phase.[7] An example of vertical transmission is seen in Drosophila melanogaster and its Wolbachia spp. symbionts.[7]

Corals have been found to form characteristic associations with symbiotic nitrogen-fixing bacteria.[8]] Corals have evolved in oligotrophic waters which are typically poor in nitrogen. Corals must therefore form a mutualistic relationship with nitrogen fixing organism, in this case the subject of this study, namely Symbiodinium. In addition to this dinoflagellate, coral also form relationships with bacteria, archae and fungi.[9] The problem is that these dinoflagellates are also nitrogen limited and must form a symbiotic relationship with another organism; here it is suggested to be diazotrophs. In addition, cyanobacteria have been found to possess genes that enable them to undergo nitrogen fixation.[8] This particular study goes further to investigate the possibility that in addition to the named dinoflagellate and certain cyanobacteria, endosymbiotic algae and the coral contain enzymes enabling them to both undergo ammonium assimilation.

Due to the small size of the genome of most endosymbionts, they are unable to exist for any length of time outside of the host cell, thereby preventing a long-term symbiotic relationship. However, in the case of the endonuclear symbiotic bacterium Holospora, it has been discovered[10] that Holospora species can maintain their infectivity for a limited time and form a symbiotic relationship with Paramecium species.

It is well accepted and understood that there is a mutualistic relationship between plants and rhizobial bacteria and mycorrhizal fungi enabling the plants to survive in an otherwise nitrogen-poor soil environment. Co-evolution is described as a situation where two organisms evolve in response to one another. In a study reported in Functional Ecology,[11] these scientists investigated whether such a mutualistic relationship conferred an evolutionary advantage to either plant or symbiont. They did not find that the rhizobial bacteria studied had any evolutionary advantage with their host but did find great genetic variation among the populations of rhizobial bacteria studied.

Organisms typically establish a symbiotic relationship due to their limited availability of resources in their habitat or due to a limitation of their food source. Triatomine vectors have only one host and therefore must establish a relationship with bacteria to enable them to obtain the nutrients required to maintain themselves.[12]

A use for symbiotic bacteria is in paratransgenesis for controlling important vectors for disease, such as the transmission of Chagas disease by Triatome kissing bugs. Symbiotic bacteria in legume roots provide the plants with ammonia in exchange for the plants' carbon and a protected home.

Symbiotic, chemosynthetic bacteria that have been discovered associated with mussels (Bathymodiolus) located near hydrothermal vents have a gene that enables them to utilize hydrogen as a source of energy, in preference to sulphur or methane as their energy source for production of energy.[4]