Since ancient times mariners have reported seeing glowing seas as their ships sailed through the night. More recently, satellites have recorded pictures of glowing seas off the eastern coast of Africa. The bioluminescent microbes responsible for such phenomena require specific conditions and high concentrations to achieve this effect, but it is similar to the algae blooms that cause red tide. There are many varieties of bacterium that can emit light, and some of them have developed symbiotic relationships with animals. Photobacterium leiognathi is one such species.
This luminescent marine bacterium lives in warm tropical waters. Unlike other species, that can multiply in colder, deeper waters P. leiognathi grows at an optimal temperature of about 30 degrees Celsius. It requires ionic sodium for growth and is best cultured on a 1.5% NaCl medium. As a facultative anaerobe it can perform both fermentative and respiratory metabolic functions. However, the microbe demonstrates less luminescence when exposed to oxygen and/or low salt concentrations. It is a gram-negative, coccobacillus 1.6 by 3.2 micrometers in size. Motility is furnished by one to three unsheathed polar flagella. Glucose, mannose, fructose, or glycerol can be used as the sole source of carbon, making the bacterium a chemoorganotroph. In order for the microbe’s bioluminescent genes to function, there must be a large concentrated population present. The regulation of the lux genes, those that code for the light emitting proteins, requires intercellular communication. As molecules termed autoinducers become concentrated in solution, they activate the lux genes. If the cultures remain undisturbed the cells will continue to glow.
The benefits of bioluminescence to the bacterium may simply be related to the attraction of other organisms and their predators. This introduces more nutrients into the environment and ensures the propagation of the species. These bacteria and others like it have also found a niche for themselves in other organisms. Although this is a common role for innumerable bacterial species, their light sets them above less flashy microbes. Several species of fish have evolved specialized compartments in which the bacteria can live. The bacteria are fed and in turn the fish can use the light for their own specialized purposes. P. leiognathi has one such host, the pony fish. These fish have adapted internal light organs that transmit the bacteria’s light to photophores on the outside of the fish’s body. The light organ of the pony fish is connected to their gut. This allows for the rejuvenation of the bacterial culture, which lives in nearly pure culture. Shutters in the fish’s photophores allow it to turn its lights off when necessary. The function of this light varies between species. Most pony fish exhibit sexual dimorphism, and males use the lights for signaling during mating. Other species produce counterilluminescence that makes them indistinguishable to predators lurking in the depths. Whatever the specific usage, P. leiognathi has literally made a place for itself in the hearts of many animals.
References
Balows, A., et al. ed. The Prokaryotes. 2nd ed. Vol. 2. Springer-Verlag New York Inc. 1992. 4 vols.
Brenner, D. J., et al. ed. Bergey’s Manual of Systematic Bacteriology. 2nd ed. Vol. 2 Part B. Bergey’s Manual Trust. New York, NY. 2005.
Eddleman, H. Isolation of Pure Cultures Of Bacteria. 1999. Indiana Biolab. 5 March 2006 <http://www.disknet.com/indiana_biolab/b203.htm>
Milky Seas from Space. 5 March 2006. <http://www.lifesci.ucsb.edu/~biolum/organism/milkysea.html>
Herring, P. J. & Widder, E. A. Bioluminescence in Plankton and Nekton. 2001. 5 March 2006. <http://www.isbc.unibo.it/Files/BC_PlanktonNekton.htm>
*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.
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