Photobacterium phosphoreum

A Microbial Biorealm page on the genus Photobacterium phosphoreum

Classification

Higher order taxa

No rank: Cellular Organisms

Superkingdom: Bacteria

Phylum: Proteobacteria

Class: Gammaproteobacteria

Order: Vibrionales

Family: Vibrionaceae

Genus: Photobacterium

Species: Photobacterium phosphoreum

Species

NCBI: Taxonomy

Proteobacterium phosphoreum

Description and significance

Photobacterium phosphoreum was first isolated from the aquatic environment in the late 1880’s by the Dutch microbiologist Martinus Beijerinck (1851-1931).[14] It is a Gammaproteobacteria which are Gram-negative, usually motile rods, are mesophilic and chemoorganotrophic, have falcultative fermentative metabolism and are found in aquatic habitats in association with eukaryotes.[14]

Photobacterium phosphoreum is one of many organisms that produce bioluminescence in marine organisms. P. phosphoreum is a light organ symbiont, living in the gut of the fish where metabolites are provided in exchange for bioluminescence, which is used for communication, prey attraction, and predator avoidance.[14] P. phosphoreum typical of deep sea fishes, which can also be found in dim and dark versions.[7] The gut of the fish, where P. phosphoreum is cultivated, is connected to some light organ on the fish which can be controlled with shutter apparatuses.[7] P. phosphoreum is psychrotolerant and often thrives in low temperatures but can be inhibited at temperatures above 25 degrees Celcius.[13, 15] Bioluminescence has been strongly linked to cell density, and bacteria living freely in the ocean are not bioluminescent as they are in the light organs of the host organism.[18]

P. phosphoreum also acts as the most important spoilage bacterium of packed chilled fish fillets. It enhances growth in packed fish products and causes the spoiled, fishy flavor.[13]

Genome structure

The gene products of luxA, α-luciferase, are necessary for the light-emitting reaction of all known luminous bacteria.[1] Unlike the Vibrio lux system, P. phosphoreum has a new gene, luxF which is located in the lux operon between luxB and luxE. The functions of the luxA to E gene products are known to be involved in the luminescent pathway, luxF likely has the same function.[17] The organization of the lux operon in P. phosphoreum is as follows: luxC luxD luxA luxB luxF luxE.[17]

In P. phosphoreum, luxA and luxB gene products are luciferase subunits and were shown to catalyze light emission in the presence of FMNH2, O2, and aldehyde. The luxC, luxD, and luxE gene products are fatty acid reductase subunits and are responsible for aldehyde biosynthesis. The new gene, luxF, was found to code for a new polypeptide of 26kDa.[16]

Cell structure and metabolism

Photobacterium phosphoreum is Gram-negative, motile rods, and is psychrophilic and chemoorganotrophic, can grow in anaerobic conditions, and emits a blue-green light.[14] Bioluminescence of Photobacterium phosphoreum is caused by an oxidation reaction. Reduced flavin mononucleotide, FMNH2, and long chain fatty acids are oxideized by molecular oxygen to flavin mononucleotide FMN and a corresponding fatty acid.[15, 17] The catalyst of this reaction is the enzyme luciferase, whose structure is heterodimeric and has molecular weights of 76,000 ± 4,000.[15] The light reaction:[10, 18]

FMNH2 + RCHO + O2 --> FMN + RCOOH + H2O + light (490-495 nm)

P. phosphoreum in laboratory settings has shown that luciferase synthesis is constitutive.[15] Luminescence has, however, been shown to be maximal in deeper waters where oxygen concentrations are lower and growth is limited. At these depths, since luciferase synthesis does not halt, the cells have higher luciferase concentration and thus display more intense luminescence.[15]

P. phosphoreum is biochemically advantageous in growth-limiting low oxygen concentrations with nonfermentable carbon sources. In these conditions, luminous bacteria use luciferase to transfer electrons, where other organisms have lost their ability to do so. By using luciferase, P. phosphoreum is able to reoxidize reduced coenzymes and other molecules for metabolism, giving it a major metabolic advantage.[8, 15]

P. phosphoreum has also been known to spoil packed, chilled salmon by using trimethylamine oxide (TMAO) as a terminal electron acceptor, facilitating its growth in packed fish products. Then, P. phosphoreum reduces TMAO to trimethylamine (TMA) which give the product its distinctive spoiled, fishy flavor.[13]

Ecology

Photobacterium phosphoreum is a common symbiont of deep sea fishes, unlike its Vibrio relatives that are commonly found in squids in shallow and medium depths.[13] There has been evidence that P. phosphoreum also has a close partnership with zooplankton, as it has been found attached to the external surfaces of zooplankton.[14] Bological benefits are gained by both members in the symbiotic relationship between bacteria and host. P. phosphoreum colonizes the light organs of the hose and plays a role through the emission of light in communication, prey attraction and predator avoidance.[14] The host has the ability to control the bioluminescence by using mechanisms such as shutters. In return, the host provides P. phosphoreum with nutrients, oxygen and a protected niche.[15]

It is believed that the spoilage of packed fish, such as salmon and cod, caused by P. phosphoreum is in part due to the bacterium’s ability to produce AHL’s (N-acylated homoserine lactones), which are communication molecules regulating bioluminescence. AHL’s were only found in nonbioluminescent strains of P. phosphoreum. This suggests that AHL may negatively regulate bioluminescence.[13] P. phosphoreum is able to remain viable on the external surfaces of migrating salmon by living under the protection of the fish’s slime.[1] The reduction of TMAO to TMA also contributes to the spoiled result of the fish’s exposure to P. phosphoreum.[13]

Pathology

Photobacterium phosphoreum has no known pathogenic activity. It is not known to cause disease in humans, plants, or animals.

Application to Biotechnology

Does this organism produce any useful compounds or enzymes? What are they and how are they used?

Current Research

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References

[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.

Edited by student of Rachel Larsen