Higher order taxa
Bacteria, Proteobacteria, Gammaproteobacteria, Vibrionales, Vibrionaceae, Photobacterium
Photobacterium is a genus of faculatively aerobic gammaproteobacteria. These gram-negative bacteria are found in various niches, such as the guts, surface, or light organs of fish, free-living throughout the marine water column, and inside decaying animal tissue. The distribution and habitat of these bacteria depends on the species. Aliivibrio and Vibrio are closely related genera to Photobacterium based on their genetic composition (Fig. 1). 
Throughout the course of evolution, bioluminescence has evolved separately among many origins and therefore, different mechanisms are utilized by different species to express the luminescent gene, luciferin. [2, 11] Some species of photobacteria are capable of bioluminescence using a similar lux-based quorum sensing mechanism as in Allivibrio. The operon is activated by the release of an autoinducer in high concentrations (Figure 2). 
Many species of marine bacteria use these luminescent bacteria for survival in the marine environment. Marine animals may use their light organ to lure prey or aid in visibility of finding prey.  They can also use this as a defense against predation by counterillumination, which camoflauges the organisms or by distracting predators via the release of bioluminescent bacteria into the water column. [9, 11]
Photobacterium leiognathi This species prefers the warm coastal waters and form symbiotic relationships in shallow-dwelling fish. They thrive at temperatures around 30 degrees C, but have the ability to grow at temperatures below 25 degrees C. .P. leiognathi can form a symbiotic relationship with leiognathi fish, Nuchequula nuchalis, a coastal water fish.  The fish have a specialized light organ that allows P. leiognathi to inhabit it via multiple chemical and physical reactions. This is initiated in the early stages of development for the fish.  Both organisms benefit from this symbiotic relationship. P. leiognathi receives shelter and an environment rich in nutrients that allows them to proliferate. Leiognathi fish benefit by using this bioluminescent light to lure and attract prey.
Photobacterium phosphoreum P. phosphoreum thrive in cold deep-water environments (5-25 degrees C).  These bacteria are free-living in the environment and are acquired in the light organ or gut of some fish. The host fish provides an environment for these bacteria to grow, reproduce, and avoid predation. The endosymbiotic bacteria emit light to aid the fish in the attraction of its prey, communication with other organisms, and predation avoidance.
Photobacterium profundum These species of photobacteria are both psychrophiles and piezophiles, meaning they have adapted the abililty to grow and reproduce in cold temperatures and high pressures, respectively. The enzymes in the SS9 strain of P. profundum are suitable for temperatures ranging from 4-15 degrees C and can survive at pressures up to 100MPa.  These adaptations are due to the fatty acid chains in the membranes of the cells. The abundance and types of fatty acids change the way the bacterium can adapt to the extreme range in temperature and pressure.  In order to adjust the membrane fluidity at low temperatures and high pressures, the bacteria can increase their abundance of monounsaturated and polyunsaturated fatty acids.  Due to these high-pressure, low-temperature environments, P. profundum must overcome the increased difficulty of motility. These bacteria have two specialized flagellum and are made up of 40 different genes encoding for the motility. More research needs to be conducted to visibly understand how their flagellum are used. 
Photobacterium damselae Originally identified as Pasteurella piscida, is a halophilic bacterium that acts as a pathogen to many marine fish and mammals. The species name was changed around 1995 based on the genetic sequencing of the 16S rRNA that identified it as more closely related to the photobacterium genus.  Pasteurellosis is a disease first discovered to infect species of white perch and has since been found in other species such as damselfish, sharks, molluscs, crustaceans, dolphins, sea turtles, and even humans. P. damselae spreads through the water column and favors high salinities. Infected fish have symptoms of lethargy, mucus production, hemorrhagic and enlarged livers, hemorrhages in abdominal cavities, increased respiratory frequencies, and ascetic liquid.  This pathogen could have very negative impacts on fish farms around the world along with human health. 
Humans can be infected with the pathogen by eating infected fish or swimming in brackish water in which the photobacterium can travel up the urinary tract of a person. Humans can experience organ failure, necrotizing fasciitis, and even death. Humans can survive between 24-72 hours with the pathogen. Antibiotics, chemotherapy, and radiation have been used to attempt to treat the infection, but have failed. The best recommendation is to amputate the infected area of the body before the pathogen spreads. 
P. phosphoreum has recently been found in packaged frozen marine fish (cod, halibut, salmon, shrimp, etc.).  This species of bacteria is the “specific spoilage organism (SSO)” responsible for growing and spoiling the packaged fish. Spoilage of fish may or may not have a direct harmful impact on human health, however, eating spoiled fish may give rise to pathogens that are toxic to human health. P. phosphoreum was discovered in the frozen fish using the amplified fragment length polymorphism (AFLP) taken from the DNA fingerprints of various photobacterium species.  These bacteria are capable of surviving in the freezing temperatures of the packaged fish.] This discovery may apply to important health factors in the food industry in determining other potentially harmful bacteria strains.
Recent studies have investigated the benefits of nonsymbiotic bacteria in the marine environment. In deeper dwelling areas of the marine environment, light is limited. Bioluminescent bacteria are attractive to predators because it is easier for the predator to find them using visual cues rather than using chemical cues. Zarubin et al. studied the use of P. leiognathi and their symbiotic relationships with other marine organisms in their research, “Bacterial bioluminescence as a lure for marine zooplankton and fish” (2011). Photobacterium leiognathi is a marine bacterium that uses their bioluminescence as bait to attract zooplankton in order to hitchhike their way up the food chain. This is an adaptation that the photobacterium use to reach their preferred environment, the gut of a fish, where there is an abundance of nutrients and the bacteria can then proliferate. Brine shrimp (Artemia salina) consume the bioluminescent bacteria and began to glow after reaching the quorum-sensing threshold.  Bacteria can then glow consistently in the presence of oxygen. Fish then became attracted to the luminescent shrimp and consumed them. The bacteria are able to survive both the digestive system of the zooplankton as well as the digestive system of the fish. Luminescent fecal pellets from the shrimp and the fish indicated that the bacteria had survived. The ultimate goal of P. leiognathi is to reach the gut of the fish where it is rich in nutrients and also provides a shelter for the bacteria. They can then reproduce and disperse back into the environment via feces. This “hitchhiking” technique is effective for the bacteria to proliferate and disperse among long distances in the ocean. 
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