This is a paper prepared for The Department of Zoology at NUIG as part of my undergraduate work. Please feel free to cite it in your work, please include the website name.
Bioluminescence Symbioses, Mechanisms and Significance John Paul Tiernan
Bioluminescence, which is the natural production of light by organisms, occurs in animals, fungi, protests and certain bacteria. Light is produced when the compound luciferin is oxidised. Larger organisms such as squid contain light organs where light production takes place, in smaller organisms such as bacteria, the whole organism luminesces (Hastings 1978).
Over 90% of species which inhabit depths greater than 700m, are thought to possess the ability to bioluminescence (Hastings et al. 1985). Due to its diversity and phylogenetic distribution, it is thought that bioluminescence may have evolved independently up to 30 times over the course of evolution (Hastings 1985). Rees et al. (1998) proposes that the luminescent substrates (luciferins) of the luminous reactions are the evolutionary core of most systems. Rees et al. (1985) also suggests that oxygen detoxifying mechanisms provide excellent foundations for the emergence of many bioluminescent systems.
In most species, luminescence is produced by the animal, however in a minority, which will be the subject of this essay; the light is produced by bacteria which function as symbionts of the animal within its light organs (Hastings et al. 1985). Bioluminescent symbiotic associations with luminous bacteria have been identified in species of teleost fish and species of loliginid and sepiolid squids (Dunlap and Kita-Tsukamoto 2006). The occurrence of bioluminescent symbiosis in pyrosomes and salps is still an unresolved issue (Dunlap and Kita-Tsukamoto 2006).
The host animals utilise the luminescence for a variety of reasons. These include: predator avoidance (through counter-illumination and flashing), mating related signalling, attracting and locating prey and navigation in dark or low-light environments (Dunlap and Kita-Tsukamoto 2006). Claes and Dunlap (2006) noted that the bioluminescent symbiosis between luminous bacteria and their hosts is a unique type of symbiosis in that the metabolic benefit for the host is light for display purposes as opposed to a nutrient for growth purposes.
Of the luminous bacteria, four species have been identified in bioluminescent symbiotic associations. They are Vibrio fischeri, V. logei, Photobacterium leiognathi and P. phosphoreum (Dunlap and Kita-Tsukamoto 2006). Genetic sequencing of bacteria involved in bioluminescent symbioses of ceratioid fish suggests that these fish contain as yet unidentified species of luminous bacteria (Haygood and Distel 1993). Luminous bacteria in the marine environment have been shown to be extremely resourceful when adapting to a varied selection of niches, including acting as saprophytes on faecal material, and pathogenic associations with marine mammals (Ruby and Lee 1998).
In this essay, I will first examine literature concerning the various mechanisms of bioluminescent symbiotic associations in both squid and fish before examining the significance of such associations.
Bioluminescent squid species contain at least three species of luminous bacteria in symbiotic associations. Sepiolid squids contain Vibrio fischeri and V. logei while Loliginid squids contain Photobacterium leiognathi (McFall-Ngai 1999). In squids, the light organs where the symbiotic associations occur are bilobed organs found ventrally within the mantle cavity, adjacent to the ink sac (Dunlap and Kita-Tsukamoto 2006).
Squids acquire their symbiotic luminous bacteria by horizontal transfer from the surrounding seawater. Based on studies on Euprymna scolopes, Wei and Young (1989) suggested that each generation of squid acquire infection from free living bacteria rather than from the egg. Most hatchlings produce light within 24 h. The bacteria must be motile to complete this transfer as a barrier of mucus and/or a current generated by cilia movement is present entering the light organ (McFall-Ngai 1999). Studies also show that the light organ habitat in E. scolopes is one of such specifity that only V. fischeri can survive (Mc-Fall-Nagi 1999). However, studies by Fidopiastis et al. (1998) revealed two separate species of luminous bacteria (V. fischeri and V. logei) co-occurring within the light organs of Sepiola spp. Temperature may be a factor influencing this, with warmer temperatures favouring V. fischeri and colder temperatures favouring V. logei (Fidopiastis et al. 1998).
Squid release their symbiotic bacteria into the surrounding water; in the case of E. scolopes, every 24 h resulting in locations which contain hosts becoming extremely enriched with the bacteria (Lee and Ruby 1994). Bacteria populations are thought to double at least once and possibly more every 24 h (Dunlap 1984). Initiation of future associations may be dependant on this exudation of bacteria (Ruby and Lee 1998). In E. scolopes, this generally takes place at dawn or first illumination (Lee and Ruby 1994). These light organ dynamics are a micro-ecology, functioning possibly to continually select more composite strains of symbiotic bacteria (Ruby and Lee 1998).
Symbiosis appears to be the normal state of E scolopes, starting immediately after hatching. However, it is not tested whether the absence of a light organ would affect its survival rate (Ruby and Lee 1998).
Another aspect of the squid – bacteria symbiotic association is the morphogenesis of the light organ due to the presence of certain bacteria. Studies by Doino and McFall-Ngai (1995) on E. scolopes and V. fischei showed that when the squid was exposed to the symbiotic bacteria for 12h, tissue regression in the light organ took place over the next few days. This morphogenesis consists of lateral ciliated epithelial appendages coalescing in the light organ into a ciliated duct.
Bioluminescent fish species contain at least three identified species of luminous bacteria in symbiotic associations; Vibrio fischeri, Photobacterium leiognathi and P. phosphoreum. The symbiotic bacteria are extra cellular and inhabit tubules and canals of the fish’s light organ (Hastings 1976). The fish keep a high density of bacteria present in their light organ, a factor which leads to continuous illumination of the organ. This is due to the synthesis of luciferase in fish necessitating a specific concentration of auto-inducer emission by the bacteria (Hastings 1978).
The light organ in fish may be located in various places (Dunlap and Kita-Tsukamoto 2006). Development of the light organ in some fish (Monocentris japonicus) may require first the acquiring of symbionts (V. fischeri) (Dunlap and Kita-Tsukamoto 2006).
As with squid, fish also acquire their luminescent symbionts through horizontal transfer from the surrounding seawater (Dunlap and Kita-Tsukamoto 2006). Studies by Wada et al. (1999) of Leiognathus nuchalis demonstrated that the fish’s offspring are without symbiotic bacteria (aposymbiotic) at hatching and require the presence of adult fish to gain symbionts, with offspring being kept apart from adult fish failing to acquire luminescence. This is due to the exudation of bacteria into the surrounding seawater from the adults. Luminescence occurs in offspring kept with adults mostly within 48 h of hatching (Wada et al. 1999). It is also necessary that the symbiotic bacteria are motile for this to occur.
Specificity of the P. leiognathi / Leiognathid fish symbiosis is maintained at the species level of the bacteria as opposed to the individual level (Dunlap et al. 2004). Symbionts in any one host are a homogenous culture and completely distinctive to another individual host of the same species (Hastings and Nealson 1981). Studies by Nealson et al. (1984) on the luminous fish Cleidopus gloriamaris and Photoblepharon palpebratus showed that the fish also exude luminous bacteria. Monocentrid fish, such as M. japonicus release bacteria directly into the seawater, whereas Leiognathid fish such as L. nuchalis release via the gut tract into the seawater (Dunlap and Kita Tsukamoto 2006).
Studies on bioluminescent symbiotic associations between P. fischeri and M. japonica have shown bacteria to produce greater luminescence under conditions of low oxygen and low growth (Ruby and Nealson 1976). Studies by Dunlap (1984) on P. leiognathi in species of Leiognathid fish compared bacteria in situ (light organ) and bacteria cultured in vivo. This showed major differences such as lack of motility and flagella in situ and 20 to 30 times slower growth in situ, which reiterates the minimal growth, maximum luminescence model.
As with squid, there may be a system of controlling the density of bacteria in Leiognathid fish in order to maximise luminescence. This is also thought to be through the autoinducer mechanism (Dunlap 1984). It is proposed that P. phosphoreum and V. fischeri metabolise glucose to pyruvate which is used in respiration by mitochondria which decreases oxygen and increases luminescence (Nealson 1979). Controlling the amount of carbon reaching bacteria forces the bacteria to use the luminescence system as an alternative respiratory pathway (to re-oxidise reduced co-enzymes) (Dunlap 1984).
Significance of Bioluminescence Symbioses.
Bioluminescence symbioses are highly significant examples of symbioses in the marine environment mainly due to their ecological processes. Bioluminescence is a widespread trait in the marine environment, particularly at depth, however in the vast majority of organisms, this luminescence is self produced. One significant aspect of bioluminescent symbioses is the small minority of species it occurs in (Dunlap and Kita Tsukamoto 2006).
Perhaps the most significant aspect of bioluminescent symbioses is its contrast with other symbioses within the marine environment with regard to its functions. In most bacteria-animal symbiotic associations, the host animal benefits from the association through the acquiring of nutrients (e.g. through nitrogen fixation by the bacteria) which are used for growth purposes by the host (Claes and Dunlop 2000). However, in bioluminescence symbioses, the host benefits through the acquisition of light, which is used for a variety of purposes, not directly linked with growth. In squid, these may be to camouflage or to deter other organisms through fright. In fish, these may be to attract predators, become invisible and to communicate (Smith and Douglas 1987). The fish/squid in turn provide the bacteria with nutrients, oxygen and shelter within the light organ (Harvey 1952). In fact, benefits to symbiotic luminous bacteria still remain unclear (Stabb 2005).
Another significant ecological aspect of bioluminescent symbiotic associations is the extent of influence the host animal has on the bacterial symbionts, which is rather great. In studies on bioluminescent symbioses in squid, Lee and Ruby (1994) noted that the abundance and distribution of luminous marine bacteria is driven primarily by its symbiotic association with the animal host. The host animals exert an influence on the bacteria in a number of ways. The low growth-high luminescence model, mentioned earlier represents a very significant control on the bacteria as the host controls the supply of oxygen and/or nutrients to the bacteria (Hastings and Nealson 1977, Nealson 1979). Also, the exudation of bacteria from the light organs of squid and fish as described by several authors (Dunlap 1984, Nealson et al. 1984, Lee and Ruby 1994) strongly controls the size, density and distribution of the bacteria populations through wide dispersal. Tight crowding leads to a lack of motility and flagella in bacteria within the light organ compared to bacteria in culture (Dunlap 1984).
Of perhaps less significance is the influence bacteria exert on their hosts. Claes and Dunlap (2000) found that the squid E. scolopes survived just as well without its luminous symbionts as animals with theirs. This is in contrast to most other bacterial symbiotic associations in the marine environment, where the bacteria would have a much greater influence on the host as the host would be nutritionally dependant on the bacteria (Dunlap and Kita-Tsukamoto 2006). The bacteria do however in some cases contribute to the development of the light organ through morphogenesis as Yamada et al. (1979) proposes that light organ development in monocentrid fish requires the presence of bacteria while Claes and Dunlap (2000) report that development of the light organ in squid is not reliant on symbionts being present.
The bioluminescent symbiotic associations, and their mechanisms, which have been examined in this essay are very unique and dynamic examples of symbioses in the marine environment. The chemical and metabolic mechanisms are unique in that the production of luminescence (luciferin) may be brought about by the bacteria being forced to use the luminescent system to respire to reoxidise reduced co-enzymes in the absence of oxygen (Hastings and Nealson 1977). The physical mechanisms are also unique in that the hosts appear to control several aspects of the bacteria’s life. The greatest significance of these mechanisms is their ecological uniqueness in the world of bacterial symbiotic associations.
Future research concentrating on these ecological processes would be beneficial, in particular the investigation of what advantages the bacteria gain from these associations as this seems to be one of the last remaining pieces in the puzzle when constructing an understanding of bioluminescence symbioses.
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