**1. Introduction**

'Bioluminescence' refers to the phenomenon of chemically induced emission of light (or other electromagnetic radiations) by a living organism. It is a common occurrence frequently observed in various organisms, ranging from simple ones like bacteria to complex animals like deep-sea fish or fireflies, and even some fungi. The first accounts of bioluminescence are found in the works of Dioscorides and even Pliny the Elder, who believed that certain bioluminescent organisms had medicinal properties [1]. There are accounts of coal-miners using dried fish skins, and even bottled fireflies as safe light sources [2]. Charles Darwin also wrote about the glowing oceans in his travails. E. N. Harvey conducted extensive studies on this phenomenon, and wrote the first detailed account of all natural bioluminescent forms. In biochemical terms, the phenomenon of bioluminescence occurs due to an interaction of a substrate luciferin with an enzyme luciferase. Shimomura et al. were the first to obtain crystalline luciferin from the sea firefly *Vargula hilgendorfii* [3].

In this chapter, we explore the origins of bioluminescence in nature, its distribution, and the many ecological roles that it plays. Furthermore, the harnessing of this phenomenon for human use and the future prospects have also been discussed in brief.

## **2. The evolution of bioluminescence**

Since bioluminescence has proven to be an energy-expensive process, the evolution of bioluminescence in nature must be of some ecological or biological significance, or must offer some evolutionary advantage to the organism. This is certainly true, because there are multiple incidences of the evolution of bioluminescence, all completely independent from each other, and showing a convergent evolution pattern [4, 5]. This trait is found in multiple species spanning different phyla. Some even show symbiotic association with microbes. All these species use this phenomenon for a diverse range of applications including evasion of predators, luring prey and even attracting mates [6–8].

Since bioluminescence is so widespread in nature, scientists have been speculating the cause of its origin and selection in the first place. The first speculation was made by E. N. Harvey himself, who believed that it had something to do with respiratory chain proteins, some of which may have had fluorescent groups or side chains [9]. Owing to the extensive research that he conducted, his theory gained some attention and credibility. It was, however, soon disproved. Some even state that bioluminescence may have merely evolved as a by-product of other metabolic functions, having no importance of its own. However, the repetitive and independent origins of bioluminescence in nature must mean that this trait does confer a significant evolutionary advantage to the species that exhibits it [10].

One theory, proposed by Seliger et al. in 1993, stated that luciferases were actually a group of mixed function oxygenases [11]. According to him, bioluminescence evolved primarily as a means of intra-specific or inter-specific interaction in the dark, deep sea biome.

Rees et al. conducted an independent study on coelentrazine, which is a marine luciferin [12]. They came to the conclusion that bioluminescence may have evolved as a biochemical pathway, mainly for the disposal of peroxide, superoxide, and other harmful oxygen species produced in the course of metabolism. This may have additionally been favored by the acute absence of illumination in the dark depths of the ocean. Bioluminescence may have undergone natural selection as these species may have progressed deeper in the dark depths of the ocean, where the selective pressure for anti-oxidant defense naturally subsided.

As is clear from the above discussion, there was a unanimous agreement among many that bioluminescence may have evolved in the deep sea ecosystem. Even today, the vast depth of the ocean abounds in various species that exhibit this trait. These may range from microbes like bacteria and dinoflagellates to complex organisms like crustaceans, molluscs, jellyfish, various bony fish, and even cartilaginous fish like sharks [10].

As of today, bioluminescence has many more purposes apart from free radical disposal, like camouflage, counter-illumination, warning colouration, predation or courtship, [10] which have been discussed in further subsections.

### **3. Distribution**

As stated earlier, bioluminescence has emerged independently in nature on multiple occasions. Nearly 700 to 800 genera spanning 13 phyla, including both prokaryotic as well as eukaryotic species, have been reported to exhibit this trait [10, 13]. The evolutionary trends of bioluminescence show exemplary convergent evolution in many cases, because of the almost similar purposes this trait serves in various species, or because of the similarity in the biochemistry of the molecules involved.

#### *The Ecology of Bioluminescence DOI: http://dx.doi.org/10.5772/intechopen.96636*

Bioluminescent organisms are found in both terrestrial as well as aquatic habitats. However, the aquatic species are exclusively limited to marine ecosystems, and a freshwater bioluminescent system is yet to be reported [10].

For the sake of simplicity, the distribution of this trait has been discussed separately for bacteria, fungi and protists, and higher animals have been discussed separately.

#### **3.1 Bacterial bioluminescence**

It is a common belief that bacterial bioluminescent systems were among the first to originate in nature. Bioluminescent bacteria are present in both terrestrial as well as aquatic habitats, and can be found all over the world. In fact, these bacteria can easily be sourced from any tissue or detritus lying on beaches, or even from uncooked seafood [4]. The glowing oceans, which are a spectacular result of these microorganisms, have been described in detail in the travails of Darwin, and can be observed, or rather enjoyed at various locations all over the world.

Bioluminescent bacteria mainly belong to the class *Gammaproteobacteria*, and are confined to three genera, namely *Vibrio*, *Photobacterium* and *Xenorhabdus*. Out of these, *Vibrio* and *Photobacterium* are mostly found in marine ecosystems, whereas *Xenorhabdus* inhabits terrestrial habitats [14]. New strains of bioluminescent bacteria are still being discovered [15]. A remarkable fact about bacterial bioluminescence is that all bacterial bioluminescent systems are exactly alike in terms of biochemistry, i.e., they all rely on flavin mononucleotide (FMN), myristic aldehyde and NADH, and also oxygen [16].

Bioluminescent bacteria may exist as free-living, symbiotic or even pathogenic forms. However, a completely obligate bacterial symbiotic system is yet to be observed in nature [8]. For example, *Vibrio fischeri* has been known to colonize specialized "light organs" [17] in the fish *Monocentris japonicus* [18], and also exhibits mutualistic relationship with Hawaiian squid *Euprymnia scolopes* [10, 14], and various species from the genus *Photobacterium* have been known to exhibit symbiosis with various fish, molluscs, etc. [19] and even cause diseases in some others [8]. However, there has been no genetic alteration in the bacterial genome for the said symbiosis. Though the animals showing the said symbiosis have developed exclusive modifications like light organs, they do not show any endosymbiotic behavior. The development of the said specialized organs may even be influenced by the presence of the symbiotic bacterial population [4]. One hypothesis accounts for the emergence of bioluminescence in bacteria because it promotes such symbiotic behavior, conferring a survival advantage to the microbes [10]. The symbiotic behavior may further be promoted because of the fact that the luminescent machinery of the bacteria is instrumental in getting rid of the reactive oxygen species produced in the host tissue [20]. The symbiotic microbes are obtained externally, and the hosts show some degree of selectivity towards the symbiont [8]. It appears that the host organisms 'choose' the colonizing symbiont according to the availability as per the depth which they inhabit. Furthermore, the said hosts can even dump the symbiont cells in order to keep their population in check [20].

Terrestrial bioluminescent bacteria are rare, and are known to infect nematodes that parasitize glowworm larvae. Upon the death of the larva, predators and scavengers ingest the carcasses, hence dispersing the bacteria as well as the nematode. Other than that, bioluminescent bacteria have been observed to inhabit various depths of the ocean, and are found even in sediments, seawaters, saline lakes, etc.

#### **3.2 Fungal bioluminescence**

Of all the bioluminescent systems that have been studied, fungal bioluminescence remains by far the most poorly investigated of them all, even though fungi are the only terrestrial eukaryotes that exhibit bioluminescence, besides animals [10]. This might be owing to the fact that most initial attempts at determining the enzymatic nature of fungal bioluminescence were failures, and have only recently been confirmed successfully [21]. The study of fungal bioluminescence has thus gained sudden prominence [22], and a genetically encodable bioluminescent system for eukaryotes has been developed [23]. Kaskova et al. conducted an extensive study of the fungal bioluminescence and colour modulation mechanisms [24].

Out of all the fungal species that have been documented till date, only about 71 [25] to 80 [26] fungal species have been known to exhibit bioluminescence. All of the said species have been unequally classified into four distinct lineages that are not so closely related [23]. "Honey Mushrooms" of the Armillaria lineage, the causative species for foxfire phenomenon, and the "Jack-o-Lantern Mushrooms" from the Omphalotus lineage are common examples of bioluminescent fungi. The origin of fungal bioluminescence can be attributed to a single evolutionary ancestry, the proof of which has been given by cross-reactions between the luciferins and luciferases of distant lineages to yield light successfully [21].

The purpose behind the emergence of fungal bioluminescence still remains elusive. Speculations have been made by Oliveira et al. that it may serve as a mode of attraction for insects, facilitating entomophilous spore dispersal, as seen in some species of *Neonothopanus* [27]. Furthermore, the same study revealed that there is some semblance of circadian control to make this entire affair more energy efficient by increasing bioluminescence at night. However, this is not true for all fungal species, wherein this trait may simply be a luminous by-product of metabolism, without a definite purpose [28]. The evolutionary feasibility of such cases is yet to be determined.

#### **3.3 Bioluminescence in protists**

Among protists, the chief groups that exhibit bioluminescence are Radiolaria (or Radiozoa), and Dinoflagellates, which are both exclusively marine. Both of these are described as follows:

#### *3.3.1 Bioluminescent radiolaria*

Among all the radiozoa, only two genera, namely *Collozoum* and *Thalassicola* are known to exhibit bioluminescence. Both of these belong to the order Collodaria, and use coelenterazine as substrate [4].

Bioluminescence has also been reported in some other deep sea species like *Aulosphaera* spp. and *Tuscaridium cygneum* [4].

#### *3.3.2 Bioluminescence in dinoflagellates*

Dinoflagellates are a group of cosmopolitan protistan organisms [29] having an ancient evolutionary history, which form one of the most important groups of phytoplankton in the aquatic ecosystems [30]. They are the only photosynthetic organisms that are capable of bioluminescence [30], and are the most dominant contributors to the occurrence of this phenomenon in the upper ocean [31]. Common phenomena like the "Red Tides" and the bioluminescent bays of Jamaica are because of the dramatic increase in the population of *Gonyaulax* and other dinoflagellate species. *Gonyaulax polyedra* is supposedly the most studied dinoflagellate species [20]. Other common bioluminescent genera are *Ceratium*, *Protoperidinium*, *Pyrocystis*, *Noctiluca*, [31] and *Alexandrianum* [29]. There have been inaccurate records of bioluminescent dinoflagellate species in the past, because of the presence of both bioluminescent as well as non-bioluminescent strains belonging to the same species. Difference in the ability has been observed even between cells of the same strain [31].

The chemical structure of dinoflagellate luciferin (sourced from *Pyrocystis lunula*) is remarkably unique [20], similar only to that found in euphausiids (krill). This perhaps is an example of dietary linkage, as krill are known to source their luciferin from the food they consume [4]. Dinoflagellate luciferin is believed to be a derivative of chlorophyll [20]. Unlike most species that are autotrophic in nature, some heterotrophic species even supplement their luciferin synthesis with chlorophyll-rich diets [4].

Dinoflagellates produce bioluminescence with the help of specialized cell organelles called "scintillons", which enable them to glow only in response to shear or physical disturbance/turbulence in the surrounding water [31]. This glow is not persistent, but occurs in brief flashes. The intensity of these flashes may be affected by various factors like exposure to prior illumination, nutritional state of the cell, or even because of a diurnal rhythm [31]. There are evidences of a circadian rhythm that is operational in dinoflagellates, and also photoinhibition of bioluminescence during daytime [29]. The synthesis and destruction of luciferin is not the only method of regulation though; cellular redistribution of luciferin has been reported to be affected by the said circadian rhythm [20]. The intensity of the flashes also differs from species to species. Dinoflagellates prioritize bioluminescence second only to reproduction, to an extent that there have been reports of cannibalism under nutritional stress in order to support bioluminescence [31].

As far as the ecological purpose of bioluminescence in dinoflagellates is concerned, we are still unclear as to why these organisms take such measures to sustain it. The exact ecological context of this trait still remains unclear, maybe because of a lack of *in-situ* studies [29]. Some studies show that the flashes of light have a startling effect on copepods (the prime predators of dinoflagellates), which dart away from the prey [32]. Another speculation, called the "Burglar Alarm" hypothesis, states that the brief flashes produced by the cells upon coming in contact with a grazer (for example, a copepod) in turn attracts a predator of higher trophic level, hence protecting the cell from its own predator. This hypothesis is widely accepted, although there are no sufficient evidences of the same [4]. Furthermore, this hypothesis does not point out any clear advantage to the dinoflagellate [31].

To conclude, bioluminescence in dinoflagellates seems to be a useful but unnecessary evolutionary trait, as an accurate ecological context is yet to be determined [30]. In order to gain more knowledge on the same, coastal blooms can be harnessed as natural laboratories to study dinoflagellate bioluminescence in further detail [29].
