The phenomenon of Bioluminescence in different organisms
Bioluminescence is the production of light by living organisms. Bioluminescence is abundant in marine organisms than terrestrial because marine habitats particularly deep regions are always darker as sunlight cannot reach at such great depths. So the basic need is of illuminated environment which is accomplished by bioluminescence. Apart from illumination, bioluminescence is used for attracting mate or prey or diverting the predators; it is also used as tool to express aposematism that is to warn off predators for defense. Terrestrial bioluminescent organisms display yellow-green, red light while blue color is predominant in marine organisms. Blue color, however often seems white or milky as human photoreceptor rod cells are unable to detect the color difference. During bioluminescence, heat is neither generated nor required; hence it is also termed as cold luminescence. Bioluminescent system is chemically almost identical in all bioluminescent species of organisms. They contain light emitting organic molecule, known as luciferin. It emits light of certain wavelength on oxidation by molecular oxygen. Oxygen is essential substrate of enzyme luciferase which catalyses the oxidation reaction. Other substrates of luciferase are reduced flavin mononucleotide (FMNH2) and saturated long chain aldehyde but they are found to be different for other organisms. FMNH2 and aldehydes are specific for luminescent bacteria. In these bacteria, FMNH2 is also oxidized to FMN (excited form) and when it attains ground state, light is emitted.
Bacteria: Bioluminescence in bacteria can be best observed at wavelength of 490nm. Vibrio fischeri (Photobacterium fischeri), V. phosphoreum and V. harveyi are predominant in marine environment. They are also ectosymbionts of marine fishes, squids and jelly fishes. Luminescent Shewanella woodyi and hanedai are also present in marine habitats. Luminescent Xenorabdus luminescens has been isolated from wound infections, soil nematodes and insects. Genetics of luminescence in bacteria is well studied. Bacterial luciferin-luciferase system is encoded by a set of genes called Lux operon. In V. fischeri, Lux operon containing five genes CDABE active in light emission and two genes RI for operon regulation have been investigated. Luminescent marine bacteria are responsible for 'Milky Seas' effect. It is produced when luminescent bacteria bloom and glow with uniform continuous light in ocean waters. Milky seas effects are recurred in Indian Ocean and have been spotted by mariners as well as satellites. Bacteria use luminescent property for communication via quorum sensing, to protect the cells from UV damage and pathogenesis related reaction in marine pathogens like V. fischeri. It has been noted that bacterial luminescence is always brighter at night than daytime; therefore it was concluded that luminescence follows circadian rhythm.
Jellyfish: Crystal jelly fish (Aequorea victoria) and alarm jelly fish (Atolla wyvillei) are two important examples of luminescent jelly fishes. Atolla uses light for help when caught by predator. This escape signal from trapped jelly fish is detected by larger community members for further actions! Bioluminescence in Aequorea is due to calcium activated photoprotein and green fluorescent protein (GFP) and in Atolla; light emitted by ingested luminescent zooplankton can be seen through its transparent stomach.
Fish: Marine fishes like viper fish, angler fish, cookie cutter shark, gulper eel, midshipman, lantern fish, hatchet fish etc are bioluminescent. This property is however is a gift of luminescent bacterial symbionts inhabiting photophores or light emitting organs of these fishes. Photophores are present over whole body or along lateral line, fins or gills. V. fischeri is known to be common symbionts of most luminescent fishes. In black loose jaw fish, light organs works like night vision goggles. We cannot see their red luminescence very brighter but they use it efficiently to locate the prey. Bioluminescent fishes use light to distract predators, to attract mate and to find prey in deep ocean compartments.
Squids: Bobtail, vampire, colossal squids are bioluminescent. Luminescent property is conferred by colonization of light organ by V. fischeri population. Light organs can control direction and brightness of illumination. Certain species of cuttlefish, krills and octopuses are also bioluminescent.
Earthworms: Species from different genera of earthworms, Diplocardia, Emlea, Fletcherodrilus, Pontodrilus, Spenceriella, Octochaetus and Diplotrema are luminescent annelids. They contain peroxidase like luciferin-luciferase bioluminescent system and luminescence is produced by special cells or chloragocytes in the form of coelomic fluid or mucus.
Snails: Snail species Dakya striata from Malaysia and Latia neritoides from New Zealand are bioluminescent. Their luciferin-luciferase is similar to that of bacteria.
Insects: We are very well familiar to luminescent insects, fireflies (Photinus pyralis and Photuris lucicrescens etc.) and glow worm (Lampyris noctiluca). Only the females of glow worm exhibit luminescent property. Fireflies have abdominal light organs and luminescence is generally used to attract mate. Some species of cockroaches, springtails, beetles and gnats also show property of luminescence. Millipedes and centipedes are also prominent bioluminescent species from terrestrial ecosystems.
Fungi: Higher fungi or mushrooms Armillaria mellea, Mycena chlorophos, Pleuorotus sp., Omphalotus nidiformis (ghost fungus) and O. olearius (Jack-o-Lantern), O. olivascens, O. illudens and Panellus stiptias are some of the examples of bioluminescent fungi. These fungi are found in humid tropical rain and temperate forests of world and decaying logs are their typical habitat. Fungal mycelia, gills and pileus emit light usually to express aposematism. Luminescence is also used for detoxification of free radicals, to attract insects which may disperse the fungal spores. Luminescent fungi contain NADH dependent reductant to reduce luciferin but luciferase is absent.
Plants: In North-East India certain species of orchids are found to be bioluminescent. Luminescent plants have been reported from Himalayan regions, Deccan plateau and Western Ghats of India. Sanjeevani booti as it is known in Ayurveda was known to exist 300 million years ago in India. It was medicinal plant used by ancient Indians for its unique characteristic: an ability to resurrect from death. Sanjeevani booti exhibited luminescent property and it was principle characteristic for identification of this plant when in wild.
The phenomenon of bioluminescence is different from phenomena chemiluminescence and iridescence. In chemiluminescence, light is given off by biological processes such as in mitotic reactions wherein cells emit light energy but that is not used by organism for any purpose. Iridescence to happen requires external light source. For example, shiny appearance of wings of sunbirds or butterflies is due to refraction of sunlight which we recognize as iridescence. Bioluminescence finds many applications in pharmacy, medical, biotechnology, taxonomy and agriculture. Scientists have proposed to enlighten our streets by genetically modified tree species containing luminescent genes from bacteria. It has been recommended to use in farming practices like monitoring and control of watering and fertigation schedules. Bioluminescent bacteria are being used as markers to assess bacterial contamination and toxicity in foods. Bioluminescent sensors are also used to monitor the progress of infections in immunocompromised patients. There would be very potential biotechnological applications of the phenomenon of bioluminescence which needs to be investigated. I expect and request the readers to let me know names and information of novel bioluminescent species of organisms, especially luminescent plants if they are known about it.
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