Bioluminescence is characterized as the emission of visible light by an organism as a result of natural chemical reactions, and is the main source of light in a large part of the oceans. A remarkable diversity of marine organisms are capable of producing their own light and use bioluminescence for vital functions ranging from defense to reproduction (WILSON; HASTINGS, 1998; HADDOCK et al., 2010). It is known that the vast majority of bioluminescent organisms reside in the oceans, and of the more than 700 known genera, around 80% are marine species (WIDDER, 2010). The marine environment has some particularly favorable characteristics for the evolution of bioluminescence. The main factors include a long uninterrupted evolutionary history, with conditions that have prevailed relatively stable over time; large areas of the habitat that receive little light, or even exist in continuous darkness; and interactions that occur between a huge variety of taxa, including predators, prey and parasites (HADDOCK et al., 2010).
From an ecological point of view, bioluminescence is an important form of communication in the marine environment, having an influence on the immense daily vertical migration, which is defined as a strategy carried out by organisms, such as zooplanktons, that aims to mitigate the risk of predation, as well as influencing the predator-prey relationship and, consequently, the flow through the food chain. Biological luminescence has been documented in a wide range of organisms, from bacteria and protists to squid and fish, with numerous phyla among them. In this context, studies have already reported bioluminescence as being produced endogenously by the organisms themselves, rather than being derived from bacterial symbiotic associations, with light production being predominantly greater in deep-water and planktonic organisms, compared to benthic or shallow-water species (NEALSON; HASTINGS, 1979; HERRING, 1987 apud HADDOCK et al., 2010).
Numerous known luminous genera were cataloged in 1987 by Herring, but additional discoveries are continually being made. It is now known that bioluminescent fish are predominant in terms of biomass, while bacteria and dinoflagellates are predominant in terms of abundance. The distribution of luminescence among the main taxonomic groups does not appear to follow simple phylogenetic or oceanographic constraints at first glance. Many studied and catalogued lineages have at least one bioluminescent species (NEALSON; HASTINGS, 1979; HADDOCK et al., 2010; WIDDER, 2010).
With regard to understanding the physiological and chemical basis of bioluminescent light, according to Osamu Shimomura (2006), natural light is generated as a result of the energy released during an oxidation reaction of luciferin, a light-emitting molecule. The reaction rate of luciferin is modulated by an enzyme called luciferase or a photoprotein variant of luciferase. Some indispensable factors for light emission (including luciferin and oxygen) are coalesced as a unit. When photoproteins bind to another ion or cofactor, they are activated to produce light, generating a change in the protein's conformational structure. This chemical process offers the organism a way of regulating its light emission (SHIMOMURA, et al., 1981; THOMSON, et al., 1998; HADDOCK et al., 2010).
Although the oxidative reaction with luciferins is responsible for a large part of the natural light emissions in the oceans, there are still many other light-emitting reactions yet to be discovered. Bacterial emissions, for example, involve the oxidation of proteins with long aldehyde chains and the enzyme luciferase. In addition, several other marine bioluminescent organisms have variations in their chemical and molecular light emission processes. In short, bioluminescence in the oceans is extremely important for the survival and perpetuation of species, as well as having a direct influence on the ecological balance of the marine habitat.
Author: Giovanna Luiza - Deputy Director of the Association
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