By Tanjim Sayeeda
In the deepest and darkest parts of the ocean where the sun does not reach, there exists a unique and extraordinary form of light known as bioluminescence. Many marine organisms such as the anglerfish, jellyfish and squid can illuminate in the dark. In addition to bioluminescence providing beautiful specks of colour within the pitch-blackness of the ocean, the creation of light is key to survival for these organisms whether it be to lure prey, attract mates or stun predators (Shimomura & Yampolsky, 2019).
Bioluminescence is a form of chemiluminescence, named so because light is produced by a chemical reaction within a living organism. Another key characteristic is its ability to produce less than 20% of thermal radiation. The chemistry of bioluminescence involves two key compounds- Luciferin and luciferase. Luciferin is a light-producing compound, which reacts with oxygen to produce oxyluciferin (and light as a by-product). This reaction is catalysed by the enzyme luciferase. Alternatively, photoproteins can replace luciferase and combine with oxygen and luciferin but would require the presence of another ion such as calcium. The various bioluminescent colours, be it the yellow in fireflies, the green in jellyfish or the blue in the bulb of the anglerfish are all determined by the arrangement of luciferin molecules (Deluca & McElroy, 2013).
The synthesis of luciferin is not always produced independently by bioluminescent organisms; it can be either absorbed through food or via a symbiotic relationship. Species of midshipman fish obtain their glow by consuming small bioluminescent crustaceans called ostracods (Bessho-Uehara et al., 2020). On the other hand, marine organisms like squid and the anglerfish have a symbiotic relationship with bioluminescent bacteria, which reside within their light organs (Anon., 2018). As for tiny marine organisms such as phytoplankton (or more specifically, dinoflagellates), light is produced independently at night causing the surface of the ocean to glow a bluish green. Spectacular sightings of bioluminescent plankton have been found in the Humacao Natural Reserve in Puerto Rico as well as warm water lagoons near openings to the sea (TSUJI, BARNES & CASE, 1972).
Despite the magical ambience the glowing waves create, the dinoflagellates produce light when they are aggravated. They briefly flash in order to attract predators towards the marine creatures either preying on them or disturbing them. The flash of light startles the predators of the dinoflagellates as well because they worry that their visibility to predators has increased (Cheryl Lyn Dybas, 2012). Similarly, bioluminescent ostracods flash their light when swallowed by cardinalfish in order to force cardinalfish to spit them out as they beam with light, or risk being spotted by predators (Bessho-Uehara et al., 2020). Bioluminescence can also be an adaption to the deep-sea environment; while surface-level squid are able to eject dark ink to blind their predators, the deep-sea vampire squid relies on bioluminescent mucus to startle predators and make a rapid escape (Golikov et al., 2019). Preys of shark, like the hatchet fish, can use counterillumination to avoid shadows being formed beneath them by sunlight, so sharks hunting below cannot see them (Paitio et al., 2020).
Although biologists are confident of the benefits bioluminescence offers to organisms, the evolution of it remains obscure. The vitality of marine organisms requiring bioluminescence for survival is highlighted by the fact that 80% of glowing organisms reside in the ocean. In addition, bioluminescence has evolved 27 different times in various species and thus the question of why many marine organisms have adapted to the ocean by producing light has intrigued researchers. One hypothesis suggests that the frequent divergence into new species amongst the bioluminescent organisms is due to the use of species-specific light patterns in order to communicate and mate. Therefore, once a specific ecological niche is found, organisms within the niche only mate amongst themselves and thus forge a new species (Annalee Newitz, 2016).
Further research into bioluminescence is ongoing and can be utilised by humans in a variety of ways. For example, the green fluorescent protein (GFP) has been used in biomedical research as a reporter gene, which is a chemical attached to a gene that is being studied. The fluorescence allows the GFP reporter gene to be identified and measured easily and thus the specific gene which is being studied. GFP has allowed researchers to monitor certain genes in order to understand its function and further the brink of scientific knowledge (Zimmer, 2002). Experimental uses of bioluminescence could involve creating bioluminescent trees that light streets and highways in cities for safety in an eco-friendly approach. Agriculture could also benefit from bioluminescent crops and plants that light up when nutrient deficient so farmers can reduce costs (Swain, 2010).
Bioluminescence is as beautiful as it is brilliant in the animal kingdom. From aiding species survival to assisting biomedical research, bioluminescence can hopefully continue to contribute to a brighter future.
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