There is much uncertainty around the origins of bioluminescence and as yet no firm conclusions have been reached. What is known is that bioluminescence is produced by a biologically controlled chemical reaction, in which energy in the form of light is produced as a by-product (Herring et al. 1989).
This reaction involves elevating the small organic molecule, luciferin, to a higher, less stable energy state. Light is only emitted when luciferin decays from its excited state, returning to a stable molecule and releasing energy in the form of light (Rees et al. 1998).
There are four main structures (Fig.4) used throughout the bioluminescent community (Haddock et al. 2010). Recent investigations would suggest that coelenterazine is the luciferin structure most widely distributed throughout bioluminescent organisms (Herring 2002).
In many organisms luciferin is taken to a higher energy level via an oxidation reaction (Fig.3). The reaction is catalysed by the presence of an enzyme generically named luciferase (Rees et al. 1998). Unlike luciferin structures there are numerous forms of luciferase, with many species having their own, unique structure (Herring 2002).
Other bioluminescent reactions do not require the presence of a substrate such as oxygen but instead use photoproteins to generate the reaction (Vysotsky & Lee 2004). The photoprotein will bind to the luciferin forming a stable complex. A co-factor, such as Ca2+, then joins the complex stimulating an intra-molecular reaction that produces enough energy to excite the luciferin to the energy state that produces light off a useful wavelength (Vysotsky & Lee 2004). Whilst the wavelength of the light emitted is dependant on the amount of energy used to excite the lucifein, the intensity of the luminescence is dependant on the amount of photoproteins present at the time of the reaction (Shimomura 1985).