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Bioluminescence
The word bioluminescence comes from Greek and Latin – bios means life in Greek and lumen means light in Latin. Bioluminescence is a specific example of chemiluminescence produced by a living organism. In case of bioluminescence, a biochemical reaction gives rise to intermediate(s) in excited states which emit the extra energy in form of light. High efficiencies of converting the chemical energy into light are common for bioluminescence; in fact, efficiencies higher than 90% are widespread.
There are many organisms in very different biotopes capable of bioluminescence – it can be the glow of bacteria on a decaying substrate, blue luminescence of protozoa on the beaches of tropical seas or the glow of fireflies (Lampyris noctiluca).
In general, bioluminescence is observed with many simple organisms – bacteria, fungi (Omphalotus nidiformis, Omphalotus olearius), insects, sea invertebrates and fish species; up to now, there haven´t been found examples of naturally occurring higher plants, reptiles, birds and mammals capable of bioluminescence.
The biologists suppose that there can be several reasons for evolution of bioluminescence:
1) Role in protection and survival
Bioluminescence can be linked to protection and survival of the organism. Some deep sea animals, like certain species of octopuses exude a glowing secret whose role is to confuse the enemy and assist in escape or frightening the predators. It is believed that bioluminescence plays a vital role in sexual communication and partner recognition.
2) Role of bioluminescence in cellular protection
It is believed that bioluminescence in lower organisms like bacteria, protozoa and fungi can be an evolutionary remnant from times when Earth´s atmosphere was low in oxygen (oxygen was toxic to primitive organisms of that time). Bioluminescence can represent an oxygen-neutralizing defence mechanism that remained conserved even after adaption to oxygen-rich atmosphere.
Molecular principles of bioluminescence
Most organisms capable of bioluminescence use enzyme-catalyzed reaction of a suitable substrate with oxygen in the presence of cofactors. During oxidation, the substrate is converted to products in excited state which emit extra energy in form of light.
The best known example is luciferin oxidation (typical for fireflies) with oxygen under luciferase catalysis in the presence of Mg2+ salts and ATP.
The sequence is depicted in Scheme 2.
In the first phase, luciferin reacts with oxygen under ATP consumption, Mg2+ assistance and luciferase catalysis to yield a reactive, highly unstable intermediate Int1. This intermediate is decomposing in the second stage into carbon dioxide and intermediate Int2 in excited state (Phase 2). Int2 undergoes deexcitation by emission of a photon with a characteristic wavelength (Phase 3). In case of luciferin, the emission maximum is at 550-570 nm.
Deep sea blue glowing organisms use a similar chemical reaction but with a slightly different substrate. It is estimated that marine organisms tend to emit mostly blue color because of evolutional reasons – blue light is less absorbed by sea water and therefore penetrates better.
Luciferin-based bioluminescence is nowadays intensively used in biology and biochemistry.
For example low concentrations of ATP can be measured by bioluminescence (because ATP is needed for luciferin oxidation). Another advanced application is detection of gene expression (luciferase gene expression) because presence of luciferase can be easily detected. This simple, yet extremely sensitive technique finds widespread use in gene technology. Bioluminescent imaging is another advanced technique that is being used for study of biological processes in living organisms. For instance, it is possible to precisely follow cancer propagation using a special line of bioluminescent cancer cells. These observations can lead to a much better understanding of the disease. Similar and other uses of bioluminescence are nowadays intensively studied and represent a dynamically developing field.