What Is Bioluminescence? Animals Making Light Without Heat

The reaction involves a light-emitting compound and an enzyme that catalyses its oxidation, with the energy released appearing as a photon rather than as heat. Several distinct chemical systems exist, which is the clearest evidence that the capacity evolved independently many times, since organisms using different compounds cannot have inherited the arrangement from a common ancestor. The efficiency is remarkable, converting a very high proportion of the chemical energy into light compared with an incandescent bulb which wastes most of it as heat, which is why the phenomenon is described as cold light. Some organisms produce the compounds themselves and others obtain them from diet or from symbiotic bacteria housed in dedicated organs, which is a substantial proportion of marine cases.

The functions differ completely between organisms and several operate in one species:

• •Attracting prey, as with the lure of an anglerfish and the glowing threads of certain cave-dwelling larvae
• •Attracting mates, which is what fireflies do with species-specific flash patterns
• •Counterillumination, where light produced on the underside matches the brightness above and erases the animal's silhouette from below
• •Startling or distracting predators, including species that release a glowing cloud and escape
• •The burglar alarm strategy, where a prey animal illuminates its attacker and attracts something larger
• •Communication and species recognition, where the pattern rather than the light itself carries the message

The overwhelming majority of luminous organisms live in the ocean and the reason is straightforward. Most of the ocean is dark, since sunlight is absorbed within a few hundred metres and the volume below that is the largest habitat on the planet, so light produced by an organism is conspicuous and useful there in a way it is not in a lit environment. Estimates suggest a substantial majority of animals in the deep open ocean produce light. On land the capacity is far rarer and is concentrated in fungi, in certain beetles including fireflies and glow worms, and in a few other groups, since ambient light during the day makes the signal useless and nights are not dark enough in many habitats. Freshwater bioluminescence is almost absent, which is a genuine puzzle with no agreed explanation.

The phenomenon is observable without any equipment in a number of ordinary situations. Disturbed seawater glows in many coastal areas at night where single-celled organisms are abundant, producing light in a boat wake, in breaking waves and around anything moving through the water, and the effect is common enough to be a tourist attraction in several places. Fireflies and glow worms are visible in season across much of the world. Certain fungi glow faintly on decaying wood, producing the effect known as foxfire, which requires full darkness and patience. Some fish and squid sold for food carry luminous bacteria and glow if kept in the dark. The common requirement is genuine darkness, which is the practical obstacle, since even modest artificial light overwhelms the effect entirely.

The biochemistry became a standard laboratory tool and the applications are substantial. A gene for a light-producing enzyme can be attached to another gene, so that anything switching the second gene on also produces light, which makes gene activity visible in a living organism and is used routinely across biology. A green fluorescent protein originally from a jellyfish, which fluoresces rather than producing light chemically, became the most widely used marker in cell biology and its discovery was recognised with a Nobel Prize. Detection of bacterial contamination uses the light-producing reaction as an indicator. Luminous bacteria are used to test water for toxicity, since the light dims when they are harmed. And engineered luminous plants and organisms appear regularly as demonstrations, with the practical applications of those considerably more limited than the coverage suggests.

An enzyme oxidises a light-emitting compound and the energy appears as a photon rather than heat, which is far more efficient than a filament. Several unrelated chemical systems exist, so the capacity evolved separately dozens of times. Most luminous organisms are marine because most of the ocean is dark, and the biochemistry became a standard tool for making gene activity visible.