Bioluminescence Explained: The Chemistry Behind Light-Producing Organisms

Fireflies blinking over a summer field and glowing waves breaking on a dark beach are both examples of the same underlying process: a living organism making its own light through chemistry rather than heat. Scientists call this bioluminescence, and it is one of the most widespread forms of biological light production on Earth.

The chemical reaction: luciferin, luciferase and oxygen

According to the Natural History Museum in London, bioluminescence happens when a molecule called luciferin reacts with oxygen and releases energy in the form of light. The reaction normally needs an enzyme, luciferase, that speeds up the process, or in some organisms a related protein called a photoprotein that works together with calcium ions instead.

The U.S. National Oceanic and Atmospheric Administration (NOAA) notes that although every bioluminescent organism relies on a reaction between an enzyme and a substrate, different species use different chemical combinations. NOAA says this diversity suggests the ability to make light evolved independently many times, rather than from a single common ancestor.

The color of the light depends on the exact structure of the luciferin and the surrounding proteins. The Natural History Museum and National Geographic Education both note that most marine organisms glow blue or green because those wavelengths travel farthest through seawater, while fireflies on land produce yellow light and a small number of deep-sea fish, such as loosejaw dragonfish, emit rare red light that most other animals in the deep cannot see.

Why organisms produce light

Researchers at NOAA and the Natural History Museum group the uses of bioluminescence into a few main categories: finding food, attracting or signaling to mates, and defense against predators. Some species combine several of these functions.

• Hunting: the deep-sea anglerfish dangles a glowing lure called an esca in front of its mouth to draw in curious prey, according to National Geographic Education.
• Mating: male fireflies flash species-specific patterns to signal to females, National Geographic Education reports.
• Defense: hatchetfish use 'counterillumination,' matching faint light on their undersides to the dim glow from above so predators looking up cannot see their silhouette, per National Geographic Education.
• Startling predators: some squid flash suddenly to startle an attacker, and the vampire squid ejects glowing mucus instead of ink, according to National Geographic Education.

Where bioluminescence is most common

NOAA states that 80 percent of the animals living between 200 and 1,000 meters (656 and 3,280 feet) deep in the ocean are bioluminescent. The Natural History Museum gives a higher estimate for a different zone, saying that up to 90 percent of life in the ocean's twilight zone produces light in some form. On land, bioluminescence is comparatively rare and best known from fireflies, some glow-worms, and roughly 75 species of fungi that give off a faint green glow, per the Natural History Museum.

NOAA's ocean-facts page points out an open scientific question: researchers have not fully explained why bioluminescence is so common in marine environments but essentially absent in freshwater lakes and rivers, nor have they pinpointed exactly how many times, or how, the trait evolved independently across different lineages.

Notable examples

Beyond fireflies and anglerfish, several other cases illustrate the range of bioluminescent life. Dinoflagellates — single-celled plankton — produce flashes of blue-green light when disturbed by waves or movement in the water, an effect sometimes visible as sparkling surf at night, according to National Geographic Education. Large, sustained patches of this glow created by colonies of luminous bacteria are known as 'milky seas' and are occasionally bright enough to be detected by satellites in tropical waters, National Geographic Education notes. Some species, such as the Hawaiian bobtail squid, do not make the light themselves at all; they host colonies of bioluminescent bacteria, including Vibrio fischeri, in a symbiotic relationship described by the Natural History Museum.

Across all these cases, the underlying chemistry is similar — a luciferin reacting with oxygen, catalyzed by luciferase or a photoprotein — but the specific molecules, colors and biological purposes differ from one lineage to the next, reflecting the many separate origins of this trait identified by NOAA and the Natural History Museum.