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Life's Uncertain

so eat dessert first!

How living light works

Bioluminescence is light made by a chemical reaction inside a living thing. No sunlight needs to be stored – which is why it works in total darkness. It is sometimes called "cold light" because it gives off very little heat.

The basic reaction

Most bioluminescence follows the same basic recipe:

  1. Luciferin – a light-emitting molecule (the fuel).
  2. Luciferase – an enzyme that speeds up the reaction (the spark).
  3. Oxygen – which reacts with the luciferin.

When luciferase helps oxygen react with luciferin, the luciferin is left in an energized, "excited" state. As it relaxes, it releases that extra energy as a particle of light. Fireflies also need ATP, the energy molecule found in every living cell.

The names come from the Latin lucifer, meaning "light-bringer". They were coined in the late 1880s by French scientist Raphael Dubois, who studied a glowing click beetle from the West Indies.

"Luciferin" is not one chemical but a family name. Fireflies, dinoflagellates, bacteria, fungi and ostracods each use a different luciferin. A molecule called coelenterazine is the most common luciferin in the sea, used by many very different animals.

Some animals, like the crystal jelly, store a ready-to-go version of the chemistry called a photoprotein, which flashes when it binds calcium.

Why most living light is blue

In seawater, blue light travels farthest, and red light is absorbed quickly. So almost all ocean creatures make blue light, and most are sensitive only to blue – many deep-sea animals cannot see red at all. That is exactly what makes the red-light dragonfish so sneaky.

On land the rules are different. Fireflies glow in shades of green to yellow-orange, glowing fungi shine green, and the railroad worm adds a red headlamp.

Why glow? Four big reasons

  • Finding food

    Lures, like the anglerfish's glowing esca, draw prey in. Searchlights, like the dragonfish's red beam, may reveal prey without being noticed.

  • Hiding

    In counter-illumination, an animal's belly glows to match the dim light from above, so its shadow disappears to anything looking up from below.

  • Defense

    A sudden flash can startle a predator. Glowing clouds act like a smoke screen. "Burglar alarm" lights may call in a bigger predator to attack the attacker.

  • Finding a mate

    Fireflies and ostracods use species-specific light codes to find partners, and some deep-sea animals are thought to use light to signal to mates.

Glowing is not the same as fluorescing

These three words are easy to mix up:

Bioluminescence
A living thing makes light from a chemical reaction. It works in total darkness.
Fluorescence
A material absorbs light of one color and instantly gives it back as another color. It stops when the light source is removed. Scorpions glowing under ultraviolet light are fluorescent, not bioluminescent.
Phosphorescence
Like fluorescence, but the material keeps glowing for a while after the light is gone – think glow-in-the-dark stickers.

People often call glowing waves "phosphorescence", but the glow in the water is almost always bioluminescent plankton.

Invented again and again

Bioluminescence did not evolve once and spread. It appears to have evolved independently many separate times across the tree of life. Older estimates put the number at about 40 or more; a 2021 review counted at least 94 separate origins.

A large survey of animals filmed in the waters of Monterey Bay, California, found that about 76 percent of the animals observed from the surface down to 3,900 meters were capable of making light.

How people use living light

  • Watching life inside cells. Green fluorescent protein (GFP), first found in the crystal jelly, lets scientists make cells, proteins and even whole organisms glow so they can be tracked under a microscope.
  • Hygiene testing. Because firefly luciferase needs ATP, it can reveal traces of living material. Food businesses and kitchens use luciferase-based swab tests to check how clean a surface is within seconds.
  • Medical and drug research. Researchers add luciferase genes to cells so that a glow shows when a particular gene switches on.
  • Glowing plants. Scientists have moved light-making genes from a glowing mushroom into plants, and after a US Department of Agriculture review, a glowing "firefly petunia" has been sold to home gardeners in the United States since 2024.