Bioluminescent beasties

It’s fun to be a glow-worm, as a glow-worm’s never glum; how can you be unhappy when the sun shines out your bum? The light that some creatures produce isn’t magic, although it may look it; it’s all down to biochemistry.

Bioluminescence, or the natural ability of living organisms to produce light, is useful for a host of reasons in nature. For example: startling predators or acting as a kind of natural burglar alarm, lighting up the predator and making them run away, as a distraction or as counterillumination, matching the light elsewhere to blend into the environment. This can lead to some spectacular sights. If you ever visit Puerto Escondido on the western coast of southern Mexico, one of the highlights is swimming in a nearby, crocodile-infested lagoon. On a moonless night, your movement will disturb the algae in the water, which will react by glowing in the dark. The result is that it looks like you’re swimming through a thousand tiny, magical sparkles.

There are several different chemicals and processes at work, depending on the species, and scientists don’t quite understand the exact processes required to produce this natural wonder. Perhaps the most researched are fireflies, famous for the colourful lights they emit due to a class of compounds called luciferins.

First isolated in the lab in 1949, crystal firefly luciferin (C₁₁H₈N₂O₃S₂) is fluorescent, meaning it emits light after absorbing it or other types of electromagnetic radiation. In the case of fireflies, a chemical reaction causes bioluminescence. Luciferin absorbs ultraviolet light – invisible to humans – which puts it in an excited energy state. Luciferin breaks down in an oxidation reaction with the help of luciferase enzymes. This produces 1,2-dioxetane, an unstable compound that decomposes into carbon dioxide and ketones, which then release energy and lose their excited state by emitting light. Without oxygen – and adenosine triphosphate (ATP) and magnesium – the reaction wouldn’t work and the firefly wouldn’t glow.

Some rival species aren’t above a little murder

Scientists have reproduced this reaction in glow sticks. These consist of a hollow tube filled with two compartments, each containing a different chemical (typically diphenyl oxalate, C₁₄H₁₀O₄, and a coloured dye in one half and hydrogen peroxide in the other). When you snap the stick, the chemicals mix, resulting in an exergonic reaction (where energy is transferred to the surroundings). This releases carbon dioxide and excites the dye, causing it to emit light as it returns to its relaxed state.

Fireflies are better than glow sticks, however, because they can turn their lights on and off. They do this by controlling how much oxygen enters the light-producing organs at the ends of their abdomens. This natural dimmer switch enables fireflies to signal to other fireflies nearby. Some species of fireflies, including the Photinus carolinus of the Appalachian Mountains in the eastern US, even synchronise their flashes with other fireflies nearby, helping males and females find each other during mating season.

Murderous mimicry

Of course, this natural light show also gives a firefly’s position away to nearby predators. That’s why some fireflies, such as the genus Photinus, also produce lucibufagins, which are defensive steroids, synthesised from chemicals in their diet. These make fireflies taste awful and can be highly toxic, naturally discouraging predators. In fact, lucibufagins are so useful that some rival species that can’t produce them, such as Photuris frontalis, aren’t above a little murder. Female Photuris fireflies mimic the flashes of Photinus females, only to eat any males of the rival species that come to mate with them. Once she’s devoured her hapless suitor, the Photuris female transfers the prey’s lucibufagin into her bloodstream, stealing the toxin for herself!