Chemiluminescence requires an exothermic reaction that produces an electronically excited intermediate. The excited molecule relaxes to its ground state by emitting a photon. Unlike thermoluminescence, no heat input is needed — the energy comes entirely from chemical bond rearrangements.
The enzyme luciferase (541 aa, ~61 kDa) binds D-luciferin, ATP, and O₂. It forms luciferyl-adenylate, which reacts with O₂ to produce the excited singlet state of oxyluciferin, which emits at 562 nm. Quantum yield ≈ 0.41 — among the highest known.
Bioluminescence has evolved independently 40–50 times — in bacteria, dinoflagellates, fungi, worms, fish, squid, and beetles. Despite this convergence, most use a luciferin/luciferase system and the common intermediate: an excited carbonyl compound.
In many deep-sea organisms, bioluminescence is shifted to longer wavelengths by fluorescence resonance energy transfer (FRET) to green fluorescent protein (GFP). Osamu Shimomura's discovery of GFP from Aequorea victoria earned the 2008 Nobel Prize in Chemistry.