Light production in the eternal darkness of the deep ocean
Below 200m — the twilight zone — sunlight vanishes entirely, yet life thrives in part by making its own light. It is estimated that 76% of deep-sea species can produce bioluminescence, making it the most common form of communication on Earth by biomass. The luciferin-luciferase reaction oxidizes a substrate to produce photons with near-perfect efficiency: luciferin + O₂ + ATP → oxyluciferin + CO₂ + light. Uses are diverse: anglerfish lure prey with photophores, dinoflagellates flash to startle predators, and firefly squid use counterillumination to match downwelling light and become invisible from below.