The sea does not glow. It flinches. Every blue flash breaking off a bow wave or a footprint in wet sand is a single-celled alga having something close to a panic attack. Dinoflagellates like Lingulodinium polyedrum, Noctiluca scintillans and Pyrocystis fusiformis carry the machinery for it in organelles called scintillons, tiny pouches slung into the cell’s outer cytoplasm and packed with two things: luciferin and luciferase. The trigger is mechanical. Shear stress on the cell membrane, roughly 0.1 to 1 newton per square metre, opens stretch-sensitive channels and fires an action potential across the membrane of the vacuole. That electrical event is not the light. It is the doorbell. - The action potential opens proton channels feeding the scintillon - Acidic vacuolar sap floods in, dropping internal pH from about 8 to 6 - Luciferase changes shape, unfolding an N-terminal domain that had been blocking its own active site - Luciferin, a tetrapyrrole cousin of chlorophyll, is oxidised...