Question

Passage:
In marine ecosystems, the nocturnal bioluminescent displays produced by unicellular dinoflagellates, such as Lingulodinium polyedra, represent one of the most widespread antipredatory defense mechanisms in the surface ocean. When mechanically stimulated by the shear stress of an approaching predator or breaking waves, these organisms emit a rapid flash of blue-green light lasting approximately one-tenth of a second. For many years, the biochemical and cellular architecture underpinning this millisecond-scale response remained poorly characterized, but recent advancements in microfluorometry and ultrastructural imaging have delineated the precise subcellular pathway responsible for light production.

At the center of dinoflagellate bioluminescence are specialized spherical organelles called scintillons, which project into the cell's acidic central vacuole from the surrounding cytoplasm. Scintillons contain three fundamental biochemical components: the substrate dinoflagellate luciferin, the enzyme luciferase, and a specialized luciferin-binding protein (LBP). In the resting state, the cytoplasm and the scintillons maintain a neutral to slightly alkaline pH of approximately 8.0. At this resting pH, the luciferin is sequestered by LBP in an inactive conformation, preventing it from interacting with luciferase, which itself remains enzymatically inactive at pH values above 7.0.

The bioluminescent cascade begins when mechanical deformation of the cell membrane activates mechanosensitive ion channels, generating an action potential that propagates along the vacuolar membrane (the tonoplast). This membrane depolarization opens voltage-gated proton channels embedded in the scintillon membrane, permitting a rapid influx of protons ($H^+$ ions) from the acidic vacuolar lumen (pH ~3.5) into the scintillon interior. As the internal pH of the scintillon drops sharply to approximately 6.3, two simultaneous biochemical shifts occur: LBP undergoes a conformational change that releases the bound luciferin, and luciferase is catalytically activated. Luciferase then catalyzes the oxidation of free luciferin by molecular oxygen ($O_2$), yielding electronically excited oxyluciferin, which emits a photon at a wavelength of 470 nanometers upon relaxing to its ground state. Following this flash, excess protons are actively pumped back across the tonoplast to restore the resting alkaline state, resetting the system for subsequent mechanical triggers.

Statement:
According to the passage, the catalytic activation of luciferase and the release of luciferin from the luciferin-binding protein both occur when the internal pH of the scintillon drops to approximately 6.3.