Why are magnesium ions needed in the firefly light-producing reaction?

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time-lapse photo of fireflies — Bioluminescence Time-lapse image of fireflies in the Catskill Mountains, New York. Firefly light is a cold light, with approximately 100 percent of the energy given off as light and only a minute amount as heat. Only light in the visible spectrum is emitted.

In the firefly light-producing reaction, ionic magnesium is required to facilitate the initial formation of a complex between adenosine triphosphate (ATP), firefly luciferase, and firefly luciferin.12

Specifically, these components react to form a luciferase-luciferyl-adenylate complex and pyrophosphate. This complex is a necessary precursor that subsequently reacts with molecular oxygen to reach a high-energy excited state, which then releases a photon of visible light as it returns to its ground state.12