Dark Priming — Nocturnal Melatonin Failure Under Nighttime Warming Triggers Bleaching

Corals are famous for 'bleaching' — turning ghostly white when stressed by heat, which happens when they expel the colorful algae living inside them that provide up to 90% of their energy. What's less appreciated is that corals aren't just passive victims of hot days; their ability to cope with daytime stress depends heavily on what happens the night before. Meanwhile, in plants, a hormone called melatonin — yes, the same one that helps humans sleep — plays a crucial role in nighttime chemical repair, acting as a kind of antioxidant janitor that cleans up damage while the organism rests. This hypothesis connects those two worlds with a provocative idea: corals and their resident algae may also rely on melatonin during the night to rebuild their chemical defenses — specifically their 'antioxidant buffer,' a toolkit of molecules that neutralize harmful reactive oxygen that builds up during heat stress. If ocean nights are getting warmer due to climate change, the theory goes, this melatonin-driven repair process gets disrupted or depleted before dawn even arrives. The coral then faces the heat of a new day essentially undefended — a phenomenon the researchers call 'dark priming' for bleaching. Think of it like a phone that never fully charges overnight. Even if daytime temperatures aren't record-breaking, a coral whose nighttime recovery was sabotaged could bleach under heat that a well-rested reef would shrug off. This reframes bleaching not just as a daytime heat problem but as a cumulative, around-the-clock stress failure — with nights playing a surprisingly central role.

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