BANGKOK , Thailand — Coral’s survival depends on a special relationship with a kind of algae.

The algae reside in the structure built by corals, and in return produce nutrients for their host by photosynthesis.

But for reasons that still elude scientists, this arrangement falls apart when seawater warms too much and the algae leave or are expelled.

The algae provide coral’s characteristic colors, and their departure leaves behind a ghostly white structure that is gradually starving.

FOURTEEN-YEAR-OLD Nannalin ‘Fleur’ Pornprasertsom surveys bleached corals during her coral conservation and citizen science course at Black Turtle Dive around Koh Tao island in the southern Thai province of Surat Thani.