Increased zooxanthellae nitric oxide synthase activity is associated with coral bleaching

Coral bleaching, the breakdown of the symbiotic relationship between host corals and their photosynthetic dinoflagellate endosymbionts, is a phenomenon of major ecological significance. The cellular and molecular mechanisms underlying bleaching are poorly understood. Here we report substantial increases in nitric oxide synthase (NOS) activity in symbionts associated with bleaching corals. Nitric oxide (NO), a product of NOS activity, is a highly reactive and membrane-permeable molecule that has the potential to exert cytotoxic effects in host cells either directly or in combination with molecular oxygen or superoxide radicals. This potential allows us to suggest that coral bleaching may be a stress response initiated by the symbiont, rather than by the host.