Molecular mechanisms underlying corals’ response to global climate change: the apoptotic pathway

Global warming and ocean acidification have recently emerged as key threats to the long-term survival of coral reefs. The oceans’ rapid warming has been held responsible for an increase in the frequency and intensity of mass bleaching events (i.e., the disconnection of the symbiotic zooxanthellae- coral association), contributing to the high rate (33%) of coral mortality worldwide. Likewise, the continuous acidification of the oceans has caused significant decreases in the concentration of carbonate ions and compromised the ability of marine calcifiers to precipitate calcium carbonate. However, some coral species are known to survive and recover from bleaching and acidic conditions. Indeed, several studies have suggested (i) that periodic global warming and coral bleaching events have occurred in the past, and (ii) that there have been periods with unfavorable conditions for calcification, characterized by absence of fossilized coral (“reef gaps”). The phylogeny of recent corals, however, places their origin in the pre-Permian extinction. These data indicate that corals have survived bleaching and mass extinction events, the latter possibly by alternating between soft bodies and calcified fossilizing forms, and are able to survive phases of ocean warming and acidification. However, the mechanisms enabling such survival are unclear. We suggest that apoptosis is the main mechanism enabling corals to survive drastic environmental changes, including thermal stress and acidifying conditions, emphasizing the importance of this process in these sessile, simple metazoans.