Coral kick out their algae partners as the oceans heat up.

Tchernova, D, H Kvitta, T Haramatyd, TS Bibbyd, MY Gorbunovd, H Rosenfeldc and PG Falkowskid. 2011. Apoptosis and the selective survival of host animals following thermal bleaching in zooxanthellate corals. Proceedings of the National Academy of the Sciences http://dx.doi.org/10.1073/pnas.1106924108.

The mystery of why some coral species lose their algae partners, bleach and die as ocean temperatures rise may be tied to tiny molecular "generals" inside the coral cells. The "generals" control harmful oxygens – called oxidants – the algae produce as the oceans warm.

In some corals, these "generals" – formally known as caspases – shift into overdrive to deal with the oxidants and eventually kill or kick out the algae they depend on for food.

Researchers report the complex story of interdependency in the journal the Proceedings of the National Academy of Sciences. The study provides new evidence as to why some corals are more susceptible than others to rising water temperatures linked to climate change.

In the study, the coral species that died the quickest under higher temperatures – some in just a week – also had the highest caspase activity. Corals that lived the longest had up to 10 times less caspase activity.

Coral reefs cover less than one percent of the ocean floor but contain more than a quarter of the ocean’s biodiversity. They are threatened by destructive fishing practices, runoff, coral mining, tourism, coastal development – and increasingly – by warming ocean temperatures.

Coral and the algae that live in them rely on each other: algae provide food for the coral while the coral protect and shelter the algae. When water temperatures rise, coral can “bleach” and lose their bright colors. The colors indicate that a healthy population of algae lives in the coral. Loss of algae is often, but not always, followed by coral death.

If water temperatures get too warm, the symbiotic algae become stressed and produce reactive oxygen species (ROS) – a term for different chemical signals that cause oxidative damage to cells. Because oxidants damage cells in humans, too, some people jump to eat foods loaded with antioxidants – molecules that can counteract the harmful ROS.

Reef-forming corals deal with excess ROS in a similar way as people do. The tiny caspase molecules in their cells start up when ROS is produced. Once activated, caspases can set off a chain reaction that can ultimately kill the cell. The excess ROS that seeps into surrounding coral tissue may send the coral cell caspases into overdrive, leading to bleaching and possible coral cell death.

To figure this out, researchers in Israel and the United States teamed up to examine the effects of temperature and sunlight on different types of corals. They raised the corals at a normal temperature of 26ºC (about 79ºF). Then they increased the water temperature by six degrees to 32ºC (about 90ºF).

The scientists found that some corals were more sensitive to bleaching than others, with one type bleaching after one week at 32ºC while another type took six times as long. Also at 32ºC, scientists found that a sensitive coral – one that quickly succumbed to bleaching – had six times as much caspase activity. In comparison, a resistant coral had 10 times less caspase activity.

When the researchers used a chemical to impair caspase activity, they were able to completely block bleaching in corals grown in elevated temperatures.

Importantly, the researchers suggest that after bleaching, some corals still have the ability to survive. Bleached corals could be repopulated by algae that are possibly more resistant to elevated water temperatures, allowing for potential recovery of coral reefs.

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