Coral reefs cover less than 1% of the ocean floor, yet support roughly a quarter of all marine species, protect coastlines from storms and provide food and income for hundreds of millions of people. They are also disappearing. Roughly half of the living coral on the world’s reefs has been lost since the 1950s, and more than 80% of reefs have been affected by bleaching caused by heat stress in the last few years.

So, what is coral? Corals are animals made up of colonies of thousands of tiny, genetically identical polyps that are relatives of jellyfish and sea anemones. Each polyp has a central mouth with a ring of stinging tentacles. They secrete a hard skeleton of calcium carbonate and these skeletons build up over centuries into coral reefs.

Although corals can catch food in their tentacles, up to 90% of their energy comes from a mutualistic relationship with microscopic algae called zooxanthellae, which live inside polyp cells in a membrane-bound compartment called a symbiosome. The algae photosynthesise and pass glucose to the coral and also give it its colour. In return, the coral provides shelter, carbon dioxide and nutrients. But the partnership only works in a narrow temperature range. A rise of just 1 to 2°C above the normal summer maximum for a few weeks stresses the algae, causing the coral to expel them. This leads to the loss of colour, known as bleaching, and also to the gradual starvation of the coral.

Researchers at Stanford University have found a surprisingly simple rescue. The team tested ten pairs of coral fragments, each pair was cut from a single coral, making the two fragments genetically identical clones: one healthy (with algae) and one bleached (with few or no algae). In five pairs the fragments touched and in the other five they were kept a few millimetres apart.

Every bleached fragment that touched its healthy partner fully recovered within seven weeks, while those kept apart died. The algae from the healthy partner moved across within three days, then divided about every four days, ten times faster than in wild corals. Under the microscope the corals recognised each other as identical and fused their tissues, forming channels that let algae and nutrients flow across, a bit like an organ transplant between identical twins and therefore with no rejection.

There is a catch: every bleached coral needs a genetically identical healthy partner, which is impractical for a whole reef. The researchers' next goal is to move the algae without the corals having to touch. One idea is to collect an algae sample from healthy corals before a marine heatwave and then add it to bleached corals afterwards so they quickly regain their algae. It won't solve climate change, but it could keep corals alive long enough for natural recovery to take over.

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Links to the specifications

AQA A-level Biology: photosynthesis and energy transfer (5.1); mutualism, populations and the effect of abiotic factors in ecosystems (7.4); the impact of climate change and human activity on biodiversity (7.4 / 7.1); cell division and recognition of self and non-self (2.2, 2.3).

OCR A-level Biology A: photosynthesis (5.2.1); biodiversity and the effects of climate change on ecosystems, and conservation (4.2.2, 6.3.2); cloning and genetic identity (6.3.1); cell signalling and recognition of cells as self (2.1, 4.1).

Discussion Points:

1. Why does bleaching lead to starvation in the coral? Use your knowledge of photosynthesis and energy transfer.
2. Why might genetically identical tissues fuse while non-identical ones do not? Think about antigens and recognition of self.
3. If we restored reefs using only clones, what risk would this pose to genetic diversity?