Growing Coral Back
Coral reefs cover only a tiny part of the ocean floor, yet they are home to a remarkably large share of marine life. In recent decades, rising sea temperatures have caused repeated episodes of bleaching, in which stressed corals expel the algae that live in their tissues. Without these algae, which supply most of their food, the corals turn white and, if conditions do not improve, slowly starve.
One response has been to grow coral deliberately, much as a forester raises seedlings. In the method known as coral gardening, divers cut small fragments from healthy colonies and hang them from underwater frames, made of plastic pipe and shaped like trees. Suspended in open water, the fragments are safe from sediment and from many of the animals that graze on reefs, and they can grow several times faster than they would on the seabed. After six to twelve months they are large enough to be fixed onto damaged reef, usually with a small amount of marine cement.
Gardening has an obvious limit: each fragment must be handled by a diver, so restoring even a single hectare is slow and expensive. A newer technique, microfragmenting, tries to speed up the process. Researchers found that when coral is cut into pieces only a centimetre or two across, the pieces grow unusually quickly, and when several fragments from the same colony are placed close together, they fuse into one larger colony. A patch that might take decades to form naturally can be covered in two or three years.
A third line of work focuses on reproduction. Many corals release their eggs and sperm on just a few nights each year, and only a tiny fraction of the resulting larvae survive. By collecting this spawn and rearing the larvae in tanks until they settle, teams can release very large numbers of young corals at once. This approach also produces genetic variety, which fragment-based methods cannot, since every fragment is a copy of its parent.
None of these methods tackles the cause of bleaching. Restoration scientists are generally frank that their work buys time rather than offering a cure, and that the long-term survival of reefs depends on limiting the warming of the oceans. Some teams are therefore breeding corals that tolerate heat better, in the hope that restored reefs will survive the next marine heatwave.