Gardening the Reef

ACoral reefs cover well under one per cent of the ocean floor yet shelter roughly a quarter of all marine species, protect coastlines from storm waves, and support the food and income of hundreds of millions of people. They are also dying at a pace visible within a single human lifetime. Marine heatwaves cause corals to expel the microscopic algae that feed and colour them — the ghostly whitening known as bleaching — and repeated bleaching kills. The world's reefs have lost around half their living coral since mid-century, and mass bleaching events that once arrived decades apart now return within a few years, faster than reefs can naturally recover.

BOut of this emergency has grown a practical, hands-in-the-water discipline: coral restoration. Its workhorse method is disarmingly like gardening. Divers collect fragments of healthy coral — sometimes pieces already broken by storms — and hang them in underwater nurseries, on frames and rope lines where water flow is good and predators are few. Freed from competition, fragments grow several times faster than they would on the reef. After months of fattening, each piece is 'outplanted': fixed to cleaned rock with cement, ties or pegs. A refinement called microfragmentation cuts slow-growing boulder corals into fingernail-sized pieces, which then regrow their tissue at up to forty times the natural rate — turning a decade of growth into a couple of years.

CThe results, at their best, are genuinely moving: bare rubble fields in Indonesia and the Caribbean transformed within a few years into dense thickets loud with fish. But honest practitioners are frank about the record. Reviews of hundreds of projects find survival of outplanted corals highly variable, with many projects monitored for only a year or two — too short to know whether a restored patch persists. Costs are high, scales are tiny: all the world's outplanting together restores each year only a small fraction of what a single serious bleaching event kills. And a garden replanted under a heat lamp will simply cook again: restoration that ignores water temperature treats the symptom while the fever climbs.

DThe field's answer has been to stop replanting the past and start engineering for the future. 'Assisted evolution' programmes, pioneered in Australia and Hawaii, search bleaching-scarred reefs for the rare colonies that survived, breed them together, and expose their offspring to controlled heat stress, selecting the toughest for outplanting. Researchers also transplant heat-adapted algal partners into corals, and practise 'assisted gene flow' — moving corals from the warmest natural lagoons into cooler areas expected to heat up. Ambitions have scaled with the crisis: Australian teams have trialled 'coral IVF', releasing millions of larvae from heat-tolerant parents onto damaged sections of the Great Barrier Reef, and cryobanks in several countries now store frozen coral sperm and larvae — a genetic ark against futures no one wants to need.

ENo serious scientist in the field claims restoration can save reefs while the water keeps warming; the consensus, repeated in paper after paper, is that it buys time and preserves options, nothing more. But the time may matter enormously. If emissions fall on anything like the promised schedules, oceans will stabilise late this century — and whether reefs revive then depends on what survives until then. Restoration, on this view, is less rebuilding than triage: keeping seed populations, genetic diversity and local livelihoods alive across the dangerous decades, so that when the fever finally breaks, there is still a patient to recover.