Coral larvae survive being frozen and thawed for the first time

A technique called cryopreservation might help save some threatened coral reefs

adult coral

DEEP FREEZE  The first coral larvae to survive being frozen and then thawed by scientists are a kind of a mushroom coral (close-up of an adult coral shown with scattered greenish tentacles).

Smithsonian Conservation Biology Institute

For the first time, researchers have quick-frozen coral larvae and then — the tough part — safely thawed them.

Swathing larvae in specks of gold and then heating them with a laser warmed the frozen coral babies in milliseconds. Thawed this way, 43 percent of 2-day-old test larvae recovered well enough to start swimming again, physiologist and cryobiologist Mary Hagedorn and her colleagues report October 24 in Scientific Reports.

This success with Fungia scutaria larvae, a colorful mushroom coral species from Hawaii, opens up much-needed conservation possibilities, says Hagedorn, of the Smithsonian Conservation Biology Institute in Front Royal, Va. Biologists knew how to freeze coral sperm, but ice crystal damage from the process can kill bigger, fat-rich structures such as coral eggs and larvae. Being able to bank more than sperm may help preserve coral genetic diversity for attempts to aid reefs that falter as the climate warms.

JUST CHILL Microscope images show the tiny cigar shape of a 3-day-old larva of a Fungia scutaria mushroom coral marked with green fluorescent protein (left) and surrounded by tiny gold particles used for quick thawing the coral (right). J. Daly et al/Scientific Reports 2018

Biologists have been freezing and storing human embryos since at least the 1980s, but the first success with any nonmammal is recent, Hagedorn says. In 2017, she, John Bischof of the University of Minnesota Twin Cities and colleagues described how warming zebrafish embryos with gold nanorods let 10 percent of the embryos survive at least 24 hours after thawing.

In a procedure loosely based on the zebrafish system, researchers enveloped larvae in an antifreeze solution with gold nanorods and froze the creatures in milliseconds with liquid nitrogen. But that wasn’t the end of the danger from ice crystals. Even during thawing, crystals can enlarge dangerously. That’s when a laser pulse quickly warmed the gold bits and thus the frozen larvae.

“It’s a bit like making a soufflé — everything has to be perfect,” Hagedorn says.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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