Adult Corals Frozen and Revived Safely by Scientists To Ensure Survival in the Face of Climate Change

All over the world, the population of coral reefs has been declining due to the effects of global warming. The unusual reduction of coral reefs is manifested at the regional level, characterized by drastic changes in the composition of coral assemblages.

Due to ocean warming and acidification, most reefs cannot maintain positive net carbonate production. As the ocean's temperatures rise, the corals find it hard to acclimate or adapt quickly, increasing the frequency and intensity of bleaching events. To prevent the projected decline in coral reefs, mitigation strategies are implemented by scientists using new and radical interventions like cryopreservation.

What is Cryopreservation?

Cryopreservation refers to preserving biological materials such as organelles, cells, and tissues by keeping them in a deep freeze for long periods without affecting the cell's viability. This method generally uses dry ice and liquid nitrogen to cool the samples to very low temperatures, commonly at -196°C.

At such low temperatures, the cells stop all their biological activities and die, so cryopreservation is a way of helping the cells survive freezing and thawing. Since the formation of ice inside the cells can break the cell membrane, scientists regulate the freezing rate and choose the freezing medium carefully.

Coral fragments are among the most complex biological systems successfully maintained through the cryopreservation and thawing processes. However, conventional cryopreservation techniques rely heavily on freezing sperm and larvae, which can only be obtained during fleeting spawning events. Since these events only occur a few days a year for a species, it limits the speed with which the corals can be successfully stored. Aside from this, the continuous warming of oceans and frequent marine heatwaves make the corals biologically stressed so that their reproductive material becomes too weak for the cryopreservation and thawing processes.

Producing Adult Corals from Frozen Larvae

At the University of Hawaiʻi at Mānoa's Hawaiʻi Institute of Marine Biology (HIMB), research biologists conducted the first successful technique of freezing and reviving entire coral fragments. This breakthrough in Kāneʻohe Bay at HIMB is a forerunner for a new age for cryopreservation and coral conservation.

Led by research biologist Mary Hagedorn, the HIMB team started research in 2019, where they cryopreserved and revived entire fragments of finger coral (Porites compressa) from Hawaii. They used a new process called isochoric vitrification to preserve fragments of about 20 individual polyps within a calcium carbonate skeleton.

In carrying out the study, the scientists searched for a healthy adult coral and harvested off a thinly sliced sample the size of a human thumbnail. The coral was bleached, and the fragment was placed into a small aluminum cylinder containing an antifreeze solution. Although the process aimed to freeze the coral piece, the researchers ensured that ice formation was avoided to save the animal's tissue from getting damaged. Finally, the cylinder was submerged in liquid nitrogen and was allowed to cool to nearly -321°F, causing the coral to freeze rapidly.

The cylinder was placed in a warm water bath for two minutes to revive the coral. Then, the coral fragments are removed and put back into the seawater.

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