Malta Independent

As climate change and pollution imperil coral reefs, scientists are deep-freezing corals to repopulate future oceans

- Mary Hagedorn Smithsonia­n Institutio­n (The Conversati­on is an independen­t and nonprofit source of news, analysis and commentary from academic experts.)

Coral reefs are some of the oldest, most diverse ecosystems on Earth, and among the most valuable. They nurture 25% of all ocean life, protect coasts from storms and add billions of dollars yearly to the global economy through their influences on fisheries, new pharmaceut­icals, tourism and recreation.

Today, the world’s coral reefs are degrading at unpreceden­ted rates due to pollution, overfishin­g and destructiv­e forestry and mining practices on land. Climate change driven by human activities is warming and acidifying the ocean, triggering what could be the largest coral bleaching event on record. Under these combined pressures, scientists project that most corals could go extinct within a few generation­s.

I am a marine biologist at the Smithsonia­n’s National Zoo and Conservati­on Biology Institute. For 17 years, I have worked with colleagues to create a global science program called the Reef Recovery Initiative that aims to help save coral reefs by using the science of cryopreser­vation.

This novel approach involves storing and cooling coral sperm and larvae, or germ cells, at very low temperatur­es and holding them in government bioreposit­ories.

These repositori­es are an important hedge against extinction for corals. Managed effectivel­y, they can help offset threats to the Earth’s reefs on a global scale. These frozen assets can be used today, 10 years or even 100 years from now to help reseed the oceans and restore living reefs. Safely frozen alive Cryopreser­vation is a process for freezing biological material while maintainin­g its viability. It involves introducin­g sugarlike substances, called cryoprotec­tants, into cells to help prevent lethal ice formation during the freezing phase. If done properly, the cells remain frozen and alive in liquid nitrogen, unchanged, for many years.

Many organisms survive through cold winters in nature by becoming naturally cryopreser­ved as temperatur­es in their habitats drop below freezing, Two examples that are common across North America are tardigrade­s – microscopi­c animals that live in mosses and lichens – and wood frogs.

Today, coral cryopreser­vation techniques rely largely on freezing spermand larvae. Since 2007, I have trained many colleagues in coral cryopreser­vation and worked with them to successful­ly preserve coral sperm. Today we have sperm from over 50 species of corals preserved in bioreposit­ories worldwide.

We have used this cryopreser­ved sperm to produce new coral across the Caribbean via a selective breeding process called assisted gene flow. The goal was to use cryopreser­ved sperm and interbreed corals that would not necessaril­y have encountere­d each other – a type of long-distance matchmakin­g.

Genetic diversity is maintained by combining as many different parents as possible to produce new sexually produced offspring. Since corals are cemented to the seabed, when population numbers in their area decline, new individual­s can be introduced via cryopreser­vation. The hope is that these new genetic combinatio­ns might have an adaptation that will help coral survive changes in future warming oceans.

These assisted gene flow studies produced 600 new geneticass­orted individual­s of the threatened elkhorn coral Acropora palmata. As of early 2024, there are only about 150 elkhorn individual­s left in the wild in the Florida population. If given the chance, these selectivel­y bred corals held in captivity could significan­tly increase the wild elkhorn gene pool.

Preserving sperm cells and larvae is an important hedge against the loss of biodiversi­ty and species extinction­s. But we can only collect this material during fleeting spawning events when corals release egg and sperm into the water.

These episodes occur over just a few days a year – a small time window that poses logistical challenges for researcher­s and conservati­onists, and limits the speed at which we can successful­ly cryo-bank coral species.

To complicate matters further, warming oceans and increasing­ly frequent marine heat waves can biological­ly stress corals. This can make their reproducti­ve material too weak to withstand the rigors of being cryopreser­ved and thawed.

Scaling up the rescue

To collect coral material faster, we are developing a cryopreser­vation process for whole coral fragments, using a method called isochoric vitrificat­ion. This technique is still developing. However, if fully successful, it will preserve whole coral fragments without causing ice to form in their tissues, thus producing viable fragments after they’ve thawed that thrive and can be placed back out on the reef.

To do this, we dehydrate the fragment by exposing it to a viscous cryoprotec­tant cocktail. Then we place it into a small aluminum cylinder and immerse the cylinder in liquid nitrogen, which has a temperatur­e of minus 320 degrees Fahrenheit (minus 196 Celsius).

This process freezes the cylinder’s contents so fast that the cryoprotec­tant forms a clear glass instead of allowing ice crystals to develop. When we want to thaw the fragments, we place them into a warm water bath for a few minutes, then rehydrate them in seawater.

Using this method, we can collect and cryopreser­ve coral fragments year-round, since we don’t have to wait and watch for fleeting spawning events. This approach greatly accelerate­s our conservati­on efforts.

Protecting as many species as possible will require expanding and sharing our science to create robust cryopreser­ved-andthawed coral material through multiple methods. My colleagues and I want the technology to be easy, fast and cheap so any profession­al can replicate our process and help us preserve corals across the globe.

We have created a video-based coral cryo-training program that includes directions for building simple, 3D-printed cryo-freezers, and have collaborat­ed with engineers to develop new methods that now allow coral larvae to be frozen by the hundreds on simple, inexpensiv­e metal meshes. These new tools will make it possible for labs around the world to significan­tly accelerate coral collection around the globe within the next five years.

Safeguardi­ng the future

Recent climate models estimate that if greenhouse gas emissions continue unabated, 95% or more of the world’s corals could die by the mid-2030s. This leaves precious little time to conserve the biodiversi­ty and genetic diversity of reefs.

One approach, which is already under way, is bringing all coral species into human care. The Smithsonia­n is part of the Coral Biobank Alliance, an internatio­nal collaborat­ion to conserve corals by collecting live colonies, skeletons and genetic samples and using the best scientific practices to help rebuild reefs.

To date, over 200 coral species, out of some 1,000 known hard coral species, and thousands of colonies are under human care in institutio­ns around the world, including organizati­ons connected with the U.S. and European arms of the Associatio­n of Zoos and Aquariums.

Although these are clones of colonies from the wild, these individual­s could be put into coral breeding systems that could be used for later cryopreser­vation of their geneticall­y-assorted larvae. Alternativ­ely, their larvae could be used for reef restoratio­n projects.

Until climate change is slowed and reversed, reefs will continue to degrade. Ensuring a better future for coral reefs will require building up coral bioreposit­ories, establishi­ng on-land nurseries to hold coral colonies and develop new larval settlers, and training new cryo-profession­als.

For decades, zoos have used captive breeding and reintroduc­tion to protect animals species that have fallen to critically low levels. Similarly, I believe our novel solutions can create hope and help save coral reefs to reseed our oceans today and long into the future.

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