April 13, 2024

As climate change and pollution endanger coral reefs, scientists are freezing corals to repopulate future oceans.

Coral reefs are some of the oldest and most diverse ecosystems on Earth and are among the most valuable. They feed 25% of all ocean life, protect coasts from storms and add billions of dollars annually to the global economy through their influence 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 oceans, producing a reef crisis that could cause the extinction of most corals within a few generations.

I’m a marine biologist at the Smithsonian’s National Zoo and Conservation Biology Institute. For 17 years, I worked with colleagues to create a global scientific program called the Reef Recovery Initiative, which aims to help save coral reefs through the use of the science of cryopreservation.

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

These repositories are an important protection against coral extinction. Managed effectively, they can help offset threats to Earth’s reefs on a global scale. These frozen assets can be used today, 10 years from now, or even 100 years from now, to help replant the oceans and restore living reefs.

Smithsonian scientists use cryopreserved coral sperm to increase the genetic diversity of Elkhorn coral.

Safely Frozen Alive

Cryopreservation is a process of freezing biological material while maintaining its viability. It involves introducing sugar-like substances, called cryoprotectants, into cells to help prevent the lethal formation of ice during the freezing phase. If done correctly, cells remain frozen and alive in liquid nitrogen, unchanged, for many years.

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

Today, coral cryopreservation techniques largely rely on freezing sperm and 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 more than 50 coral species preserved in biorepositories around the world.

We use this cryopreserved sperm to produce new corals throughout the Caribbean through a selective breeding process called assisted gene flow. The aim was to use cryopreserved sperm and crossed corals that wouldn’t necessarily have met – a type of long-distance matchmaking.

Genetic diversity is maintained by combining as many different parents as possible to produce new sexually produced offspring. As corals are cemented to the seabed, when the number of populations in their area decreases, new individuals can be introduced through cryopreservation. The hope is that these new genetic combinations may have an adaptation that helps corals survive changes in future ocean warming.

Corals in Kaneohe Bay, Hawaii, during 2014 and 2015 warming events in which more than 80% of corals were affected. Some species and individuals, like the coral on the left, were resistant to warming.
Claire Lager, SmithsonianCC BY-ND

These assisted gene flow studies produced 600 new genetically varied individuals of the endangered Elkhorn coral. Acropora palmata. As of early 2024, there are only about 150 Elkhorn individuals left in the wild in the Florida population. Given the opportunity, these selectively bred and captive corals could significantly increase the wild elk antler gene pool.

The preservation of sperm and larvae is an important protection against biodiversity loss and species extinction. But we can only collect this material during fleeting spawning events, when corals release eggs and sperm into the water.

These episodes occur for just a few days a year – a small window of time that presents logistical challenges for researchers and conservationists, and limits the speed at which we can successfully stock coral species.

To further complicate the situation, warming oceans and increasingly frequent marine heat waves can cause biological stress to corals. This can make your reproductive material too weak to withstand the rigors of cryopreservation and thawing.

An Elkhorn coral produced through assisted gene flow, showing vigorous growth and development.
Cody EngelsmaCC BY-ND

Expanding the rescue

To collect coral material more quickly, we are developing a cryopreservation process for whole coral fragments, using a method called isochoric vitrification. This technique is still under development. However, if completely successful, it will preserve entire fragments of coral without causing ice to form in their tissues, thus producing viable fragments once thawed that thrive and can be placed back on the reef.

To do this, we dehydrated the fragment by exposing it to a viscous cryoprotectant cocktail. We then placed it in a small aluminum cylinder and immersed the cylinder in liquid nitrogen, which has a temperature of minus 320 degrees Fahrenheit (minus 196 Celsius).

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

Using this method, we can collect and cryopreserve coral fragments year-round, as we do not need to wait and observe 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 and thawed coral material through multiple methods. My colleagues and I want the technology to be easy, fast and cheap so that any professional can replicate our process and help us preserve corals around the world.

We created a video-based coral cryogenic training program that includes instructions for building simple 3D-printed cryofreezers, and we collaborated with engineers to develop new methods that now allow coral larvae to be frozen by the hundreds simply and cheaply. metal meshes. These new tools will allow laboratories around the world to significantly accelerate the collection of corals around the world over the next five years.

Without coral reefs, the world would lose a valuable source of food, coastal protection, medicines and income – and some of the world’s most beautiful and unique ecosystems.

Protecting 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 reef biodiversity and genetic diversity .

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

To date, more than 200 coral species, out of approximately 1,000 known hard coral species, and thousands of colonies are in human care in institutions around the world, including organizations linked to the North American and European branches of the Association of Zoos and Aquarius. Although these are clones from wild colonies, these individuals could be placed in coral breeding systems that could be used for further cryopreservation of their genetically varied larvae. Alternatively, their larvae could be used in reef restoration projects.

Until climate change is slowed and reversed, reefs will continue to degrade. Securing a better future for coral reefs will require the construction of coral biorepositories, the establishment of land-based nurseries to maintain coral colonies and develop new larval colonizers, and the training of new cryo-professionals.

For decades, zoos have used captive breeding and reintroduction to protect animal species that have fallen to critically low levels. Likewise, I believe our new solutions can create hope and help save coral reefs to replant our oceans today and in the future.

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