Why are Antarctic fish not afraid of freezing? | Science Museum

Antarctica is the coldest place on Earth, and the Southern Ocean surrounding Antarctica is the coldest water on Earth. Since the Oligocene 32 million years ago, the Antarctic Circumpolar Current has hindered the heat exchange between the Antarctic Ocean and other seas, causing the water temperature of the Southern Ocean to gradually drop. The seawater temperature around the Antarctic continental shelf remains below 0°C all year round and is covered with ice and snow.

The extremely low temperature has caused the ocean in this area to be "frozen for thousands of miles". This is a frozen "little world". Animals living here often have thick fur and fat. For example, penguins have thick subcutaneous fat, like a "thick coat". These fats provide penguins with energy to resist the cold.

But not all creatures living in Antarctica have the penguins' "thick coats", such as the fish living in Antarctica.

According to researchers' observations, fish living in Antarctica do not have as much fat as penguins, but they still do not freeze. So how do they resist the cold without a "thick coat"?

Why don’t fish in Antarctica freeze to death?

Generally speaking, fish are cold-blooded animals, and their body temperature changes with the water temperature, becoming the same as the water temperature, in order to adapt to the surrounding environment. Research results on fish physiology show that the freezing point of fish blood is generally around -1°C. In other words, if the temperature drops below -1°C, the fish will be frozen into "popsicles". The temperature of Antarctic seawater is below this freezing point all year round, and ordinary fish cannot survive here.

However, scientists have discovered that the fish in Antarctica are not only not frozen into ice, but also swim around and reproduce as if nothing has happened. So why are the fish here not afraid of freezing?

Take the Antarctic cod, the world's least cold-resistant fish, for example. The Antarctic cod is thicker and fatter, and its skin is a natural silver-gray color with black and brown spots. It mainly lives in the relatively cold waters near Antarctica, and can even be found near the Ross Ice Shelf at 82° south latitude.

Scientists have found that compared with fish in other regions, fish living in Antarctica have a special component in their blood - glycogen. A special chemical substance with glycogen as the main component can help these fish cope with the cold. Scientists call this thing antifreeze protein. This antifreeze protein is a high molecular protein that can interact with ice or water to reduce the temperature at which water freezes. In addition, it can automatically adjust with changes in seawater temperature. In summer, antifreeze protein stops producing and continues to work its "magic" until winter.

Antifreeze proteins have two main functions:

The first is thermal hysteresis activity (THA). Antifreeze proteins reduce the freezing point of aqueous solutions in a non-colligative manner and have little effect on their melting point (MP), resulting in a difference between the melting point (MP) and freezing point (FP) of aqueous solutions. That is, they can change the internal environment of icefish, reduce the freezing point of blood, and prevent the formation of ice crystals.

The second is ice recrystallization inhibition (IRI). Ice recrystallization (IR) explains a thermodynamically favorable process in which larger ice crystals are formed at the expense of smaller ice crystals. Larger ice crystals can be lethal to frozen cells and organisms living in polar or cold regions, and antifreeze proteins can inhibit ice recrystallization at very low concentrations. In other words, if ice crystals are really formed, antifreeze proteins will directly stick to the open surface of the ice crystals. This is a direct interference that intervenes in the changes of ice crystals at a physical level, making it impossible for them to expand, melt or refreeze easily. This ensures a safe in vivo environment and keeps the number of ice crystals very small and stable.

This protein in Antarctic fish allows them to remain in a non-freezing state in waters below zero degrees. It is with this "talent" that Antarctic fish can swim freely in cold waters.

What can specific proteins be used for?

With the progress of various scientific and technological advances, the genetic basis of polar fish physiological adaptation to low temperature environments has been revealed in the past 10 years. The thermal hysteresis activity and ice recrystallization inhibition activity of antifreeze proteins make them important in cryopreservation. They can be used to store various types of cells, tissues and organs, fresh and cold foods, etc. at extremely low temperatures. They have become key technologies in the fields of life sciences, medicine and health, food, agriculture, etc.

In the field of reproductive medicine, cryopreservation is one of the key technologies that can solve problems such as infertility and delayed childbirth. Antifreeze protein can be used to cryopreserve tissues and organs such as sperm, eggs and embryos; in the field of surgery, cryosurgery is a key technology for tumor treatment. Studies have reported that injecting a small amount of antifreeze protein into the tissue to be removed before surgery can improve the success rate of the operation and reduce the problem of freezing damage to the target tissue after cryosurgery; in the food field, antifreeze protein as a food additive can inhibit the growth of ice crystals and improve the quality of frozen food; in the agricultural field, antifreeze protein transgenic technology improves the cold resistance of animals and plants, and effectively improves the cold resistance of aquatic economic fish.

Although Antarctic fish antifreeze proteins have good application prospects in the fields of medicine, food, agriculture, etc., there are still some limitations and shortcomings in production costs and industrialization, and production is limited to research and special applications. Therefore, how to produce highly active antifreeze proteins at low cost is the key to future application development.

Creative team: China Science and Technology Museum New Media Team

Review expert: Li Weiyang, a well-known popular science writer