Think about this: The freezing point of pure water is 0°C , and our cells are made up of 70% water, then how do fish in the arctic and Antarctic regions not end up freezing, but have a thriving life in the sub zero temperature regions? Well, there are multiple factors involved, let us have a look at them.
The freezing point of water
We know that the freezing point of pure water is 0°C. However, this freezing point depends on multiple factors including, density, salinity and atmospheric pressure.
You may have noticed that in lakes, the top layer freezes first, whereas the water down below is still liquid and the fish and other aquatic life survives in them. But why don’t the lower layers of water freeze?
Unlike other liquids that expand when heated, water doesn’t expand till it is heated above 4°C . At 4°C water has the least volume and maximum density (not 0°C as you may be likely to think). This property of water is called “Anomalous expansion”.
Freezing of water in a lake
When the surface water reaches 4°C, it attains maximum density and hence sinks down and displaces the lower layers to the surface. When the newly emerged layers also reach 4°C, they sink down and displace the layers below. When the temperature reaches 0°C, the surface water freezes into ice, whereas, the bottom layers are still at 4°C and in liquid state. During this process, oxygen gets trapped below the surface, so the aquatic creatures still have an oxygen supply.
In the oceans, as we go deeper, the temperature of the water drops, and the oxygen content also decreases. In such cases, the organisms adapt to their surroundings and have slower metabolic rates that allow them to survive the extreme conditions.
Anti-Freeze Proteins (AFPs) or Ice Binding Proteins (IBPs)
The freezing point of fish blood is about -0.9°C. It is possible for the surrounding regions in the arctic and Antarctic regions to go much below that temperature. It has been found that special proteins called Anti-Freeze Proteins or Ice Restructuring Proteins are found in some organisms that prevent the formation of ice crystals in the fish blood. ( fungi, microalgae, crustaceans fish)
During the process of ice formation, a tiny ice crystal forms which later grows rapidly and leads to all of the surrounding water turning into ice. Anti-Freeze Proteins get adsorbed on to the surface of the tiny ice crystal and do not allow it grow further, thus inhibiting the freezing of the blood or the cells of the organisms.
Now that we know that there are special proteins in fishes that prevent their, freezing, one may ask, will they freeze if we put them in the refrigerator? Yes, they will freeze. the antifreeze proteins have a limit till where they can prevent freezing, Thermal hysteresis is the difference between the freezing and melting point of a liquid. Anti Freeze proteins increase the thermal hysteresis, but not beyond a certain point. If the refrigerator is set at -20°C, the fish will freeze. Moreover, by the time the fish reach your refrigerator, they will most probably be dead and the proteins will be inactive.
Application of AFPs
Researchers are now looking to use the properties of AFPs for human welfare, by using them as cryoprotectants (i.e protection from cold). It is being tested out for the storage of various biological tissues.
In Conclusion, Anti-Freeze Proteins (AFPs) or Ice Binding Proteins (IBPs) in Fish, The Freezing Point of Water and it’s Anomalous Expansion prevent fish from freezing in ice cold water.
Kim, H. J., Lee, J. H., Hur, Y. B., Lee, C. W., Park, S. H., & Koo, B. W. (2017). Marine antifreeze proteins: structure, function, and application to cryopreservation as a potential cryoprotectant. Marine drugs, 15(2), 27.
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Saunri Dhodi Lobo is pursuing M.Sc in Life Sciences with specialization in Neurobiology. Her interests include writing poetry, going for nature walks and swimming. Currently she is involved in research on Alzheimer’s Disease in fruit flies.
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