Icefish blood contains "antifreeze" proteins that are effective down to -2.2°C, according to our partner
The Conversation
.
Wood frogs avoid freezing by recycling urea.
This molecule acts as a cryoprotectant because its accumulation in cells lowers their freezing point.
This analysis was conducted by
Juliette Ravaux
, lecturer in animal biology at the Sorbonne University, and
Sébastien Duperron
, professor of microbial ecotoxicology at the National Museum of Natural History (MNHN).
The major problem for species exposed to extreme cold is the freezing of bodily fluids.
An acute problem, in particular, for those who do not regulate their internal temperature.
Freezing of body fluids severely damages the cells, which risks crushing or bursting, and should be avoided.
The ice fish have found the parade.
These bony fishes inhabit the southern waters of Antarctica and the tip of South America, and belong to the remarkably cold-adapted group of notothenioids.
In their environment, the sea water is around -2°C most of the year, and the ubiquitous ice crystals deposit on their skin, their gills, and enter their bodies when they feed and drink from sea water. Despite this, ice fish do not freeze, and thus live down to -2.2°C.
How do they do ?
Their resistance to cold is linked to the presence of "antifreeze" proteins in their blood and body fluids.
These proteins have the particularity of being bristling with tiny hydrophobic tips which, like a key in a lock, fit perfectly into the nanometric holes formed naturally by the arrangement of water molecules on the surface of the crystals. of ice.
Once bound to the crystals, the proteins prevent them from growing and icing up any liquid present.
They thus lower the temperature at which ice forms below the natural freezing point of bodily fluids (–0.7 to –1°C).
They therefore act as protective agents against freezing, also called cryoprotectants.
By their ability to control the formation of ice, these antifreeze proteins have potential for application in many fields: agriculture, to develop frost-resistant plants, the food industry to preserve the structure of original frozen foods animal or plant, or medicine for freezing living tissues or cell cultures.
Paradoxically, this protective mechanism could be fatal to Antarctic fish.
Antifreeze proteins bind irreversibly to ice crystals, and they stabilize them so that they only melt at relatively high temperatures.
However, in the habitat of these fish, these are never reached;
monitoring over more than a decade between 2000 and 2013 reports values oscillating between – 2 and – 0.5°C.
The stabilized ice crystals therefore accumulate in the animal's tissues, invading the blood, the digestive system, and even the spleen.
We do not yet know how the animal eliminates or stores these deleterious crystals… Antifreeze proteins therefore do not on their own explain the resistance of Antarctic fish to cold.
A frog sorbet
Unlike Antarctic fish or Arctic springtails which fight against the formation of ice, other animals resist extreme cold… by freezing!
These animals are said to be frost tolerant.
This is the case of the wood frog
Rana sylvatica
, which inhabits the boreal forests of Alaska and Canada.
In the fall, it builds a shelter for hibernation: a simple hole in the forest floor covered with leaves and debris, then snow.
She takes refuge there until spring, enduring sub-zero temperatures for seven months with minimums of -15°C inside the shelter, while the temperature outside drops to -40°C.
The frog manages the dehydration of its tissues and the formation of ice crystals, without however completely avoiding freezing.
She thus survives the Arctic winter by tolerating that her body freezes up to 60% of its volume!
How does the wood frog resist this shock treatment?
Its survival is partly due to its ability to accumulate and recycle a metabolic waste product: urea.
In the fall, she stops urinating and stores urea in her tissues.
This molecule acts as a cryoprotectant, since its accumulation in cells lowers their freezing point.
In addition, it retains water in the cells and thus prevents them from becoming dehydrated.
Indeed, when ice crystals form in the liquids surrounding the cells, the latter become saltier since the quantity of liquid water decreases.
As water moves naturally from the least salty environment to the most salty, it will be gradually drained out of the cells.
The presence of urea counters this phenomenon, because by increasing the concentration of small molecules in the cell, it restores a balance between the inside and the outside.
There is then no more water leakage, and the cells can remain intact although ice crystals form in the extracellular fluids.
In addition to its protective role, this nitrogen-rich waste also serves as a source of energy.
The gut of the wood frog is home to a rich population of bacteria capable of degrading urea.
These bacteria are much more active and efficient during hibernation.
They produce twice as much urease, the enzyme that degrades urea, during the dormant phase, and the enzyme then has an activity three times greater than that measured in active frogs.
This degradation of urea releases nitrogen which is used to renew cellular components.
This recycling of nitrogen from urea provides a nutrient supply that helps the frog survive hibernation, and restart its metabolism in the spring before it begins to feed again.
In autumn, the frog produces another cryoprotective molecule, glucose.
The liver then releases large amounts of glucose from glycogen stores, which leads to an increase in the sugar content of the tissues.
Glucose accumulates in the cells, and is added to the action of urea to retain water and avoid the concomitant dehydration with the formation of ice in the extracellular liquids.
The amounts of glucose are five times higher in the muscles of the thighs, and thirty times higher in the heart, in Alaskan frogs frozen in the laboratory compared to non-frozen frogs.
And these quantities increase further by a factor of ten in frogs frozen in their natural environment.
This exceptional accumulation is explained by the alternation of partial freezing and thawing cycles during the fall.
In the forests of Alaska, as of October, the night temperature frequently drops below -1°C, the freezing point of the wood frog.
During this first month of autumn, the batrachian experiences an average of twelve cycles of freezing and thawing.
OUR “EVOLUTION” FILE
The freezing phases lead to chronic production of glucose by the liver, whereas partial thawing is not accompanied by a resumption of the metabolism which would consume this glucose.
In winter, the soil of arctic forests thus hides frozen frogs, rich in urea and glucose, and therefore bitter and sweet!
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