The worm that can survive to intracellular freezing
It is known that the Antarctica is one of earth’s harshest environments, challenging its inhabitants with extreme cold and drought, requiring extreme adaptation mechanisms, but a lot of organism have adapted to live in freezing cold conditions, but not many are able to survive intracellular ice formation . An example of this is Panagrolaimus davidi, a bacterial feeding anhydrobiotic nematode.
Panagrolaimus sp. is an Antarctic nematode living associated with moss and algae in terrestrial habitats on the Victoria Land coast that are free of snow and ice for part of the year. It has to survive very variable thermal and hydric environments where liquid water and temperatures suitable for growth are only periodically available. Panagrolaimus sp. can survive complete water loss and is the only organism that has been shown to survive intracellular ice formation throughout its tissues.
Usually intracellular ice formation is fatal because mechanical disruption may be caused by the expansion of water as it freezes, the puncturing of membranes by ice crystals and the redistribution of ice crystals after freezing and during defrosting.
The recent study in the Antarctic shows the capacity of Panagrolaimus sp. to withstand ice formation intracellularly and survive, and been able to produce progeny once defrosted.
Panagrolaimus sp. has a thick cuticle, which allowed to survive in sub-zero conditions, but the cuticle is insufficient because ice can penetrate intracellular compartments trough the excretory pore and other orifices and unlike other organisms that can live in low temperatures, there is no evidence to show that Panagrolaimus sp. possesses ice-active proteins.
By exploring patterns of gene expression, researchers were able to show how nematodes are molecularly active in the frozen state, highlighting certain key genes that allow them to withstand such extreme physical state.
So, how is it possible that this nematode survives to intracellular freezing? Well this still a not fully understood. For now it is known that Panagrolaimus sp. uses a cryoprotective dehydration technique, which means that if freezing of the nematode’s surroundings occurs at a high sub-zero temperature, the force for inoculative freezing into the body of the nematode is insufficient for freezing to occur and the nematode remains unfrozen. Water is then lost to the surrounding ice, since super-cooled water has a higher vapour pressure than ice at the same temperature, and the nematode survives. However, even if Panagrolaimus sp. is cooled faster, it is frozen while hydrated can experience intracellular ice formation and survive. So this nematode has more than the usual techniques to maintain life while intracellularly frozen, which now still unknown.
The most accepted answer, conclude by Wharton (2003) is that the extreme cold tolerance of Panagrolaimus sp. compared to other nematodes may arise from a combination of rapid freezing, trehalose production and a recrystallization inhibiting ice-active protein, But this protein has not been syntetize in lab studies yet.
Nowadays still not clear how the organism can survive to intracellular freezing but it is known that is related to the amount of nutritional reserves in the body. An study by Raymond and Wharton (2013) show that the ability to survive a freezing exposure that is likely to produce intracellular freezing declines with culture age. In cultures that are fed regularly, the ability to survive freezing at −10 °C increases, but in starved cultures freezing survival declines.
Panagrolaimus sp. is the most studied nematode that can survive freezing, but nobody has proved the technique that this worm uses to survive to intracellular freezing.