Even though 99% of the Antarctic land mass is smothered in ice, it is one of the driest places on earth. Few plants and animals have risen to the challenge of surviving at the continent's bitterly cold temperatures, but even though the continent's temperature can fall forty degrees below freezing, some nooks and crannies manage to reach temperatures that would be pleasing even in temperate zones. However, surviving the daily freeze/thaw cycle adds another challenge to microscopic nematodes that must protect themselves from the extensive damage wreacked by ice crystals. David Wharton is fascinated by how animals survive in the planet's harshest environments, and recently became interested in how Antarctic nematodes survive freezing. Amazingly, Panagrolaimus davidi is one of the few creatures that can survive intracellular freezing, but when Wharton looked at the way that the nematode responds at temperatures close to zero, he found that rather than freezing solid, the nematodes survive by freeze-drying(p. 215).
Wharton describes how he found the nematodes in a coastal valley when he visited Antarctica in 1989. The nematodes were living on a patch of algae that were growing in a snow-melt stream. When he returned to his New Zealand lab,he realised that the nematodes were remarkably easy to keep in culture,flourishing at 25°C and producing another generation every week, as opposed to the year that it takes the worms to reproduce in their natural environment.
Fascinated by how the worms survived freezing, Wharton had watched ice crystals form in the worm's cells as he dropped the temperature, but he had also realised that some of the worms didn't freeze; they shrivelled up and dried out instead. Keen to know why some worms froze while others desiccated,Wharton began watching how the worms fared as ice formed around them at different temperatures.
Wharton suspended thousands of the nematodes in water, and sandwiched the drop between two coverslips so that he could film the worms as the water froze. After lowering the temperature, he initiated freezing by touching the edge of the droplet with an ice crystal, and watched as the ice slowly encased the nematodes. After 30 minutes Wharton recorded the fraction of frozen nematodes versus dehydrated worms, and found that even though the water was completely frozen at -1°C, all of the worms had dehydrated, while at-5°C, all of the worms froze, and only one third had survived when they thawed.
He also tested how the nematodes reacted as he cooled the forming ice at different rates and found that when the worms iced up quickly, they couldn't escape freezing, but if he cooled the forming ice slowly, again the worms dehydrated.
But how do the worms escape freezing at some temperatures and cooling rates and not others? Wharton explains that as the water surrounding the nematode freezes, the external vapour pressure falls below the vapour pressure of the liquid water still inside the worm's cells. If the worm is cooling slowly, or it is only just below freezing, the water is gently pulled out of the worm. But if the temperature drops too quickly before the dehydrating pressure gradient is established the worm cannot escape, and it freezes.
Wharton explains that some earthworms also protect their eggs from subzero temperatures by dehydration, but the Antarctic nematodes have capitalised on both approaches to thrive at a pedestrian rate in the coldest place on earth.