Marine Matters: To Freeze or Not to Freeze
Thursday, December 15, 2016 10:46 AM
Yes, it’s getting a little nippy outside. Our hemisphere is tilted as far from the sun as it will be this year, the jet stream is drawing bitter air down from Canada, and the wind off the ocean is fierce and strong. Without my wool socks, warm pants, and assorted fleece layers, I wouldn’t last outdoors for 20 minutes, more or less. The below-freezing air would creep into my water-filled tissues and they slowly would begin to form tiny ice crystals. As those ice crystals grew they would draw water inexorably from my cells until, one by one, the cells essentially dissolved. Sounds awful. With any luck, I’d be comatose from the cold well before then.
So how do other animals survive the cold that would do me in so speedily? Atlantic cod don’t wear fleece. Ocean pout don’t carry space heaters. Salt water occasionally dips below 32°F as the formation of sea ice increases its salinity and its freezing temperature drops as a result. Yet the water-filled cells of North Atlantic fish don’t freeze even then.
They are saved by protein. Not your everyday type of protein, but a specific protein that binds to ice called antifreeze glycoproteins. These proteins are present in fish and other species, like insects, in the north and south polar regions. When the antifreeze glycoproteins in the blood and tissues of certain fish species encounter a tiny ice crystal, they stick to the crystal and cause it to stop growing. That’s important because ice crystals are aggressive.
When ice begins to form in water, it starts out as many small crystals. As the cold continues, a few small crystals will begin to grow larger than the others by sucking water molecules from their colleagues. Those larger ice crystals are the ones that can cause mortal harm to cells. Antifreeze glycoproteins attach to ice crystals in the body when they are small and keep them in check. The fish still have ice in their systems but it’s harmless ice.
There are more than a hundred different species of fish in the Antarctic carrying antifreeze in their blood and body. Similarly, fish in the Arctic region evolved to have similar antifreeze glycoproteins despite living thousands of miles apart, an example of co-emergent evolution.
Fish are not the only ones who can flourish in below-freezing temperatures. Insects like the snow flea use a combination of glycerol and alcohol molecules to freeze but not freeze solid. Glycerol, a lipid, allows the liquid around the animal’s cells to freeze but prevents the interior of the cells from following suit. The insect will appear to be a chunk of ice during the most bitter days of winter yet return to life without damage when the weather warms.
Then there’s the Upis beetle of Alaska. This little critter can survive -60°C temperatures and prosper. Just recently a team of researchers isolated the animal’s secret. The Upis beetle doesn’t produce a protein antifreeze like other polar creatures; it somehow manufactures a wildly complex sugar called xylomannan that keeps its cells from freezing even at such a low temperature. Don’t look for xylomannan in the store; this is not the type of sugar you put in your coffee.
The antifreeze glycoproteins found in fish may also have found their way into our food. The ability to keep ice crystals from growing too large is a nifty trait for such products as ice cream. Unilever produces low-fat ice cream lines (Breyer’s Light, for example) to which these antifreeze proteins are added in order to limit ice formation. The proteins keep the ice cream from turning into an unpalatable mash of chunky ice if it thaws and then refreezes. The company doesn’t draw the protein from polar fish; it makes it by fermenting genetically modified yeast. The compound is called an ice-structuring protein, which sounds much better than fish antifreeze.
Unfortunately, the world and its oceans are warming up, nowhere more rapidly than in the Arctic. The change in climate will make it hard on these sea creatures who have evolved over the millennia rare methods of staying alive in the deep freeze.