Ice-fishing in Antarcticaby Rebecca Priestley
Understanding how natural fish antifreeze binds to ice may have benefits for agriculture and industry.
‘Okay, folks, the fish are under the ice. The bad news is the ice is two metres thick.” We’re going ice-fishing with University of Auckland biologist Clive Evans on the annual sea ice not far from Scott Base. Paul Cziko, from the University of Oregon, drove us here in the Pisten Bully, a compact tracked vehicle packed with drill bits, buckets, fishing rods and survival bags. It was a bone-rattling ride, with six of us packed into the heated cab. Today, we’re after Trematomus pennellii, a small benthic fish – a bottom-feeder. It’s part of a remarkable order of Antarctic fish that carry an antifreeze protein which allows them to survive in waters that average -1.9°C. In water this cold, most fish would freeze solid. “How many are we after?” I ask Paul, who’s trying to determine at what temperatures ice crystals melt inside the fish. “All of ’em,” he replies, with a grin.
He’s wearing jeans, an earflap cap and a woollen jersey. I feel like a novice in my Antarctica New Zealand-issue layers of polypropylene, wool, nylon and down, but it’s cold. The air temperature is hovering around 0°C, but the wind chill is making it feel like -15°C. Clive is used to taking students out onto the ice and quickly has us doing all the work. We use an ice drill to bore three holes through the ice. As we reach the pristine water of the Ross Sea, a flood of slushy ice water surges out of the hole. I bait my hook with a cube of frozen fish and drop the line down to the sea floor 12m below. With three hand-lines between us, we’re soon populating the cylindrical chillybins of icy seawater with small brown fish that peer up at us. “Shall I put the lid on?” I ask Clive. “Yep, keep them warm,” he says. In this climate, the polystyrene bins are used to stop the cold getting in. We move to a new ice hole, and Clive drives the Pisten Bully up alongside to protect us from the wind.
This is Clive’s 20th trip to the ice, and this time he’s here for two months. Each morning he and his team go out fishing, and each day they drive back to Scott Base and deposit the fish in their aquarium. Afternoons and evenings are for lab work. They’ll stay as long as they can, but when the sea ice breaks up, they can’t fish any more. It’s December, and there are already cracks forming parallel to the shore, about 20m from us. “We keep an eye on things,” says Clive. “We look out for seals. When you see a seal, you know you’ve got a hole. There are little warning signs.” Apart from the occasional excitement of a tug on the line, ice-fishing is a tranquil job. A seal pokes its head up in a big crack in the ice a few metres from where we’re fishing. It looks at us, snorts, then disappears.
A few snowflakes begin to fall. I ask Clive about the fish antifreeze, discovered in the 1960s by American biologist Art DeVries, a veteran of Antarctic science who is working with Clive this season. The question now, for scientists like Clive and Art, is how the antifreeze protects the fish. Antifreeze only works if ice is present, explains Clive. “It doesn’t stop ice crystals from forming, it stops them from growing. That means that antifreeze will only work if the fish has ice inside it. And ice inside a super-cooled fish can be lethal.” Recent research by Clive, Art and their international colleagues has revealed that ice crystals accumulate in the spleen, “so they’re away from the circulation”. Circulating ice crystals would act like tiny clots and could get trapped in fine blood vessels, damaging the gills or killing the fish. “So they sequester them inside the spleen. And unless they melt, which means the environment has got to warm up, they’re always going to be there. Ultimately, it might be too big a burden for the fish and it might kill them.”
Other scientists are using x-ray crystall ography to look at the crystal structures of the antifreeze and ice and see how they fit together. “If you can figure out exactly how the antifreeze binds to ice, you can create synthetic analogues and use them in other applications like protecting crops from freezing or in industrial applications where ice-crystal growth might be a problem.” In the US, “ice-structuring proteins”, based on antifreeze from Arctic fish, are already being used in ice cream and yoghurt. The antifreeze found in Antarctic fish is a glycoprotein, a huge molecule made from a string of amino acids and sugars. For years, one challenge for scientists was to understand how the antifreeze glycoprotein, which is made in the exocrine pancreas where digestive enzymes are made, found its way in huge concentrations into the blood. In a paper just published in Antarctic Science, Clive, Art and colleagues tell why they believe they’ve solved the problem.
Although most glycoproteins that reach the gut would be digested, they say, this one is different. “It survives all the acids and enzymes in the gut and finds its way to the rectum, where all of a sudden it’s sucked up and spat out into the blood intact. From the blood it goes into the liver, then into the bile and the gall bladder and then gets discharged back into the gut. So you have this nice cycle, which is energyconservative. These fish live right on the edge and they need to make sure they don’t waste energy. If they were pooing out all this antifreeze, it would be a total waste, so they suck it up again and recycle it.” It turns out 15 fish are enough for today, and by late morning we’re driving back to Scott Base with the fish barrels nestled between our knees. The fish are deposited into an aquarium at the lab – an insulated container filled with microscopes, centrifuges, sterile cabinets and fridges.
This is Clive’s last season on a project funded by the Human Frontier Science Program, but he still has plenty of unanswered questions about how the antifreeze makes its way, intact, from the gut to the blood. “Is there a receptor in the gut for this antifreeze, and if there is, can we characterise that receptor?” The next step, he says, would be to see whether mammals have a similar receptor. If so, it could offer new opportunities, the receptor being a portal for drug delivery via the rectum. Is there a time limit on this research? “Like other species that have evolved in Antarctic waters, our little fish is likely to be faced with challenges arising from the increasing loss of sea ice should their environment continue to warm. Antarctic fishes have evolved to live with ice over millions of years. Despite the problems this brings, it is now an integral component of their life cycle. Our work is about to be turned on its head: it’s not the problem of ice but the lack of it we now need to address.”
Rebecca Priestley travelled to Scott Base on Antarctic New Zealand’s media programme.
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