While most vertebrates can survive only a few minutes under anoxia, crucian carp routinely spend the entire four to five-month winter swimming in iced-over ponds. How do they survive the long, dark, and frigid winter of Northern Europe? Alcohol, of course! Carassius fish use their muscles to make ethanol in a stunning feat of biochemical adaptation to their extreme environment.
Without a steady supply of oxygen, mitochondria, the powerhouse of cells, can’t do their job of providing bundles of energy. Anoxic cells are instead forced to rely on oxygen-independent, or anaerobic, metabolism outside the mitochondria to convert energy into a usable form. This alternative pathway is highly inefficient and produces the toxic metabolic by-product lactic acid, the stuff that makes your muscles scream during a sprint. Carp under the ice face quite a predicament: they need energy to power their body, but somehow also need to avoid poisoning their own blood with lactic acid.
That’s where the booze comes in. By converting lactic acid into ethanol and then dumping that ethanol into the water, carp can survive for months with very little oxygen. And do they ever survive. Anoxic crucian carp party it up with blood alcohol contents reaching more than 50 mg per 100 milliliters over the winter. That’s above the legal limit for driving in Scotland and northern European countries, but not the USA or Canada (80 mg per 100 milliliters). They’re basically drunk for three months of the year.
Alcohol is not a normal metabolic by-product for a vertebrate. If it was, we wouldn’t need breweries, distilleries, and backwoods cooper stills to facilitate happy hour. Exactly how Carassius manage this strange feat of biochemistry has mystified scientists ever since the original paper describing their signature trick was published in 1980. Historically, biochemists assumed that the protein responsible for feeding the mitochondria, pyruvate dehydrogenase, malfunctioned under anoxia. This left the fuel, acetaldehyde, vulnerable to another protein, alcohol dehydrogenase, which converted it into ethanol instead. source
