Animals do the most amazing things. Read about them in this series by Janaki Lenin.
Depriving animals of oxygen is sentencing them to death. But there are a few exceptions. A group of Eurasian carp (Carassius sp.) including goldfish is known to survive for several months without the life-sustaining gas. When starved of oxygen – precipitating a condition called anoxia – these fish suffuse their blood with booze. They don’t drink the stuff; they brew it in their blood. This is the secret of pet goldfish surviving in aquaria without pumped oxygen.
In the northern latitudes of Europe, ice cakes the surface of ponds during winter, depriving the water below of oxygen. Crucian carp, a native wild species, and its close relative, the goldfish, can survive in such ponds for up to five months. When their tissues burn glucose without oxygen, they release lactic acid. The accumulation of lactates changes the pH of blood, developing into a medical emergency. But crucian carp and goldfish don’t show any signs of distress.
In 1980, Eric Shoubridge and Peter Hochachka of the University of British Columbia, Canada, wrote that the fish’s muscles prevent lactic acid from building up by converting it into the non-acidic ethanol. Despite booze running through their bodies, their brains and hearts somehow manage to function normally.
How do the fish convert lactates to ethanol? A multidisciplinary collaboration of scientists from Norway and the UK set out to find the answer.
“The project started several years ago when all [the collaborators] except one worked at the department of biosciences in Oslo but with different experimental expertise, from ecophysiology to biochemistry and molecular biology,” Göran Erik Nilsson, the senior author of the study, told The Wire. “I first heard of ethanol production in crucian carp and goldfish in the 1980s and have worked on it since then. It is obviously of interest to find out how evolution has solved the problem of surviving without oxygen since anoxia-related diseases [in humans] like stroke and heart infarction are very common.” Nilsson works at the University of Oslo, Norway.
The researchers compared the gene transcripts of tissues from the 15-cm-long crucian carp living in normal and oxygen-depleted waters. The red and white muscles of fish in no-oxygen water had up to two times more of an enzyme called pyruvate decarboxylase than in the brains, hearts and livers. This enzyme is similar to brewer’s yeast and is the catalyst for turning lactates into ethanol.
Indeed, when yeast live in oxygen-free environments, it produces ethanol. “But the enzyme pyruvate decarboxylase evolved independently in yeast and crucian carp/goldfish,” says Nilsson.
How much alcohol courses through the fish’s blood? “Blood alcohol concentrations in crucian carp can reach more than 50 mg per 100 millilitres, which is above the drink drive limit in these [north European] countries,” Michael Berenbrink, an evolutionary physiologist at the University of Liverpool, said in a press release.
This doesn’t mean fish are intoxicated. “Their blood alcohol level is about 0.05-0.1%,” says Nilsson – similar to a human who has consumed a pint or two of beer. The fish release the ethanol to the water before their alcohol levels rise any higher.
The researchers faced a challenge in the course of this study. “The crucian carp genome has not been sequenced so each gene we studied had to be sequenced by us before we could measure its expression,” says Nilsson. And they were in for another surprise.
Crucian carp and goldfish duplicated their genome so they have additional copies of the enzyme complex called pyruvate dehydrogenase, of which decarboxylase is one. One copy functions under normal oxygen-rich conditions. When ice crystallises and creates a severe oxygen crisis, the other copy that produces the brewer’s yeast-like enzyme is activated.
“This genome duplication occurred in an ancestor to the crucian carp and goldfish some eight million years ago,” says Nilsson. “[It gave] evolution the tools to come up with new functions for old proteins by working on the extra copies of these genes.”
A few other animals, including two groups of freshwater turtles and naked mole-rats, also boast of this ability to survive oxygen depletion. But they use different mechanisms. These fish are the first vertebrates known to produce pyruvate dehydrogenase and ethanol, until now noticed only in yeast and plants.
The closely related group of Cyprinus carp cannot cope with low oxygen waters. The levels of the enzyme in its tissue are no different from the levels in the brain of crucian carp and its relatives.
Crucian carp and goldfish cannot survive without oxygen in this manner indefinitely or without costs – although the latter’s tolerance for such extreme conditions is lower than its relative’s. Animals convert food into glycogen, a form of glucose, and store it in the walls of their livers.
Crucian carp put away enormous reserves of glycogen to burn through winter. By late autumn, glycogen makes up 30% of their liver mass, probably the largest among vertebrates. As oxygen levels drop, the fish slow their movements by up to 75% to save energy. Although they continue to be mentally alert, they become seasonally blind. But they have little to fear since no other fish inhabit these waters.
“The ethanol production allows the crucian carp to be the only fish species surviving and exploiting these harsh environments, thereby avoiding competition and escaping predation by other fish species with which they normally interact in better oxygenated waters,” said lead author Cathrine Elisabeth Fagernes in the release.
But by excreting ethanol, the carp waste a lot of this fuel. The colder it is, the longer the carp can live. At higher temperatures, they burn through their glycogen stores faster.
There’s still a key element of this process to unravel. “We still do not know how the ‘no oxygen’ signal is transduced to the enzymes, activating the ethanol production,” says Nilsson.
Although the fish may not be intoxicated, it’s likely they enjoy a mild buzz.
The study was published in the journal Scientific Reports on August 11, 2017.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.