Goldfish turn to alcohol to get through winter in icy ponds

In August 2017, a study published in the journal Scientific Reports detailed the molecular mechanism that allows goldfish and their wild relatives, the crucian carp, to survive for months in oxygen-deprived water under ice. Researchers from the University of Liverpool and the University of Oslo identified a unique biological pathway that lets the fish produce ethanol, or drinking alcohol, to avoid a lethal buildup of metabolic waste.

This ability provides a solution to a critical problem faced by animals in anoxic environments. When oxygen is unavailable for respiration, cells switch to an anaerobic process that produces energy but also generates lactic acid as a byproduct. In most vertebrates, including humans, accumulating lactic acid quickly becomes toxic, leading to cell death.

A Unique Vertebrate Adaptation

Goldfish and crucian carp evolved a method to circumvent this fate. The scientific team found that these fish possess a second set of enzymes that, when oxygen levels plummet, activate to convert the accumulating lactic acid into ethanol. This alcohol can then be safely diffused from the body through the gills into the surrounding water. This process is highly unusual for a vertebrate and is more commonly associated with microorganisms like brewer’s yeast.

According to the report, this adaptation gives the fish a significant survival advantage in frozen ponds and lakes, where oxygen can be completely depleted for long periods. While other species might perish, the carp and goldfish can endure these conditions for months at a time, entering a low-energy state sustained by this anaerobic process.

The Molecular Pathway for Ethanol Production

The key to this ability lies in a modification of the proteins that channel carbohydrates into the mitochondria, the powerhouses of the cell. Under normal, oxygen-rich conditions, one set of proteins functions as it does in other animals. However, in the absence of oxygen, a specialized second set of proteins activates.

This second enzyme set intercepts the metabolic process before lactic acid can build to dangerous levels. It reroutes the byproducts into a pathway that creates ethanol outside the mitochondria. This prevents the acidosis that would otherwise prove fatal within minutes, allowing the fish to maintain basic cellular function without oxygen.

An Ancient Genetic Event

Genetic analysis performed by the researchers traced the origin of this trait to a whole genome duplication event that occurred in a common ancestor of goldfish and crucian carp approximately 8 million years ago. Such events provide redundant genetic material that can evolve new functions without disrupting existing, essential ones.

Over time, one set of the duplicated genes retained its original function for aerobic energy production. The second set mutated to acquire its new role in anaerobic ethanol production, becoming active only when triggered by the lack of oxygen. This evolutionary history explains why the trait is so rare, as it depended on a specific and large-scale genetic change in the distant past.

Survival Under Anoxic Conditions

The production of alcohol allows the fish to survive, but it does result in significant levels of ethanol in their bodies. It was reported at the time that blood alcohol concentrations in crucian carp could reach more than 50 milligrams per 100 milliliters. For context, this is above the legal drink-driving limit in many countries.

Michael Berenbrink, a researcher at the University of Liverpool and a co-author of the study, noted that the behavior of the fish appeared to change during this process. However, it remained an open question whether these changes were a direct consequence of intoxication or part of a more complex, energy-conserving survival strategy for enduring the harsh winter.

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