In the absence of oxygen, many vertebrates, including humans, die in minutes. But this is not the case for goldfish (Carassius auratus) and carps (Carassius carassius). These two species of the same genus are indeed able to survive between 5 and 6 months without oxygen. European researchers have discovered how such a feat is possible. They shared their results on August 11, 2017 on the Scientific Reports.
A backup metabolic pathway
These fish are able to transform the lactic acid (“waste” that forms in the tissues when the cells lack oxygen) into ethanol and to release it subsequently into the water via their gills. This process avoids the dangerous accumulation of lactic acid in the muscles and thus prevents these animals from poisoning themselves. But this alternative metabolic pathway does not only allow the transformation of lactic acid. It also allows these fish to survive for several months without breathing. “These animals use the same mechanism as other vertebrates to breathe, the one that requires oxygen, explains Cathrine Fagernes, co-author of the study.
However, without oxygen, they must produce energy. In other words, instead of producing lactic acid at the end of the metabolic process, they produce alcohol “which will serve them as” fuel “. So even though these fish get less energy than when they breathe, they stay alive, though in far less ideal conditions, “they get smaller, or even stop some processes that consume energy, such as vision,” says the researcher. But these “savings” allow them to survive.
A very high blood alcohol level in the absence of oxygen
In this study, the researchers discovered that all this is possible thanks to the duplication of a portion of the genome coding for an enzyme called pyruvate dehydrogenase. During the course of evolution, some copies of this portion mutated allowing later coding of another enzyme: pyruvate decarboxylase. It is the latter that, specialized in alcoholic fermentation, can synthesize ethanol.
“During the period when they evolve in the waters without dioxygen-covered ponds covered with ice – which can last several months in the north of Europe – the alcoholemia (blood alcohol level) can reach more than 50 milligrams per 100 milliliters, which is above the allowed limit for driving in these countries,” says Dr. Michael Berenbrink, another of the authors of the study.