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Neurobiology

Counterintuitive? Fish make more brain cells in water rich with carbon dioxide

Reference: Wang, Xiaojie, et al. “Brain regions of marine medaka activated by acute and short-term ocean acidification.” Science of The Total Environment 720 (2020): 137279. DOI: 10.1016/j.scitotenv.2020.137279

A strange, poisonous gas has been spreading around. Its inhalation is unavoidable. After breathing it for a while, everyone starts acting bizarre. Some confuse left and right; others ignore warning noises and walk right into dangerous traffic. Children can’t seem to remember simple things. This sounds like a horror movie scenario, but it is an actual looming, malignant possibility – just not for humans. The gas is carbon dioxide, and its victims are fish in the ocean.

Medaka, also called Japanese rice fish, are a popular aquarium breed that are also used extensively in research. Photo by Wikimedia commons.

Our civilization requires lots of energy for heat, industry, and travel. Most of this energy comes from burning fossil fuels, releasing carbon dioxide into the atmosphere. About a third of this carbon dioxide is absorbed by ocean waters. Saltwater is basic, and carbon dioxide dissolved in water is acidic, so the buildup of carbon dioxide in the ocean, called hypercapnia, acidifies water by making it inch towards neutrality. 

While ocean acidification doesn’t actually render the water as acidic as lemon juice, it still holds plenty of detrimental potential for marine life. A team of researchers at Shanghai Ocean University, China, studied how hypercapnia affects the brains of the marine medaka fish. They discovered that fish who spent a week in acidified water started making more brain cells – and possibly developed anxiety.

The acid test

New nerve cells (neurons) are continuously born in the adult brains of many species, including humans, but this process, known as neurogenesis, is especially prevalent in fish. Brand-new brain cells are important for making new memories and controlling emotions – but can life in acidified water change that?

In many species, from fish to humans, the adult brain continues to generate new neurons. Photo by Robina Weermeijer on Unsplash

The researchers kept the medaka in tanks with acidified water for up to a week. They then collected the brains from half the fish and compared them to the brain of medaka who had been kept in normal basic water. To identify new neurons, they looked for a protein called doublecortin, which marks newborn nerve cells. 

Surprisingly, the acidified water-dwelling medaka had much more doublecortin in their brains than the normal water-dwelling medaka, meaning that their brains had been busy making new neurons. Could hypercapnia be promoting neurogenesis?

Scaredy fish

As hypercapnia is known to change how fish react to noise and predators, the researchers wanted to know if a week in acidified water changed the medakas’ behavior. They took the remaining half of the medakas and subjected them to a light-dark preference test, which is often used to measure anxiety in laboratory animals.

The light-dark preference test uses a tank that’s painted half in black and half in white. Because fish are curious and like to explore new spaces, normal fish would scout around everywhere. But a dark corner is a great hiding spot for fish from their enemies in the ocean, so nervous fish may show a preference for dark space.

Indeed, the medaka from acidified water were quick to scurry towards the supposed safety of the dark half of the tank, staying there for two minutes before coming out and swimming all around the tank. By comparison, the medaka from normal, basic water enjoyed the entire tank from the very first moment. This urge of acidified water-dwelling medaka to skulk in the dark may mean that hypercapnia elevates anxiety in fish.

A grain of salt

The discovery that acidified water activated neurogenesis is rather surprising, especially when coupled with the finding that it made the medaka anxious. These results seem to contradict the known role of neurogenesis, which helps overcome behavioral disorders like depression and anxiety. But much of neurogenesis research was done not in fish but in mice, and it may be hard to compare how new neurons affect the behavior of the two drastically distinct species. 

Despite this discrepancy, there is no doubt that ocean acidification affects fish. The influx of carbon dioxide is changing their underwater world, and their world is changing them. After all, even lots of new brain cells don’t guarantee a happy life if a fish can’t escape breathing in carbon dioxide through its gills.

I am a PhD candidate at Northeastern University in Boston. I study regeneration of the nervous system in water salamanders called axolotls. In my free time, I like to read science fiction, bake, go on walks around Boston, and dig up cool science articles.

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