Zebrafish experiments demonstrate how vulnerable freshwater and marine species can be affected by climate change.
When the climate changes, which organisms survive and which die? A tiny fish larva offers unexpected insight into how the brain responds to rising temperatures.
“It was pretty amazing, actually. The whole brain lit up,” said Anna Andreassen, a Ph.D. candidate at the Norwegian University of Science and Technology (NTNU).
Living organisms, whether fish or humans, tend to deteriorate as the temperature rises. Many people have probably experienced this on a slightly too hot summer day. But what exactly happens inside the body when it becomes uncomfortably hot?
In order to find the answer, biologists in the NTNU Department of Biology combined genetic technology with neurophysiological techniques.
“We wanted to look at the mechanisms that limit the thermal tolerance of organisms. Which animals will survive when the Earth’s temperature increases due to climate change, and why? We chose to look at the brain,” says Andreassen.
Climate change causes heat waves
Animals that live in the water are experiencing temperatures that are rising to deadly levels, and heat waves that cross continents are becoming more frequent. To predict how species will adapt to climate change, it is essential to understand what limits survival at very high temperatures.
“Thermal tolerance has been a topic of research for decades, and the idea that temperature affects brain activity is old. What’s new is that we can now use genetic technology and neurophysiology to study the phenomenon,” explains Andreassen.
NTNU researchers in Trondheim studied the brain activity of newly hatched zebrafish larvae while gradually increasing the temperature around the fish larvae.
“These fish have been genetically modified so that neurons in the brain emit fluorescent light when they are active. We can see this light under the microscope as the larvae swim. These larval fish also have the advantage of being transparent. We can look directly into the brains of living larvae,” says Andreassen.
Losing the ability to respond
This way, researchers can track brain activity while gradually increasing the temperature of the water in which the fish are swimming.
“We can see how the larvae behave as it warms up. When it starts to get extremely hot, they lose their balance and start swimming in circles with their bellies in the air.
The researchers pricked the fish larvae to check their response. They pushed the tail of the larvae, which normally triggers a swimming reaction.
“At a certain temperature, the fish stopped reacting to blows. They were still alive, but from an ecological point of view they could be considered dead. In this condition in nature, they could not swim away from predators or find their way to colder waters,” says Andreassen, who adds that this condition is only temporary in small experimental fish.
“They’re just as fit as soon as we put them back in cooler waters,” says Andreassen.
Heat turns off the brain
So far, the experiments have gone as the researchers had planned. By shining light in front of the fish’s eyes, they could also test whether the brain was picking up visual impressions. When the temperature rose, the brain completely stopped responding to stimuli and was completely inactive. But then, when they turned the temperature up a bit more, something happened.
“The whole brain lit up. The closest I can describe to what we saw was some sort of seizure,” says Andreassen.
Normally, you only see brain activity as small specks of light in defined parts of the brain. The amazed researchers could now observe under a microscope how the fluorescent light spread in seconds and covered the entire brain of the tiny fish larva.
“We know that the zebrafish brain has a lot in common with the human brain – 70% of the genetic material is the same – and the researchers speculated that there could be a connection between what we saw in these fish larvae and what you see in the brains of children who have a fever,” says Andreassen.
Next, the researchers want to examine a special type of brain cell, glial cells, under the microscope.
“What we are excited to study here is the activity of glial cells during heating. These cells play a central role in the supply of oxygen to the brain – they both control the level of oxygen and regulate blood flow and therefore the supply of oxygen. Because we can see that oxygen levels affect thermal tolerance, one hypothesis is that the brain stops functioning because glial cells are no longer able to regulate oxygen levels.
Differences drive evolution
In order to take a closer look at what happened, the Trondheim researchers began manipulating the amount of oxygen in the water the fish were swimming in, while increasing the temperature.
“To our surprise, we found that oxygen level played a role in controlling thermal tolerance. When we added supplemental oxygen, fish larvae performed better at higher temperatures, had higher brain activity, and also recovered faster after being exposed to higher thermal limits compared to fish at low oxygen.
Studies on other species have given mixed results when testing the effect of oxygen concentration on thermal tolerance.
“Being ‘insensitive’ to fluctuating oxygen levels could therefore be an evolutionary advantage as the temperature on Earth increases.
“The results show that thermal tolerance is something that varies from species to species. It could be a characteristic that determines whether a species is able to adapt to climate change or will succumb to rising temperatures. Many organisms live in oxygen-poor environments where temperatures can quickly become higher than normal. They will be particularly vulnerable,” says Andreassen.
She gives as an example organisms that live in shallow freshwater areas, in rivers or in the intertidal zone.
“These are habitats where large fluctuations in oxygen levels can occur, often at the same time as temperature fluctuations. In these habitats, fish whose thermal tolerance is limited by oxygen level are likely to struggle more than fish that are not affected by it. Animals that manage to maintain nerve function under low oxygen levels might be the ones that tolerate high temperatures the best,” says Andreassen.
Reference: “Brain Dysfunction During Rewarming Is Linked to Oxygen Limitation in Larval Zebrafish” by Anna H. Andreassen, Petter Hall, Pouya Khatibzadeh, Fredrik Jutfelt, and Florence Kermen, September 19, 2022, Proceedings of the National Academy of Sciences.