Even in the harshest environments on our planet, life has found a way to exist, from giant tube worms that live in the extreme heat and pressure of deep-sea vents to wood frogs that allow themselves to freeze over winter in the Arctic. Similarly, sulfur mollies (Poecilia sulphuraria) and widemouth gambusia (Gambusia eurystoma) live in sulfur-rich hot springs in Mexico, where the water is exceptionally hot and contains little to no oxygen. Generally, lower amounts of oxygen make it harder for fish to survive in hot water; however, these fishes are used to living in the extreme conditions in the hot springs and may have unique coping mechanisms that allow them to persist. This realization led Korbinian Pacher, at Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Germany, and a group of colleagues from universities in Germany and Mexico to study these two remarkable fishes. The researchers predicted that the fish may do what so many other animals are incapable of – be able to survive in extreme heat no matter how much oxygen is in the water.

To test this, the team collected both species of fish from the hot springs and measured the maximum temperature the fish could tolerate, or their heat tolerance, in water that was fully saturated with oxygen and in water without any oxygen in it. To the team's surprise, the mollies were slightly better at tolerating the heat when there was no oxygen in the water than they were in fully oxygenated water. However, the gambusia had worse heat tolerance when there was no oxygen in the water, just like most other fishes. The researchers speculated that the mollies might use unusual methods, such as breathing air at the water's surface, to compensate for the lack of oxygen in the water and maintain their heat tolerance.

Next, the team wanted to use the information they collected about the fishes’ heat tolerance to figure out how close each species lives to the maximum temperatures it can survive in. But Pacher and the team had a problem: the longer that fish are in hot water, the harder it is for them to tolerate higher temperatures. So, they created mathematical models to estimate how each species’ heat tolerance changes depending on how long the fish are in hot water. The team also recorded the water temperature in the hot springs for an entire year and compared their models with the actual water temperatures the fish live in. Surprisingly, both species of fish regularly encounter water temperatures that exceed their heat tolerance. Although both species of fish have mastered survival in extreme conditions, it appears that they are living right at the edge of what they can tolerate.

In fact, while the team was running their experiments, a sudden influx of warm water from upstream rapidly raised water temperatures in the hot springs by 1°C. What the researchers observed was shocking – this small, abrupt change in water temperature caused several fish to perish. Given that climate change is predicted to further increase water temperature in the hot springs and that even small temperature changes have drastic effects, the future of these fish is in our hands. Their survival will depend on how hard we fight climate change and whether we allow it to render their hot spring homes truly uninhabitable.

Pacher
,
K.
,
Hernandez-Roman
,
N.
,
Juarez-Lopez
,
A.
,
Jimenez-Jimenez
,
J. E.
,
Lukas
,
J.
,
Sevinchan
,
Y.
,
Krause
,
J.
,
Arias-Rodriguez
,
L.
and
Bierbach
,
D.
(
2023
).
Increased thermal tolerance under anoxic conditions in an extremophile fish from hot sulfur springs in Mexico
.
bioRxiv
,
2023
-
07
.