Scientists Had Never Seen Complex Life Divide Above 60°C. Then Came the Fire Amoeba

For decades, 60°C marked an awkward frontier in biology. Bacteria and archaea had been found reproducing at far higher temperatures, but no known eukaryote — the branch of life whose cells contain a nucleus and other internal structures — had convincingly crossed it.

Then researchers began turning up the heat on an amoeba collected from a geothermal stream in California.

At 57°C it grew vigorously. At 60°C it kept going. At 63°C, under the microscope, the cell was still carrying out mitosis: copying and separating its genetic material and dividing into two.

The organism, newly named Incendiamoeba cascadensis, or roughly the “fire amoeba from the Cascades”, has now set a new experimentally demonstrated upper temperature record for eukaryotic reproduction. The result, published in Cell, does not establish a new absolute limit for complex cellular life. It does something more interesting: it shows that the old one was not a limit after all.

“As far as we know, there is no reason why 63° is the hard limit,” Angela Oliverio of Syracuse University, one of the study’s senior researchers, told Science News. “We don’t really know what the upper temperature limits are.”

Breaking the 60°C barrier

The distinction between surviving heat and reproducing in it is crucial.

Many organisms can endure conditions in which they cannot actively live. Spores, cysts and dormant cells can wait out extremes that would stop normal growth. But cell division is a much sterner test. A eukaryotic cell must keep membranes functioning, preserve proteins in workable shapes, protect and copy its DNA, organise its chromosomes and operate the microscopic machinery that pulls them apart.

All of that has to happen at once.

Heat is particularly destructive because it attacks several of those systems together. Proteins can unfold or clump. Membranes become harder to regulate. DNA and the systems responsible for repairing it come under stress. The elaborate internal organisation that distinguishes eukaryotic cells from bacteria and archaea becomes increasingly difficult to maintain.

That is why the previous boundary mattered. A handful of fungi and red algae could reproduce at around 60°C, but for decades nothing with the cellular architecture of a eukaryote had been shown replicating beyond it.

The fire amoeba changed that by three degrees.

Three degrees sounds trivial until the scale is biological rather than meteorological. The researchers tested the organism at 17 temperatures between 30°C and 64°C, with repeated cultures at each temperature. It would not grow at 40°C or below. Its preferred range was around 55°C to 57°C. At 63°C it could still reproduce.

At 64°C reproduction stopped, but the cells could still move. NASA, which supported the research, says the amoeba remained partly active at temperatures as high as 66°C. When researchers exposed it to 70°C for five minutes and then returned it to cooler conditions, it recovered. Exposure to 80°C proved fatal.

The important number is therefore not 70°C. The scientific record is 63°C, because that is where active cellular reproduction was demonstrated.

A life built around heat

The organism was collected in Lassen Volcanic National Park in northern California, at the southern end of the Cascade volcanic range. Between 2023 and 2025, researchers sampled geothermal sites where water temperatures reached roughly 65°C.

The amoeba turned up repeatedly. It was isolated from 14 of 20 sampling locations along a tributary of Hot Springs Creek, suggesting that it was not a freak cell that happened to survive an unusual patch of water. It was part of the local ecosystem.

And unlike an organism merely tolerating an occasional heatwave, I. cascadensis appears to require heat. Its failure to grow at 40°C or below makes it an obligate thermophile: for this organism, temperatures that would be dangerously hot for humans are not an emergency. They are home.

It also has more than one physical form. The amoeba can adopt a slower shape suited to feeding and a more elongated form associated with faster movement. When temperatures become too severe for normal activity, it can form a protective cyst-like state.

That combination offers an elegant survival strategy in a geothermal environment. When conditions are favourable, it feeds and divides. When they change, it can move. When they become too extreme, it can retreat into a more protected state and wait.

The amoeba feeds on heat-adapted bacteria, placing it near the top of a microscopic food web in an environment from which most familiar eukaryotes are excluded. Heat is not simply something it has learned to withstand. Its entire ecological niche is organised around it.

How does a complex cell survive?

The researchers then looked inside the organism for clues.

Genome comparisons showed an enrichment of genes associated with proteostasis — the systems cells use to make, fold, maintain and dispose of proteins — as well as genes connected with genome stability and environmental sensing. When the amoeba was placed under greater heat stress, pathways involved in protein maintenance, DNA repair and membrane trafficking became more active.

Its predicted proteins also carried an unusual chemical signature: more positively charged residues on their surfaces. Similar features have been seen in heat-loving bacteria and archaea and may help proteins remain stable instead of unfolding or sticking together as temperatures rise.

That resemblance is intriguing because bacteria, archaea and eukaryotes represent very different forms of cellular organisation. If similar physical solutions have evolved independently, it would suggest that life has only a limited number of ways to keep proteins functional under extreme heat.

But this part of the story remains unfinished. The genomic patterns identify promising mechanisms; they do not prove that any single gene, protein property or cellular pathway explains the fire amoeba’s record. The next stage will be to determine which adaptations are essential and which are simply associated with a life spent in hot water.

The more unsettling possibility

The discovery becomes more interesting when the researchers stop looking at California.

They searched more than 31,000 environmental metagenomes — databases containing genetic material recovered from soils, waters and microbial communities around the world — for sequences resembling those of I. cascadensis.

Related genetic signatures appeared in geothermal material from Yellowstone National Park and from New Zealand’s Taupō Volcanic Zone. They did not appear randomly across ordinary environments.

That raises the possibility that the fire amoeba is not a singular evolutionary oddity. It may be one member of a poorly sampled group of heat-loving eukaryotes scattered through geothermal systems.

If so, biology may have confused the edge of its catalogue with the edge of what life can do.

That is the deeper significance of the 63°C result. The old 60°C boundary was never a law of physics. It was an empirical frontier: the hottest temperature at which researchers had managed to demonstrate eukaryotic growth. One newly cultured organism has now moved it.

There may be others waiting above it.

Why NASA is interested

NASA’s involvement inevitably brings the search for extraterrestrial life into the story, but the implications need care.

The fire amoeba does not show that complex life exists on Mars, beneath the ice of Europa or anywhere else beyond Earth. Nor does heat tolerance alone make an environment habitable. The amoeba needs liquid water, suitable chemistry and food. It depends on other organisms in the geothermal ecosystem that supports it.

What extremophiles provide instead is a way of testing assumptions.

Astrobiologists cannot begin with a catalogue of alien organisms; no such catalogue exists. They can study the only life known and ask how far its chemistry can be pushed. Every organism that thrives beyond an expected boundary slightly alters the range of environments that cannot be dismissed on temperature, pressure, acidity, radiation or salinity alone.

In that sense the fire amoeba matters less because it points towards a particular planet than because it exposes a weakness in the way biological limits are sometimes imagined.

Life’s boundaries are often presented as numbers. But a number established from the organisms scientists have already found is not necessarily the same thing as a hard boundary imposed by chemistry.

At Lassen, researchers found an amoeba, put it in increasingly hot incubators and watched one such number fail.