A tiny amoeba breaks the assumed heat ceiling for complex life
In a steaming creek at Lassen Volcanic National Park in California, a single-celled eukaryote has set a new record for heat tolerance, challenging long-held limits for complex organisms. A research team from Syracuse University reports that this amoeba divided at roughly 145°F (63°C), surpassing the previously accepted upper bound near 140°F that has held for decades.
A new kind of heat-loving organism
Led by microbiologist Beryl Rappaport, then a PhD candidate at Syracuse University, the team studied heat-tolerant eukaryotic microbes from volcanic hot spring environments. They collected samples from a tributary of Hot Springs Creek near Boiling Springs Lake, a hydrothermal basin in the park. The nearby Boiling Springs Lake maintains about 125°F year-round. The organism in question belongs to Amoebozoa, a broad group of amoebae known for their flexible cell shapes. It propels itself and feeds by extending temporary projections that allow it to glide across mineral-rich surfaces.
"Eukaryotes can grow at higher temperatures than we previously believed,â Rappaport stated. The researchers gradually increased incubator temperatures until they observed stable cell division at the new upper limit.
Why heat harms most cells
Extreme heat disrupts proteins, causing them to lose their structure and halt cellular functions. Beyond a certain point, membranes become leaky and enzymes stop working. Eukaryotes have internal compartments, which can heighten their vulnerability to heat. Even hardy red algae, such as those in the Cyanidiales group, cap out at around 133°F in volcanic springs.
To counter heat, many organisms deploy chaperone proteins that help refold damaged proteins and prevent clumping. Cells may also maintain more rigid membranes by using tightly packed lipids to resist leaks when temperatures rise. Nucleic acids can fray with heat, threatening accurate replication. The success of the new amoeba suggests it may stabilize DNA and RNA in previously unseen ways.
Complex cells in hot springs
This finding reshapes how scientists define the limits of complex cells in natural hot springs, showing that some eukaryotes can adapt to temperatures once thought unattainable. The team plans to sequence the amoebaâs genome to identify heat-adapted proteins and membrane features, and they will test which genes activate as temperatures rise in controlled cultures.
"This discovery opens the door to what eukaryotes might still be capable of," said Rappaport.
How researchers determine heat limits
To pinpoint a ceiling, scientists typically culture cells across a range of temperatures, then monitor division, appearance, and survival over days in consistent media. The Lassen team moved from field samples to stable lab lines, using gradual heating to separate short-term survival from true reproduction at a target temperature. Lower-heat controls establish a baseline for growth and health, while replicate experiments reduce the risk that a single lucky lineage drives the result.
The search for life beyond Earth
While many microbes without nuclei (archaea) already thrive at high temperatures, one archaeal strain can multiply at 252°F. This kind of extreme heat tolerance informs how we search for life in hydrothermal environments on other worlds. It encourages mission planners to consider hot brines, steep chemical gradients, and enduring heat when evaluating potential habitats.
The Lassen amoeba also invites broader surveys of hot streams and pools globally. Researchers will likely revisit sites where earlier sampling missed quickly moving cells in thin films. Field safety is crucial near boiling water, and national park trails help keep visitors on solid ground. The surrounding basin is fragile; stepping off trails can damage crusts that vent scalding steam.
Broader implications
What stabilizes this cell at such high temperatures remains a key question. A plausible explanation points to a toolkit of heat-shock proteins that protect other proteins during thermal spikes. The membranes may incorporate more saturated lipids to resist softening, and the organism could adjust ion balance to keep enzymes functioning under heat.
If these strategies hold, they could inspire industrial applications where heat-tolerant enzymes and biomaterials reduce cooling costs and simplify sterilization.
Finally, this discovery highlights how overlooked hot springs in smaller parks can yield groundbreaking science. Big discoveries can emerge from tiny creeks just a few steps from a trail.
The study is published on bioRxiv (preprint): https://www.biorxiv.org/content/10.1101/2025.11.24.690213v1
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