Fire Amoeba Reproduces at 145°F, Pushing Back the Heat Limit for Complex Life

In the hot spring waters of Lassen Volcanic National Park in California, researchers have watched a single-celled organism do something complex cells were not expected to do: divide and multiply at 145 degrees Fahrenheit, or 63 degrees Celsius. The organism is an amoeba named Incendiamoeba cascadensis, nicknamed the Fire Amoeba, and the NASA-funded work describing it appears in the journal Cell.

The number matters because of what the amoeba is. It is a eukaryote, meaning its cells have a nucleus and a set of membrane-bound internal structures. Heat damages proteins, breaks down cell membranes and denatures other organic molecules, and the delicate internal architecture of eukaryotic cells has long made them look poorly suited to extreme environments. Before this finding, the highest temperature at which a eukaryote was known to function was around 140°F (60°C), a limit associated with some fungi and certain red algae. The Fire Amoeba is now the hottest eukaryote known to reproduce.

Its tolerance does not end at the reproductive limit. At 145°F the amoeba stops dividing, but it stays active in water above 147°F (64°C), moving around and searching for food. The researchers see that gap between staying active and being able to reproduce as another handle for studying where life's limits lie.

Why had no one found it sooner? The scientists suggest part of the answer lies in how biologists hunt for life in extreme places. If eukaryotes were assumed unable to live near such temperatures, few people looked for them there. Beryl Rappaport, a graduate student at Syracuse University and one of the study's authors, said that assumptions about membrane stability had limited earlier searches for high-temperature eukaryotes.

To understand how the amoeba copes, the team sequenced its genome and tracked how its genes behaved at different temperatures. Several strategies emerged. Some protect DNA and guard against its breakdown under heat stress. Others help the cell respond to changes in its surroundings. Genes responsible for keeping proteins in working condition at high temperatures showed increased activity, which matters because heat can cause proteins to lose their structure. The researchers also found proteins with very high surface charge, a feature previously noted in thermophilic bacteria and archaea. That suggests some survival tactics may be shared across very distantly related forms of life.

The team also compared the amoeba's genome with data from other geothermal sites and found related DNA in places ranging from Yellowstone National Park to New Zealand. Researchers say similar organisms may therefore exist in hot springs worldwide.

The finding also interests astrobiologists, who study extremophiles to learn the practical limits of life. Alison Olcott, program scientist for Exobiology at NASA Headquarters, says such research helps define the biochemical and physiological limits of life as it functions on Earth, and those limits guide the search for life elsewhere. The caveat is firm: this is not a sign of extraterrestrial life. Temperature is only one requirement, alongside water, acidity, oxygen, pressure, nutrients and a surrounding ecosystem. Rappaport cautioned that the amoeba could not simply be placed on another planet and expected to survive.

What the discovery does show is that the boundary once drawn around complex life was less fixed than assumed. The next question for researchers is what else is living in other hot springs that was previously presumed impossible.