The discovery of a “fire amoeba” capable of surviving temperatures typically impossible for complex life has expanded scientists’ knowledge of extremophiles, creatures that live in Earth’s most inhospitable regions.
The small single-celled amoeba is considered a “complex” life form due to its nucleus and internal structures, despite being a single-celled organism, placing it in the category known as eukaryotes. The hardiest extremophiles are typically bacteria and other forms of simple life lacking a nucleus, causing researchers to reconsider just what extremes may be survivable for eukaryotic life.
Discovering an Extremophile
“We need to rethink what’s possible for a eukaryotic cell in a significant way,” said co-author Angela Oliverio, a microbiologist at Syracuse University, in a recent Nature article.
Among extremophiles, many subcategories exist, divided by what particular type of extremes a life form can survive. For example, a psychrophile can survive temperatures of 15°C and below, while radioresistant organisms can tolerate high doses of radiation. In this case, a team of researchers has discovered a thermophile, a category of life capable of withstanding temperatures of 45 °C or higher. The new research is currently available as a preprint on bioRxiv while awaiting peer review.
The strange amoeba was discovered in Lassen Volcanic National Park, in California’s Cascade Mountain Range. The name the researchers gave to their find was Incendiamoeba cascadensis, Latin for “fire amoeba from the cascades.”
Lassen Volcanic National Park is home to unusual hydrothermal features, including acidic lakes and geothermal pools that glow with incandescence. Yet it wasn’t in one of these intriguing features where the unusual extremophile was discovered. Instead, a simple pH-neutral hot stream served as the host for the fire amoeba.
“It’s the most uninteresting geothermal feature you’ll find in Lassen,” said first author H. Beryl Rappaport, also a Syracuse University microbiologist, in a statement.
Growing the Fire Amoeba
Upon the researcher’s initial examination, there wasn’t much to see. No evidence of life was readily apparent under the microscope, but the team cultured the sample with nutrients anyway. However, after being left alone to grow and kept at a steady 57 °C (within the stream’s normal temperature range), something interesting began to happen, and subsequent observations revealed an amoeba growing in the cultured sample.
The team began to experiment to see how far they could push the organism, beginning by raising the culture’s temperature until it exceeded 60 °C, the highest known threshold for eukaryotic survival. Finding the fire amoeba still thriving, the researchers continued, noting that it could still divide at 63 °C, and was still moving at a temperature of 64 °C.
Even beyond these temperatures, I. cascadensis remained alive. Observations at 70 °C showed the cells turning into dormant cysts that were inactive, but still alive. When the temperature was reduced, those cysts sprang back to life.
Testing the Limits of Survival
While the results break any previous record for eukaryotic survival, simpler organisms like bacteria and archaea have still been observed thriving in much hotter environments. The hottest known survival temperature is attribtued to the thermophile Methanopyrus kandleri, which can withstand temperatures of an astounding 122 °C. Among eukaryotes, only a few species of fungi and red algae were able to cling to life at the earlier 60°C record. Eukaryotic cells belonging to humans and other types of mammals top out around 43 °C.
The researchers say that existing work has not paid close attention to determining how extreme of environments eukaryotes can withstand. Only further research will be able to determine what other eukaryotic extremophiles lie hidden in Earth’s most inhospitable regions, and the team says that searching for them will likely prove beneficial to science.
“We looked in one stream,” Oliverio concluded. “Maybe we got extremely lucky and there’s nothing else out there, but we really don’t think that’s the case.”
The paper, “A Geothermal Amoeba Sets a New Upper Temperature Limit for Eukaryotes,” appeared at the bioRxiv website on November 24, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.