The amoeba that raised the thermal limit for eukaryotes

An amoeba isolated from a neutral-pH geothermal tributary near Hot Springs Creek in Lassen Volcanic National Park has set a new benchmark for eukaryotic life. Incendiamoeba cascadensis divides at 63°C; at 64°C, it still moves and captures food, although division has not been demonstrated. The discovery was published online by Cell on September 22.

The finding matters because eukaryotes are organisms whose cells contain a nucleus and organelles, structures that depend on proteins, membranes and coordinated internal transport. Until now, known eukaryotic growth reached ~60°C in red algae and fungi, while the record for amoebas was 57°C. The new species therefore expands the range in which this kind of cellular organization can not only persist but also produce offspring.

To distinguish temporary activity from growth, H. Beryl Rappaport, Natalie Petek-Seoane, Tomas Tyml, Felix Mikus, Angela Oliverio and colleagues isolated the microorganism and cultured it at different temperatures. Under each condition, they observed division, movement and feeding; after more severe exposures, they checked whether the cells resumed their functions. This sequence established demonstrated growth at 63°C and only behavioral and feeding activity at 64°C.

The team also examined cellular responses and gene expression—that is, which genes were used more or less intensively under heat. They found associations with DNA repair, transport of components within the cell and protein maintenance. These measurements indicate a coordinated response to heat stress, but they do not, on their own, identify a single adaptation responsible for the observed tolerance.

The amoeba also adds an ecological dimension to the record. It is a microbial predator that consumes thermophilic bacteria adapted to hot environments. Its presence shows that geothermal habitats can support not only resilient producers or decomposers, but also feeding relationships organized around a single-celled eukaryote.

There is a crucial limit to interpreting the study’s highest number: the cells recovered after a brief exposure to 70°C, but that does not mean they grew at that temperature. The data come from a single species maintained in the laboratory, and researchers still need to determine how much of its performance occurs in the natural environment and which mechanisms are indispensable. The solid advance is more specific: division at 63°C, with movement and feeding still present at 64°C.