Tenacious hot springs amoeba sets heat-tolerance record

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Sept 22 : Lassen Volcanic National Park offers a wondrous landscape of hot springs and other geothermal features in the Cascade mountains of California, fueled by molten rock deep below the surface. In one of these hot springs, scientists have discovered a microbe that surely is one of the toughest found anywhere on Earth.

This previously unknown species of amoeba — a type of single-celled organism known for its ability to change shapes freely — was found to survive and thrive at temperatures that would kill all other known complex life.

The species, named Incendiamoeba cascadensis, was able to perform cellular division, called mitosis, at up to 145.4 degrees Fahrenheit (63 degrees Celsius) and stay active at up to 147.2 F (64 C), continuing to find food and explore its environment. When exposed to heat up to 158 F (70 C), the amoeba was able to recover after being returned to lower temperatures.

Its heat tolerance is a record for any eukaryote, the expansive category of organisms on Earth covering people and the entire animal kingdom, plants, fungi and a vast array of microbes called protists including amoebae. Eukaryotes are considered complex life because their cells contain a membrane-bound nucleus and other membrane-enclosed subcellular structures called organelles lacking in simpler organisms like bacteria.

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"These temperatures surpass what was previously thought possible for any eukaryote and reopens questions about the limits of complex life," said biologist Beryl Rappaport of Syracuse University in Syracuse, New York, lead author of the research published on Tuesday in the journal Cell.

Its scientific name honors its heat tolerance, meaning "fire amoeba from the Cascades."

Amoebas are highly successful organisms, owing to their structural flexibility, numerous survival mechanisms and evolutionary adaptability. They can continuously change shape, often crawling by extending structures called pseudopods, or "false feet," and have prospered for hundreds of millions of years across a vast range of environments.

Until now, a few species of red algae, including Cyanidioschyzon merolae, and fungi, including Thermomyces lanuginosus, had held the heat-tolerance record for eukaryotes, at around 140 F (60 C).

There are some microbes that are not eukaryotes such as bacteria and archaea, as well as viruses — all lacking defined nuclei and all possessing smaller genomes. An archaeon called Methanopyrus kandleri holds the heat-tolerance record for all life on Earth, enduring temperatures of 252 F (122 C) — well above the boiling point for water — around hydrothermal vents in the deep ocean.

A MICROSCOPIC PREDATOR

The researchers discovered Incendiamoeba in a tributary off Hot Springs Creek, a thermal stream close to Lassen's Drakesbad area, according to Syracuse University biologist and study senior author Angela Oliverio. It was one of the few pH-neutral geothermal sites at Lassen, meaning neither acidic nor alkaline, and Incendiamoeba flourished in that setting.

The ability to withstand high temperatures has benefited Incendiamoeba. It is the top predator in its hot springs environment, feeding on heat-loving bacteria.

"One potential advantage for Incendiamoeba cascadensis to have this degree of heat tolerance is that it can avoid competition for food or other resources with other species by being the only organism of its kind in its environment. Because no organisms that feed on amoebae, like microscopic animals, have been found at these temperatures, it also avoids predators," Rappaport said.

The researchers documented how this amoeba protects itself at high temperatures by changing shape and forming a protective outer layer, and detected increased gene activity relating to DNA repair and the cell's internal shipping system under these conditions.

The discovery underscores the resilience of Earth's organisms.

"I think the most significant implications of this research is that it challenges our understanding of how and where eukaryotic life can persist. That has implications for understanding evolutionary biology in terms of fundamental constraints on complex cells and how they can evolve, and it expands the search for life on Earth and elsewhere in the universe," Oliverio said.

"I find it to be incredibly hopeful every time we expand our understanding of the myriad of ways in which life can persist in seemingly inhospitable environments," Oliverio said.

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