A New “Fire Amoeba” Reproduces at 63 °C, Widening the Search for Life Beyond Earth

September 30 (IT Home) — Scientists have found a multicellular extremophile living in hot springs in Lassen Volcanic National Park, California, surviving at temperatures never seen before in such an organism.

The discovery is potentially significant for the search for extraterrestrial life, widening the range of environments that might harbour complex organisms.

Organisms that can survive in extreme environments are collectively known as extremophiles. When the extreme condition is mainly high temperature, the organisms are called thermophiles. The newly discovered thermophile from volcanic waters has been named Incendiamoeba cascadensis, nicknamed the “fire amoeba.” Experiments found that it can reproduce by cell division at 63 °C; at 64 °C it remains fully active but stops reproducing. Even at 66 °C the amoeba retains some physiological activity, and at 70 °C it halts its life processes but can later recover. Once the temperature reaches 80 °C, however, the fire amoeba dies.

Many single-celled organisms had already been found to survive at far higher temperatures. Methanopyrus kandleri, for example, is a single-celled organism that can survive and reproduce at up to 122 °C. But single-celled prokaryotes such as M. kandleri have no nucleus and no cell membrane — structures that are easily destroyed at high temperatures. It is precisely the absence of these cellular structures that allows them to survive above the boiling point of water.

Complex multicellular life — the “eukaryotes” in the classification of life — tolerates heat far less well. Unlike prokaryotes, eukaryotic cells have a nucleus and a cell membrane. Before this study, the highest temperature at which a multicellular organism had been shown to reproduce was 60 °C, a record set by certain fungi and red algae. Researchers had grown rather pessimistic, believing that eukaryotes could hardly survive and reproduce at higher temperatures.

“Research on eukaryotes has to some extent been limited by the assumption people made about the upper limit of cell membrane stability,” said Beryl Rappaport, a graduate student at Syracuse University and the lead scientist of the study, in a statement. “We hope the discovery of Incendiamoeba cascadensis will encourage more researchers to keep looking for heat-tolerant eukaryotes.”

The clues to the fire amoeba’s heat tolerance lie in its genetic sequence. Rappaport’s team identified a set of genes that remain stable and protect DNA at 63 °C, as well as genes that help the amoeba sense its surroundings and the temperature around it. Some genes even become more active in hot conditions — for instance, helping the amoeba fold proteins so that it can carry out various physiological functions.

“We found many survival mechanisms that help Incendiamoeba cascadensis live at high temperatures, and some of these strategies may be shared by all thermophiles,” Rappaport said. “For example, some proteins in the fire amoeba carry a strong positive surface charge, which helps keep their structure stable. This charge signature is very similar to what has been observed in thermophilic bacteria and archaea” — single-celled microorganisms that include prokaryotes.

Interestingly, the amoeba’s DNA contains gene fragments similar to DNA sequences detected in geothermal samples from places as far away as New Zealand and Yellowstone National Park in the United States. That suggests Incendiamoeba cascadensis itself, or close relatives of it, may be spread around the world without having been discovered yet.

Similar thermophiles may also exist beyond Earth. No other planet in the solar system has the mild environment of Earth: they are either too hot or too cold, too radioactive or too dimly lit, too acidic or covered in toxic substances. And outside the solar system, humans have yet to find a planet with an environment like Earth’s. Some extraterrestrial environments could in theory support such extremophiles, however — Mars, for example, or Europa, Jupiter’s moon with an underground ocean. The discovery of Incendiamoeba cascadensis therefore widens the temperature range in which eukaryotic life (and humans are eukaryotes too) could survive on other planets.

Rappaport also offered a caution: “Complex life like Incendiamoeba cascadensis could certainly survive on other planets. But as far as we know, only Earth meets all the conditions such an organism needs to live comfortably. Temperature is not the only measure; the environment must also have the right acidity, oxygen levels, pressure, water and food sources. Incendiamoeba cascadensis cannot survive on its own — it also needs other life to support it.”

So even though Mercury is very hot, that airless, barren planet could not support thermophiles. On some planets, however — ones that orbit closer to their star than the conventional habitable zone, or that are volcanically very active, and that also have liquid water — thermophiles might find a home. Io, Jupiter’s volcanically active moon, does not meet the requirement of having water.

The new research was published in the journal Cell on 22 September.