LASSEN VOLCANIC NATIONAL PARK — Scientists discovered a new species of amoeba named Incendiamoeba cascadensis in samples collected from a tributary of Hot Springs Creek in Lassen Volcanic National Park, California. The organism grows and divides at temperatures up to 145 degrees Fahrenheit (63 degrees Celsius), setting the record for the highest temperature at which eukaryotic cells are known to replicate.

Prior to this discovery, no complex eukaryotic cells were known to replicate at temperatures above 140 degrees Fahrenheit (60 degrees Celsius). Previous eukaryotic heat tolerance records were held by species of fungi, red algae, and the amoeba Echinamoeba thermarum. The study findings were published in the journal Cell on September 22.

Eukaryotes are organisms that store their DNA inside a membrane-bound nucleus and include plants, animals, fungi, and protists. Prokaryotes, such as bacteria and archaea, are single-celled organisms that do not have a nucleus or membrane-bound organelles. Some prokaryotes, such as the archaeon Methanopyrus kandleri, can survive temperatures as high as 250 degrees Fahrenheit (122 degrees Celsius).

Beryl Rappaport is a microbiologist and graduate student at Syracuse University in New York and the first author of the study. Angela Oliverio is a microbial ecologist at Syracuse University in New York. Rappaport is a Ph.D. student in Oliverio’s lab, where her research focuses on microbial ecology and extremophile adaptation mechanisms. "study first author Beryl Rappaport said," Beryl Rappaport said.

"I was very surprised," Rappaport said. "We definitely had to go back and check to make sure that our incubators were calibrated correctly. It really was amazing." The amoeba thrives at temperatures between 131 and 135 degrees Fahrenheit (55 and 57 degrees Celsius) and stops growing below 108 degrees Fahrenheit (42 degrees Celsius).

Incendendoeba cascadensis remains active and moves to search for food at temperatures up to 147 degrees Fahrenheit (64 degrees Celsius). The organism can pivot between two body shapes to manage environmental stress. "One slower form seems suited to foraging and one faster one seems useful for escaping if temperatures become suboptimal," she said.

The amoeba can survive brief exposure to temperatures up to 158 degrees Fahrenheit (70 degrees Celsius) by entering a dormant state with protective shells. Cells recovered after being held at 158 degrees Fahrenheit (70 degrees Celsius) for five minutes and then cooled to 140 degrees Fahrenheit. However, Incendendoeba cascadensis did not survive heating to 176 degrees Fahrenheit (80 degrees Celsius).

Researchers sequenced the Incendendoeba cascadensis genome and studied gene expression at multiple temperatures. Genetic analysis revealed genes that help stabilize DNA, protect it from breakdown, sense the external environment, and maintain protein folding at high temperatures. Incendendoeba cascadensis proteins have positively charged amino acids on their surface.

The positive surface charge on the proteins may help keep them stable at high temperatures, similar to adaptations in heat-loving prokaryotes. "We were able to uncover many strategies that could help I. cascadensis survive at high temperatures, and some of these strategies could be used by thermophiles across all life," she said. "For instance, some proteins in I. cascadensis have a high positive surface charge that could help them remain stable. These protein charges are similar to those found in thermophilic bacteria and archaea."

Debashish Bhattacharya is an evolutionary biologist at Rutgers University in New Jersey who was not involved in the study. He noted the structural differences that complicate heat adaptation in complex cells. "When you're a bacterium … you can reshuffle your genome by scooping up DNA from other prokaryotes," Bhattacharya said.

"That ability helps prokaryotes adapt relatively quickly to stressful environments. But because DNA in eukaryotic cells is confined in the nucleus, eukaryotes are not able to simply grab a bunch of genes and switch their lifestyle at the same rates as prokaryotes. It takes far, far more evolutionary change to turn a eukaryote into an extremophile."

Why It Matters

The identification of Incendiamoeba cascadensis expands the known thermal boundaries for complex life, which has implications for astrobiology. Alison Olcott is the program scientist for Exobiology at NASA Headquarters in Washington. She explained how such findings inform the search for life beyond Earth.

"Studying extremophiles helps us better understand the biochemical and physiological limitations of life as we know it on Earth," Olcott said. "This information, in turn, helps guide NASA’s search for life as it expands the range of conditions we think life could potentially be inhabiting elsewhere." The discovery demonstrates that eukaryotic life can persist in conditions previously thought to be exclusive to prokaryotes.

What's New

Oliverio addressed the uncertainty surrounding the maximum thermal threshold for eukaryotes. "As far as we know, there is no reason why 63° is the hard limit," Oliverio said. "We don't really know what the upper temperature limits are." She noted that methodological assumptions may have previously limited the scope of research in this area.

"In part, studies on eukaryotes may have been limited because of assumptions about membrane stability," she said. "We are hoping that the discovery of I. cascadensis encourages others to keep searching for high temperature eukaryotes."

The broader geographic distribution of the species was clarified through large-scale data analysis. The study analyzed 31,093 publicly available metagenomes globally and identified Incendiamoeba cascadensis sequences in three samples from Yellowstone National Park, USA, indicating its presence in geothermal areas of the United States.

Incendiamoeba cascadensis was discovered in a tributary of Hot Springs Creek in Lassen Volcanic National Park, not in one of the park’s famous acidic pools, showing the potential for undiscovered extremophiles in less-studied geothermal sites. Researchers found DNA sequences similar to Incendendoeba cascadensis in geothermal samples from New Zealand and Yellowstone National Park.

How Sources Differ

Sources differ on the specific details regarding the location and identification of Incendiamoeba cascadensis. Another source states that Incendiamoeba cascadensis was found in samples collected from a tributary of Hot Springs Creek in Lassen Volcanic National Park, California.