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An amoeba from Lassen's hot springs divides three degrees above the old eukaryote heat limit
Incendiamoeba cascadensis, from Lassen Volcanic National Park, divides at 63 degrees Celsius and still hunts for food at 64, one degree past a membrane-stability limit its lead author says may have narrowed the search.
The Scientist · Science desk
What happened
- NASA-supported scientists watched an amoeba reproduce by division at 63 degrees Celsius in the heated waters of California's Lassen Volcanic National Park, a record for any known eukaryote.
- The description appeared Tuesday in the journal Cell, from a team whose lead author is Beryl Rappaport, a graduate student at Syracuse University.
- A standing suggestion in the field held that organelle membranes in eukaryotes could not remain stable above 62 degrees Celsius, and the new organism breaks it.
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Why it matters
- constraint Rappaport says assumptions about membrane stability may have limited studies of eukaryotes, so the 63-degree figure partly measures where microbiologists have been willing to culture them at all.
- precedent Habitability arguments that treat complex cells as the fragile limiting case now have to account for a 63-degree observation and a failed 62-degree membrane prediction, both from one geothermal site.
- capability Extremophiles are already mined for proteins useful in industry and medicine, and this genome gives protein engineers a named set of heat-induced folding genes to draw on.
Two different ceilings were in play here, and they came from different kinds of evidence. The 60 C figure was an observation, the hottest at which any eukaryote had been seen to reproduce, held by a few species of fungi and red algae [4]. The 62 C figure was a prediction: organelle membranes in eukaryotes had been suggested to lose stability above it [6]. The amoeba divides three degrees above the observation and one degree above the prediction [15][16].
The prediction may have done more to the field than the record did. "In part, studies on eukaryotes may have been limited because of assumptions about membrane stability," Rappaport said [7]. "We are hoping that the discovery of I. cascadensis encourages others to keep searching for high temperature eukaryotes," Rappaport said [8].
Reproduction is what fails first. Above 63 C the amoeba stops dividing but remains active, moving around in search of food at up to 64 C [3]. For scale, the working definition of a true thermophile is an organism that can replicate, move, eat and survive above 45 C [11], so division at 63 C sits 18 degrees above that threshold [21].
The team sequenced the genome and measured gene expression at multiple temperatures. They found many genes that help stabilize DNA and protect it from breaking down, plus others that let the organism sense its external environment [9]. Expression of certain genes rose at high temperatures, including genes involved in maintaining protein folding [10]. Those are the candidates for how the cell holds together at 63 C.
NASA's account of the work says the more complex cells of eukaryotes might be more durable than previously thought, and could help scientists understand locations in the universe where complex life could survive [13]. Astrobiologists study the boundaries of survival on Earth partly to judge how organisms might live on worlds like Mars [20]. The evidence in hand is one single-celled amoeba from heated water in one national park [2]. Eukaryotes also include plants and human beings [19], and at these temperatures the study looked only at the amoeba.
Previous extremophile research has mostly focused on single-celled bacteria and archaea [12]. The known eukaryote ceiling is a record of the hottest conditions in which anyone has watched a eukaryote divide, and I would expect it to move again before anyone measures a physical ceiling for eukaryotic cell division.
What to watch
- Whether other groups culturing hot-spring eukaryotes report division above 63 C. Those reports would show how much of the old ceiling was sampling effort.
- Functional follow-up on the DNA-stabilizing and protein-folding genes, tying a specific gene to the temperature at which division stops.
- Whether direct lipid measurements on I. cascadensis revise the 62 C membrane stability figure.