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Until fairly recently, it was widely believed that the highest temperature to which life on Earth could possibly adapt was about 60 degrees Centigrade (°C). Above this temperature, it was assumed, essential biomolecules would be destroyed, and life of any kind would, therefore, also be destroyed. In the last two decades, however, this assumption has been proved to be wrong. Many new species of bacteria and archaea have been discovered in a wide range of habitats that have ambient temperatures well above 60 °C. These organisms, known as "thermophiles" (heat lovers), are thriving in hot springs around the world. The hottest of them are living in submarine hot springs that may reach temperatures of 113 °C (hydrostatic pressure prevents boiling). The discovery of these hyperthermophilic microbes has had important scientific and practical consequences. Scientifically, it has expanded our knowledge of the diversity of organisms on Earth, and it has provided important insights into mechanisms for thermostabilization of essential biomolecules. This information is of interest to the NASA Astrobiology program goals of "establishing the limits for life in environments that provide analogues for conditions on other worlds" and "how life evolves on the molecular, organismic, and ecosystem levels."
While many new species of hyperthermophilic bacteria and archaea have been discovered in the last 20 years, few species of thermophilic Eukarya have been discovered. One of the three major branches on the tree of life, Eukarya includes all of the more familiar life forms (figure 1). For Eukarya, the upper temperature limit remains about 60 °C, a level set by some species of fungi found living in self-heating compost piles. Figure 2 shows the current estimates for the temperature range in the universe, the range in which life as we know it exists, and the ranges to which each of the three major divisions of life have adapted. It is not yet clear, however, if the unimpressive upper temperature limits of Eukarya reflect an inability of these organisms to adapt to high-
temperature habitats or the inability of scientists so far to find Eukarya living in these habitats. The scientific objective of this study was to answer the fundamental question: What are the upper temperature limits for macroscopic Eukarya on Earth?
To answer this question, researchers at Ames used a small, robust, submersible video camera and lighting system to hunt for macroscopic Eukarya in the hot springs of Yellowstone National Park. The temperatures of these springs ranged from 31 to
120 °C. It seems likely that there are many undiscovered thermophilic Eukarya lurking in the depths of these hot springs. Like other NASA systems probing distant planets, NASA's video system allows observations in otherwise inaccessible habitats. To date, Eukarya have been observed in springs up to 40 °C, but the search continues for higher temperatures. Ultimately, thermophilic Eukarya will be trapped and brought under scrutiny in the laboratory. It seems likely that new species of extremely thermophilic Eukarya will reveal molecular adaptations to high temperatures that have not been observed in the microbial systems currently being investigated. Such adaptations will expand our view of how complex organisms cope with extreme environments and provide insights into what kinds of organisms may inhabit the hypothetical hydrothermal communities in the subsurface on Mars or the oceans of Europa. For Astrobiology, the search for thermophilic Eukarya will provide guidance for search strategies for life elsewhere in the universe, and their discovery here will raise some intriguing possibilities for finding complex life in hydrothermal systems beyond Earth.
Point of Contact: J. Trent
(650) 604-3686
jtrent@mail.arc.nasa.gov
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Fig. 1. A stylized representation of the three groups that make up the tree of life, indicating the differences in cell types (background), and showing that bacteria and archaea are microscopic single-celled organisms (balloons), while most Eukarya are macroscopic multicellular organisms. Note: Bacteria and archaea used to be classified together as "prokaryotes" based on structural features of their cells, but recent analysis of critical biomolecules indicates that dividing "prokaryotes" into two separate groups gives a more accurate representation of phylogeny. By both cell structure and molecular criteria, bacteria and archaea are distinct from Eukarya.
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Fig. 2. The temperature range which terrestrial life forms prefer is just a small portion of the complete known temperature range (shown in degrees kelvin on the far left) in the universe. The temperature range in which the three major divisions of terrestrial life forms (eukarya, bacteria, and archaea) have been found living is shown in degrees centigrade on the right in the box. This narrow range is divided into three, based on where organisms have optimal growth; organisms are then categorized according to their preferred range: psychrophiles prefer <15 °C; mesophiles between 15-50 °C; and thermophiles
>50 °C. The question marks indicate that we do not yet know the upper limits at which organisms can live.
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