Based on fossil evidence, prokaryotes were the first inhabitants on Earth, appearing 3. These organisms are abundant and ubiquitous; that is, they are present everywhere. In addition to inhabiting moderate environments, they are found in extreme conditions: from boiling springs to permanently frozen environments in Antarctica; from salty environments like the Dead Sea to environments under tremendous pressure, such as the depths of the ocean; and from areas without oxygen, such as a waste management plant, to radioactively-contaminated regions, such as Chernobyl.
Prokaryotes reside in the human digestive system and on the skin, are responsible for certain illnesses, and serve an important role in the preparation of many foods.
Learning Objectives Discuss the origins of prokaryotic organisms in terms of the geologic timeline. Key Points All living things can be classified into three main groups called domains; these include the Archaea, the Bacteria, and the Eukarya.
Prokaryotes arose during the Precambrian Period 3. However, it is still likely that other parameters affect microorganisms, as suggested by the change in bacterial diversity with elevation at Mount Fuji Japan Singh et al.
In the atmosphere, microorganisms have to contend with multiple hazards, including UV-C and cosmic radiation, low temperatures, desiccation, and oxidants DasSarma and DasSarma, , and it is unlikely that decreasing pressure plays the most significant role in microbial community diversity Amato et al.
Under these conditions, sporulation, resting stages, and biofilm formation are strategies used to withstand the multiple extremes Delort et al. Radiation sources include UV radiation, X-rays, gamma rays and more generally, cosmic rays.
These different types of ionizing radiation, in particular UV and gamma rays, can impact microbial cells via direct and indirect e. One of the first radiation-resistant microorganisms isolated was Deinococcus radiodurans , which has been well-studied and regarded as a model organism for understanding radiation tolerance Krisko and Radman, Additionally, these microorganisms are often polyextremophiles Table 4 Fredrickson et al.
Many ecosystems on Earth are affected by some type of radiation, with the most extreme radiation emanating from human-made radioactive-contaminated sites.
These range from 0. Radiation can additionally be found in subsurface environments, due to the radioactive decay of radiogenic isotopes e. Indeed, a hyperthermophilic and radiation-tolerant archaeon was isolated Thermococcus gammatolerans EJ3 from a deep-sea hydrothermal environment located at the East Pacific Rise, where natural radioactivity occurs Pb, Po, Rn Jolivet et al. There are several isolated microorganisms which can survive exposure to extreme radiation kGy , including exposure to space conditions for hundreds of days De Vera et al.
UV radiation likely influenced the evolution of life, especially during the Archean, when the ozone layer had yet to develop in the upper atmosphere due to a lack of atmospheric O 2. During this time, there were also intervals in which a photochemically produced organic haze would form, creating a UV shield Arney et al.
As such, the earliest life would have to contend with periods of intense UV radiation until enough O 2 was produced by oxygenic phototrophs after the Great Oxidation Event ca. It is likely that microorganisms had to develop the necessary resistance to both UV and ionizing radiation.
Indeed, model simulations demonstrate that the — nm wavelength range were several orders of magnitude higher about 4—3. Microbial adaptions to radiation include more genome copies for genome redundancy Anitori, , chapter 2 , changes in DNA repair functions Byrne et al.
These adaptations are seen throughout the microbial tree of life; for example, two mutants of Halobacterium sp. Although radiation resistance has been observed throughout Archaea, Bacteria, and Eukarya, the origins and evolution of such adaptations to radiation has yet to be determined. Since the first extremophile discoveries in , each decade of exploration has broadened our view of the boundaries of microbial environmental habitability.
Therefore, it is likely that the true limits of life have yet to be found. The ability of life to adapt and thrive under extreme conditions can be further supported by the analysis of the communities adapted to pH changes caused by human activity, including the dumping of mine drainage and steel slag. Similar to pH, the current pressure range of microbial life P range 0. Although there are many poly extremophiles currently in culture see Table 4 for some examples of notable polyextremophiles , data concerning the ability to withstand multiple stressors are extremely limited Harrison et al.
Moreover, the number of cultured microorganisms is tiny if compared to the diversity of uncultured clades Hug et al. The number of uncultured microorganisms at the genus level has been recently estimated to be on average 7. These uncultivated microorganisms are very likely to include poly extremophiles and will aid in expanding our understanding of the boundary conditions of life. Several studies have also demonstrated the growth of microorganisms under lab-simulated planetary conditions, including Mars-like Nicholson et al.
In this context, defining the boundary limits of life on Earth is a crucial step in identifying the conditions likely to originate or support life on other planetary bodies. Therefore, studies on the limits of life are important to understand four areas: 1 the potential for panspermia, 2 forward contamination due to human exploration ventures, 3 planetary colonization by humans, and 4 the exploration of extinct and extant life.
Similar to Earth, other planetary bodies might have different environments with varying ranges for each parameter. Since our knowledge of individual niches or habitats is extremely limited for other planetary bodies, we considered the range of each parameter temperature, salinity, pH, and pressure across three planetary layers: 1 atmosphere, 2 surface, and 3 subsurface Table 5. Many planetary bodies studied thus far have the potential for extinct or extant life, based on our knowledge of life on Earth.
Depending on the planetary body, different poly extremophiles could persist. For example, halopsychrophiles might be able to persist on Titan, Ceres, and Europa, which likely have saline subsurface oceans Grindrod et al. These lifeforms would also need to withstand high pressures. For example, the hydrostatic pressure of the subsurface ocean at Titan ranges from to MPa Sohl et al. Table 5. Boundary conditions for different planetary bodies of astrobiological interest compared to Earth , split into atmosphere, surface, and subsurface layers.
The atmospheres of some planetary bodies could potentially harbor life as well. Other planetary bodies presented in Table 5 have transient or tenuous atmospheres that have extremely low pressures and likely cannot support life. In comparison, on Earth, microorganisms have been observed and cultured from the upper atmosphere, although stresses such as UV-C radiation, low temperatures, and oxidants make it difficult to survive DasSarma and DasSarma, Similar strategies may be needed on other planetary bodies.
The surface of other planetary bodies, such as Ceres, Europa, and Mars, experience high levels of radiation, and thus, may be unsuitable to support life. However, shielding from UV-C radiation increases the chance of survival and includes shielding by atmospheric dust or burial Barbier et al. Shielding is also necessary against charged particle radiation and can be achieved by burial at only centimeter depths below the surface.
However, any subsurface aquifer deeper than a few meters would be protected from damaging radiation. Dartnell et al. At the surface, E. Compared to E. These survival times are, in fact, lower limits in light of recent measurements by the Radiation Assessment Detector onboard the Mars Science Laboratory Hassler et al.
Ehresmann et al. In addition to radiation, the surface of other planetary bodies is generally extremely cold. This indicates the physiology of radiation-tolerant psychrophiles is important for understanding the potential of life on the surface of other planetary bodies, such as the production of a fibril network, cell aggregation, and cold shock proteins Reid et al.
This suggests the subsurface is one of the most important locations in the search for extinct and extant extraterrestrial life Jones et al. The subsurface of other planetary bodies is potentially warmer than the surface and atmosphere Table 5 , influenced by geothermal processes [e. Several planetary bodies Enceladus, Titan, Ceres, and Europa likely have subsurface oceans, and Mars could potentially have a limited supply of groundwater Clifford et al.
Potential communities in these extraterrestrial subsurface environments are unlikely to be supported by surface exports of organic carbon like on our planet Kallmeyer et al. The abiotic production of H 2 can occur through a variety of mechanisms, including the radiolysis of water Lin et al.
Thus, serpentinization may have played a role in the origins of life on Earth Russell et al. Several planetary bodies could have ongoing serpentinization in a subsurface ocean, including Enceladus, Titan, Ceres, and Europa Table 5 , and serpentinization reactions could be widespread in the cosmos Holm et al.
Mars might also have serpentinization occurring in the subsurface or had serpentinization occurring millions of years ago, as indicated by the observation of hydrated minerals, such as serpentine phases, on the surface of Mars Ehlmann et al. Serpentinite-hosted sites on planetary bodies could likely support chemoautotrophic life, such as methanogens McCollom, For example, the piezotolerant thermophile Methanothermococcus okinawensis was capable of growing under Enceladus-like conditions up to 5 MPa Taubner et al.
In contrast to serpentinization, radiolysis consists of radionuclides decay, such as uranium, thorium, and radioactive potassium, decomposing water molecules into oxidizing radicals that then react with oxidizable substrates, such as pyrite, generating the necessary chemical energy for life to survive.
It is possible that radiolysis could support such life on other planetary bodies, including the Europan ocean Altair et al. It is important to note that the presence of liquid water or other liquid solvent is the main indicator to consider the possibility of extinct or extant life on a planetary body. In places with low water activity, desiccation-tolerance could become an important factor in determining the survivability of organisms, coupled with the transient availability of water over time either by precipitation, moisture, fog, or atmospheric humidity.
For example, desiccation tolerant organisms may be able to survive under Mars-like surface conditions Johnson et al. Given the limited understanding of the processes that have led to life on our planet, discussions regarding the conditions under which life might originate on other planets remains rather speculative McKay, An additional point to keep in mind while discussing the origin—and long-term persistence—of life on a planetary body is the necessity of elemental cycling on planetary scales Jelen et al.
Extremophiles have pushed our understanding of the boundaries of life in all directions since they were first discovered. As already highlighted by Harrison et al. Despite this, there is a fundamental lack of studies addressing the tolerance of microorganisms to multiple extremes Rothschild and Mancinelli, ; Harrison et al.
In the past 50 years of extremophile research it has become apparent that the limit of life varies when organisms face co-occurring multiple extremes. Future research will need to focus more on the interaction factor between multiple parameters.
While considering the basic requirements of life discussed in the introduction namely, energy, solvent, and building blocks , it is possible that the true limits of life are actually controlled by practical implications of these requirements.
For example, the current theoretical limits of life regarding temperature, pressure, and salinity are directly linked to the water activity or the stability of biological molecules under such conditions Price and Sowers, Despite ongoing scientific investigations of our planet for most of recorded human history, we still find life in unexpected places, and given the number of Earth ecosystems that still need to be explored in detail, we expect the current boundary of life to be pushed even further.
Taken together, these observations suggest that the true shape of the terrestrial biosphere remains undefined. Moreover, the astonishing diversity of planetary bodies and exoplanets Seager, will most likely expand the combinatorial space of environmental conditions, allowing us to speculate wildly about possible extraterrestrial lifeforms.
While considering the possibility for life to originate and exist on other planetary bodies, it is important to consider the variability of Earth local conditions when compared to the planetary mean Tables 2 , 5. The majority of parameters considered in this review are unlikely to be extreme over an entire planet, and local or transient conditions might still support life.
An outstanding example are communities present in microbialites in the Atacama Desert, where seasonal water deliquescence on salt grains was sufficient to sustain a productive and diverse community Davila et al. Therefore, it is unlikely that time-limited, coarse-grained observation of any extraterrestrial environment will be enough to definitely rule out the existence of life or conditions within the boundary space of Earth life, at least transiently.
NM conducted literature search, created figures, and wrote the manuscript. DG devised the topic, supervised manuscript structure and data collection, conducted literature search, created figures, and wrote the manuscript.
The opinions expressed in this publication are those of the authors and do not necessarily reflect the views of the John Templeton Foundation.
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There are also groups of organisms belonging to the same phylogenetic family that have adapted to very diverse extreme or moderately extreme conditions. Over the last few decades, the fast development of molecular biology techniques has led to significant advances in the field, allowing us to investigate intriguing questions on the nature of extremophiles with unprecedented precision. In particular, new high-throughput DNA sequencing technologies have revolutionized how we explore extreme microbiology, revealing microbial ecosystems with unexpectedly high levels of diversity and complexity.
Nevertheless, a thorough knowledge of the physiology of organisms in culture is essential to complement genomic or transcriptomic studies and cannot be replaced by any other approach. These findings have made the study of life in extreme environments one of the most exciting areas of research, and can tell us much about the fundaments of life. The mechanisms by which different organisms adapt to extreme environments provide a unique perspective on the fundamental characteristics of biological processes, such as the biochemical limits to macromolecular stability and the genetic instructions for constructing macromolecules that stabilize in one or more extreme conditions.
These organisms present a wide and versatile metabolic diversity coupled with extraordinary physiological capacities to colonize extreme environments. In addition to the familiar metabolic pathway of photosynthesis, extremophiles possess metabolisms based upon methane, sulfur, and even iron.
Although the molecular strategies employed for survival in such environments are still not fully clarified, it is known that these organisms have adapted biomolecules and peculiar biochemical pathways which are of great interest for biotechnological purposes.
Their stability and activity at extreme conditions make them useful alternatives to labile mesophilic molecules. This is particularly true for their enzymes, which remain catalytically active under extremes of temperature, salinity, pH, and solvent conditions. Interestingly, some of these enzymes display polyextremophilicity i. From an evolutionary and phylogenetic perspective, an important achievement that has emerged from studies involving extremophiles is that some of these organisms form a cluster on the base of the tree of life.
For this reason, extremophiles are critical for evolutionary studies related to the origins of life. It is also important to point out that the third domain of life, the archaea, was discovered partly due to the first studies on extremophiles, with profound consequences for evolutionary biology.
Furthermore, the study of extreme environments has become a key area of research for astrobiology. Understanding the biology of extremophiles and their ecosystems permits developing hypotheses regarding the conditions required for the origin and evolution of life elsewhere in the universe.
Consequently, extremophiles may be considered as model organisms when exploring the existence of extraterrestrial life in planets and moons of the Solar System and beyond. Microbial ecosystems found in extreme environments like the Atacama Desert, the Antarctic Dry Valleys and the Rio Tinto may be analogous to potential life forms adapted to Martian conditions.
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