This action creates a polarized cell membrane. A separate protein complex harnesses the flow of protons trying to reenter the cell to provide energy—similar to the way a mill wheel harnesses the current of a river to do useful work. In archaea, the purple-tinted bacteriorhodopsin proteins cluster in a specialized region of the cell surface called the purple membrane, where they enable the harvest of light energy for growth under conditions where oxygen is scarce.
In classic experiments, Stoeckenius and others showed that light could drive the synthesis of adenosine triphosphate or ATP—the cellular currency for energy transactions—in reconstituted spheres that contained bacteriorhodopsin and ATP synthase. The latter is a key enzyme found in mitochondria, those "powerhouse" organelles present in eukaryotic cells. This work provided irrefutable proof of chemiosmotic coupling, the mechanism of energy generation used by all cellular life.
Despite its name more on this later , an organism that was known as " Halobacterium species NRC-1" was the first haloarchaeon —and one of the first archaea of any kind—to have its genome studied. Ford Doolittle at Dalhousie University and my research group then at the University of Massachusetts Amherst conducted those early experiments.
NRC-1 is in most respects a typical haloarchaeon, widely distributed in hypersaline environments such as the Great Salt Lake. The genome of this species has the unusual property of being spontaneously unstable, such that entire physiological systems, such as the phototrophic purple membrane and buoyant gas-filled vesicles, are sometimes mutated.
This curiosity led us to identify a large number of mobile genetic elements—similar to the "jumping genes" described by pioneering geneticist Barbara McClintock in maize. These elements were the first to be discovered in any archaeon. Figure 4. The retinal pigment in bacteriorhodopsin absorbs light of a wavelength that human beings perceive as green, but reflects light at the red and, to a lesser extent, the violet ends of the spectrum, a pattern that yields a purple appearance purple line.
By contrast, photosynthetic chlorophyll pigments absorb indigo and red and reflect green green line. This mirror-image relation suggests that chlorophyll evolved to exploit parts of the spectrum left unused by the purple pigment.
The carotenoid pigments orange line shield haloarchaea from high-energy violet and ultraviolet light waves but reflect lower-energy orangish-red colors, giving rise to the scarlet shades seen in many salterns.
The absorbance spectra have been scaled for comparison. The sequence of the NRC-1 genome was completed in the summer of It was the first complete genome to be sequenced with funds from the U. National Science Foundation. The genome consists of a large, circular chromosome 2, kilobases and the two smaller DNA hoops, called plasmids or replicons: pNRC kilobases and pNRC kilobases. The pNRC replicons contain many of the DNA repeats that enable genomic rearrangements, including 69 of 91 mobile elements, 33 to 39 kilobases of so-called inverted repeats, which can flip or invert portions of the circles, and kilobases of sequence that are identical in both plasmids.
Figure 5. NRC-1 and other haloarchaea contain extremely acidic proteins, as indicated by low values for a quantity called the isoelectric point, or p I. Based on sequences predicted from genomic data, the isoelectric profile of NRC-1 proteins red line is much more acidic than those of other single-cell organisms, including Bacillus subtilis Bsu , Escherichia coli Eco , Methanococcus jannaschii Mja , Methanothermobacter thermautotrophicus Mth , and Saccharomyces cerevisiae Sce.
The highly acidic side chains on NRC-1 proteins probably enable them to stay in solution even in concentrated brines. All this repetition confounded the computer programs tasked with organizing the overlapping fragments of DNA sequence into a seamless genome. Later in , we summoned an international consortium of 12 laboratories to meet in Amherst over the winter holidays to identify familiar elements in the genome.
Such an analysis was a Herculean task in those days. The computational tools used to analyze genomes were still in their infancy, so we had to write our own computer scripts and manually inspect much of the data. One of the most exciting discoveries gleaned from that sequence was that the 2, predicted proteins were, on average, much more acidic than those of other organisms.
The average isoelectric point a measure of acidity for predicted Halobacterium proteins is only 4. By contrast, the values for nearly all non-haloarchaeal species are close to neutral, a 7 on this scale. Acidic proteins carry strong negative charges, which ought to repel other negatively charged molecules in the cell, such as DNA and RNA. But even proteins tasked with binding DNA itself an acid —including pieces of the complex machine called the transcription apparatus, which creates RNA from DNA—turned out to be acidic.
Figure 6. The shape of a DNA-binding protein complex is similar in Halobacterium NRC-1 a and b and Homo sapiens c and d , even after hundreds of millions of years of divergent evolution. However, the surface charges of the proteins are starkly different. In this image, red indicates the negative charges of acidic parts, blue the positive charges of basic parts.
In the center of this protein complex made of two proteins, TBP and TFB is the DNA molecule here, green portions indicate the active gene, whereas pink and orange denote noncoding areas. Negative charges present a problem for attracting and binding DNA—which, as an acid, also carries a net negative charge. The solution is probably a layer of positive ions sandwiched between the otherwise repellant molecules. The views in a and c depict the protein complex riding the coil of DNA into the page, whereas b and d show the same complex coming out of the page.
Our recent experiments have shown that acidic transcription factors bind DNA with ease in the hypersaline environment inside haloarchaeal cells or isolated in test tubes filled with saline solution. This attraction is remarkable because these proteins and DNA—both negatively charged—ought to repel each other. One possible explanation is that the proteins and DNA work together by sandwiching positively charged ions between nearby, negatively charged side groups. Mutual repulsion between acidic groups helps acidic halophilic proteins to remain in solution under conditions in which neutral, non-halophilic proteins would precipitate or "salt out.
In recent years, the full sequence of the NRC-1 genome has been followed by the publication of five additional haloarchaeal genomes: Haloarcula marismortui , a metabolically versatile species from the Dead Sea; Natronomonas pharaonis, an alkali high p H -loving species from the alkaline soda lakes of the Sinai; Haloferax volcanii, a moderately halophilic species from Dead Sea mud; Haloquadratum walsbyi, a square-shaped organism common in salterns; and Halorubrum lacusprofundi, a cold-adapted species from an Antarctic lake.
These six genomes currently provide us with an excellent view of haloarchaeal diversity. The unveiling of the NRC-1 genome spawned questions about as well as insights into the evolutionary history, or phylogeny, of haloarchaea. Although the data confirmed NRC-1 as a true archaeon, the gene that most scientists use as an evolutionary chronometer, the so-called 16S ribosomal RNA, had a unique sequence that prevented easy categorization of the species.
This contradiction presented a challenge to the people who name microbes for a living. Laboratory studies indicate that not all species can survive the freezing and thawing process, and many species are killed when frozen, especially if they are in the exponential growth phase. This minimum temperature for growth appears to be determined by the fluidity of cell membranes and the availability of liquid water.
If an organism cannot desaturate its membrane lipids, the cellular transport of substrates ceases. The freezing property of the liquid within and immediately adjacent to the cell also comes into play. Either factor can prevent the cell from growing. Psychrotrophs may survive at the surface temperatures of Europa, as indicated by current techniques that employ freezing for preserving microbial cells.
Surviving microbes might have extreme difficulty initiating growth owing to the absence of organic matter for heterotrophic growth and their inability to metabolize at K. The absence of an organic matter energy source does not, however, rule out the possibility of psychrotrophic chemoautotrophic growth if the organism can reach subsurface liquid water.
Barophiles are microorganisms that thrive under conditions of high hydrostatic pressure, and all known examples inhabit marine environments. Studies indicate that most organisms cannot grow when the pressure exceeds 60 MPa, and many are indeed killed at that pressure. Radiation-resistant organisms are of particular relevance to any discussion of the forward contamination of Europa.
Because there are no known radioactive environments that can explain the evolution of D. The consensus view is that the mechanisms that evolved to permit survival in very dry environments also confer resistance to radiation. It is possible that other desiccation-resistant microorganisms, not yet described as radiation-resistant, could pose a threat to the europan biosphere.
Such organisms can only pose a threat if they can survive a multi-year journey to Europa. On Europa, life-sustaining, near-surface environments may exist within or under regions of water ice, since ice will provide microbes with some degree of radiation protection. In addition to requiring water in the liquid state, genetic repair would certainly also be dependent on the presence of a source of carbon and energy.
Nevertheless, the presence of carbon in material recycled from the interior via geologic processes or in cometary and meteoritic debris cannot be discounted. The ability or inability of terrestrial organisms to adapt to, and survive and multiply in, extreme terrestrial environments reveals much about the resilience of life in stressful circumstances. Given that these organisms have had millions of years to come to terms with their particular physical and chemical environments, their ability to cope provides some insight into the problems facing terrestrial organisms suddenly introduced into extraterrestrial environments.
Antarctic cryptoendolithic environments exist where communities of microorganisms have colonized the surface layers of porous rocks to depths of a few millimeters.
Photosynthetic members of the community utilize sunlight that penetrates the translucent rock crust. The cryptoendolithic colonies obtain water from snow, which melts when it falls on warm and dry rock surfaces.
Some scientists are drawn to the novelty of the organisms, searching for ones that are undescribed or that might harbour useful enzymes for industrial processes or antibiotics to save lives. Others simply find that the best organism for their scientific questions happens to have extreme preferences.
Scientists glimpse oddball microbe that could help explain rise of complex life. To identify, culture, genetically manipulate and observe extremophiles, researchers often tweak the methods used in more run-of-the-mill organisms. Whereas some techniques can be easily transferred — from one thermophile to other heat-lovers, say — others have to be adapted for each new organism. The researchers hope to find genes that might indicate how extremophiles survive and whether they might make compounds that could work as antibiotics.
So Tighe and the XMP team developed a six-enzyme cocktail — now commercially available as MetaPolyzyme — to break down any cell surface they might come across, adding detergents and organic solvents to collect nucleic acid on magnetic beads 1.
Thus armed, the researchers are working through a freezer full of samples, says Tighe. A UK research team built this heated microscope chamber to obtain live-cell images of the heat-loving organism Sulfolobus acidocaldarius. Credit: A. Pulschen et al. CC BY 4. Studying living extremophiles in the lab creates challenges, too.
Some such organisms are easy to cultivate, she says: just add salt. DiRuggiero has also worked with anaerobic hyperthermophiles, which grow at high heat and in the absence of oxygen. Standard agar in plastic Petri dishes would melt, so scientists turn to glass dishes packed with a gellan-gum derivative called Gelrite, which can withstand the heat.
The tools and techniques that work in lab favourites such as Escherichia coli — including the plasmids that are used to transfer genetic material, methods to insert that material into new microbes and compounds to select microorganisms that have successfully integrated new genes — are frequently ill-suited to high levels of salt, roasting temperatures and other extreme environments. Scientists often create the genes they want to use in E. Some extremophiles can take up nucleic acids from the surrounding media, says Carrie Eckert, a synthetic biologist at the University of Colorado Boulder and the National Renewable Energy Laboratory in Golden, Colorado.
Extremophiles: Life at the deep end. Scientists are learning to modify those methylation systems in E. 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. Likewise, hyperthermophilic microorganisms present in hot springs, hydrothermal vents and other sites heated by volcanic activity in terrestrial or marine areas may resemble potential life forms existing in other extraterrestrial environments.
Recently, the introduction of novel techniques such as Raman spectroscopy into the search of life signs using extremophilic organisms as models has open further perspectives that might be very useful in astrobiology.
With these groundbreaking discoveries and recent advances in the world of exthemophiles, which have profound implications for different branches of life sciences, our knowledge about the biosphere has grown and the putative boundaries of life have expanded. However, despite the latest advances we are just at the beginning of exploring and characterizing the world of extremophiles. This special issue discusses several aspects of these fascinating organisms, exploring their habitats, biodiversity, ecology, evolution, genetics, biochemistry, and biotechnological applications in a collection of exciting reviews and original articles written by leading experts and research groups in the field.
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