One of the most remarkable biogeochemical phenomena in arid desert regions of the world is desert varnish. Although it may be only a hundredth of a millimeter in thickness, desert varnish often colors entire desert mountain ranges black or reddish brown. Desert varnish is a thin coating (patina) of manganese, iron and clays on the surface of sun-baked boulders. Desert varnish is formed by colonies of microscopic bacteria living on the rock surface for thousands of years. The bacteria absorb trace amounts of manganese and iron from the atmosphere and precipitate it as a black layer of manganese oxide or reddish iron oxide on the rock surfaces. This thin layer also includes cemented clay particles which help to shield the bacteria against desiccation, extreme heat and intense solar radiation. It has been estimated that up to 10,000 years are required for a complete varnish coating to form on boulders in extreme arid desert regions.

Although they survive in some amazing environments, the previous examples of bacteria could not survive in the oxygen deficient atmosphere of Mars. However, there are bacteria that can survive without free oxygen under some of the most extreme environmental conditions on earth. Could any of these or related archaebacteria survive on the surface of Mars?

The five-kingdom system of classification for living organisms, including the prokaryotic Monera and the eukaryotic Protista, Fungi, Plantae and Animalia is complicated by the discovery of archaebacteria. The prokaryotic Monera include three major divisions: The regular bacteria or eubacteria; the cyanobacteria (also called blue-green algae); and the archaebacteria. Lipids of archaebacterial cell membranes differ considerably from those of both prokaryotic and eukaryotic cells, as do the composition of their cell walls and the sequence of their ribosomal RNA subunits. In addition, recent studies have shown that archaebacterial RNA polymerases resemble the eukaryotic enzymes, not the eubacterial RNA polymerase.

Archaebacteria also have introns in some genes, an advanced eukaryotic characteristic that was previously unknown among prokaryotes. In eukaryotic cells, the initial messenger RNA (M-RNA) transcribed from the DNA (gene) is modified (shortened) before it leaves the nucleus. Sections of the M-RNA strand called introns are removed, and the remaining portions called exons are spliced together to form a shortened (edited) strand of mature M-RNA that leaves the nucleus and travels to the ribosome for translation into protein. This process is known as "gene editing." Some authorities hypothesize that eukaryotic organisms may have evolved from ancient archaebacteria (archae = ancient) rather than from the common and cosmopolitan eubacteria. The archaebacteria could have flourished more than 3 billion years ago under conditions previously thought to be uninhabitable to all known life forms. Although many conservative references place the archaebacteria in a separate division within the kingdom Monera, some authorities now recognize them as a 6th kingdom--The kingdom Archaebacteria.

In recent years, the traditional 5-kingdom or 6-kingdom system of classification has been challenged by authorities. Data from DNA and RNA comparisons indicate that archaebacteria are so different that they should not even be called a type of bacteria. Systematists have devised a classification level higher than a kingdom, called a domain or "superkingdom," to accomodate the archaebacteria. These remarkable organisms are now placed in the domain Archaea. Other prokaryotes, including eubacteria and cyanobacteria, are placed in the domain Bacteria. All of the traditional eukaryotic kingdoms are placed in the domain Eukarya. The 3-domain system of classification is shown in the following table:

Methanogenic bacteria live in marshes, swamps and your gastrointestinal tract. In fact, they are responsible for some intestinal gas, particularly the combustible component of flatulence. They produce methane gas anaerobically (without oxygen) by removing the electrons from hydrogen gas (H₂):

The electrons and H+ ions from hydrogen gas are used to reduce carbon dioxide (CO₂) to methane (CH₄). In the reaction, the H+ ions combine with the oxygen from CO₂ to form water (H₂O). During this process, the electrons are shuttled through an anaerobic electron transport system within the bacterial membrane which results in the phosphorylation of ADP (adenosine diphosphate) to form ATP (adenosine triphosphate). ATP is the vital energy molecule of all living systems which is absolutely necessary for key biochemical reactions within the cells. In fact, the varnish bacteria make their ATP in a similar fashion, only the electrons are coming from the aerobic oxidation of iron and manganese. During the oxidation process, the electrons are shuttled through an iron-containing cytochrome enzyme system on the inner bacterial membrane. The actual synthesis of ATP from the coupling of ADP (adenosine diphosphate) with phosphate (PO₄) is a lot more complicated and involves a mechanism called chemiosmosis. The electron flow generates a higher concentration (charge) of positively-charged hydrogen (H+) ions (or protons) on one side of the membrane. When one side of the membrane is sufficiently "charged," these protons recross the membrane through special channels (pores) containing the enzyme ATP synthetase, as molecules of ATP are produced. In eukaryotic cells, including the cells of your body, ATP is produced by a similar process within special membrane-bound organelles called mitochondria. In fact, some biologists believe that mitochondria (and chloroplasts) within eukaryotic animal and plant cells may have originated from ancient symbiotic bacteria that were once captured by other cells in the distant geologic past. This fascinating idea is called the "Endosymbiont Hypothesis."

Extreme thermophilic bacteria live in boiling water of hot springs where no other living cells can possibly survive (i.e. without their proteins becoming completely denatured). They have also been discovered in (or near) hot water emanating from sulfide chimneys ("black smokers") thousands of feet deep at the bottom of the ocean (near the Galapagos Islands). Surviving in the wild near steam vents where the temperature reaches 350 degrees Celsius under tremendous pressure, the bacteria have also been cultured under extreme heat and pressure in the laboratory. Although their optimal "operating temperature" is just over 100 degrees Celsius, no one would have believed that a living cell containing DNA, RNA and protein could survive anaerobically in a pressure cooker at 250 degrees Celsius, more than twice the temperature of boiling water. These cells could easily have flourished on a young earth under conditions previously thought to be uninhabitable to all known life forms.

Boiling hot springs in Yellowstone National Park are colored by colonies of thermophilic cyanobacteria, eubacteria and archaebacteria. Orange-colored cyanobacteria generally occur in water that has cooled below 73 degrees Celsius (163 degrees F). The green chlorophylls in these photosynthetic bacteria are masked by orange carotenoid pigments. Like the bright red halobacteria of salt lakes, carotenoids protect the delicate cells from intense solar radiation, especially during the summer months. Warmer, whitish areas of the ponds contain stringy masses of nonphotosynthetic eubacteria. Thermus aquaticus survives in temperatures too high for photosynthetic bacteria, up to 80 degrees Celsius (176 degrees F). Thermus aquaticus is heterotrophic and survives on minute amounts of organic matter in the water. TAQ polymerase used in the amplification of DNA using the polymerase chain reaction (PCR) was originally isolated from a colony of T. aquaticus collected in a hot spring at Yellowstone National Park.

Archaebacteria thrive in boiling water at Yellowstone National Park, at temperatures of 92 degrees Celsius (198 degrees F). These bacteria also thrive near steam vents at the bottom of the ocean at temperatures exceeding 115 degrees Celsius (239 degrees F). Scientists from throughout the world are studying the amazing bacteria flora at Yellowstone National Park. This is one of the best places on earth to study these organisms in their natural protected habitats. In other parts of the world, similar hot springs have been destoyed for the production of geothermal energy.

Acid hot springs in Yellowstone National Park with a pH of below 4.0 support the eukaryotic alga Cyanidium caldarum. This remarkable photosynthetic alga can even survive in a pH of zero! Some acidophilic hot springs bacteria utilize the oxidation of sulfur and iron for the synthesis of ATP. Alkaline hot springs support colonies of bacteria that utilize hydrogen sulfide for their energy source.

Life as we know it may have first arisen more than three billion years ago in a high temperature environment of boiling water. Thermophilic bacteria in hot springs of Yellowstone National Park may be relict populations of the first life on earth. In fact, these thermophilic bacteria may be the ancestors of all other life forms, including humans!

The third group of archaebacteria (called halobacteria) are especially interesting because they color the salt flats of desert playas and evaporation ponds a spectacular pinkish-red. This is especially evident in Owens Lake in the arid Owens Valley of California. Owens Lake was once a vast blue lake, before it was drained (by diverting Owens River) to provide the city of Los Angeles with water. Today it is a pinkish-red, dry lake bed or playa teaming with salt-loving archaebacteria. Solar evaporation ponds in the salt flats along US Highway 395 sometimes become a deep vermilion red. A drop of brine contains millions of tiny rod-shaped bacteria swimming among cuboidal crystals of sodium chloride (NaCl). Two flagellated halophilic green algae (Dunaliella and Dangeardinella) are often mixed with the rod-shaped halobacteria. Although Dunaliella in some regions of the world is also colored bright red, the populations in Owens Lake are green. The bacteria thrive in saturated brine up to 30 percent salinity (9 times the salinity of sea water). They can also be found embedded in the thick, pinkish-red salt crust literally baking in the desert sun. Because of their high internal osmotic concentration of potassium and sodium ions they are able to survive in the brine, otherwise they would rapidly become plasmolyzed and shrivel up. In fact, they cannot survive if the salt concentration drops below 10 percent. The bright red carotenoid pigment protects the cells from intense solar radiation. In some arid salt flats (such as Australia) the carotene-rich halobacteria and halophilic algae are harvested as a source of B-carotene, the precursor of vitamin A.

Owens Lake

Another remarkable pigment in the cell membrane of halobacteria called (bacteriorhodopsin) enables them to utilize sunlight for energy. Like photosynthetic chloroplasts in plant cells, the halobacteria produce their own ATP; but unlike green plants, they utilize bacteriorhodopsin instead of chlorophyll. The exact mechanism of ATP production is complicated and beyond the scope of this article, but it involves a "proton pump" across their cell membrane similar to the chemiosmotic mechanism for ATP synthesis in the chloroplasts and mitochondria of eukaryotic cells in higher organisms. Positively-charged hydrogen ions (protons), forced to one side of the membrane, flow back through special channels (pores) in the membrane as ATP (adenosine triphosphate) is enzymatically produced from ADP (adenosine diphosphate) and P (phosphate). These bacteria are especially interesting because the chemiosmotic mechanism for generating ATP does not require an electron transport system as in other photosynthetic bacteria and higher plants. Strains of these amazing bacteria have also been shown to survive anaerobically without free atmospheric oxygen while deeply embedded in thick salt crust. Bacteriorhodopsin is remarkably similar to the light sensitive pigment (rhodopsin) in the rod cells of human eyes which enables us to see in dim light. Thus, when we enter a dimly lighted room, it takes about 30 minutes for our eyes to adjust fully as the rhodopsin gradually increases in concentration. Of course, a flash of light can instantaneously break down your rhodopsin level, much to the chagrin of star-gazers who have become accustomed to the darkness.

To appreciate the tenacity of halophilic cells, I once sent some pieces of salt crust from Owens Lake to Dr. Richard Norris, an expert on flagellates at the University of Witwatersrand in Johannesburg, South Africa. Dr. Norris was able to dissolve the salt crystals and extract the algal cells which he identified as Dangeardinella saltitrix. This species has a slender, pear-shaped cell with two peculiar flagella, one extending forward and one trailing behind. Extreme halophilic algal cells such as Dangeardinella and pinkish-red halobacteria (Halobacterium) can survive completely encased in salt crust for extended periods of time. In fact, bacterial spores of the genus Bacillus were isolated from pockets (inclusions) in salt crystals harvested from an underground salt bed 2,000 feet below the surface. The salt deposits were formed from an ancient sea in a geologic formation that dates back about 250 million years.

In 1984, a 4.2-pound, potato-sized meteorite called ALH84001 was discovered in Allan Hills ice field, Antarctica, by an expedition of the National Science Foundation's Anarchic Meteorite Program. Since its discovery, the meteorite has been analyzed by a NASA research team at the Johnson Space Center and at Stanford University using sophisticated high-resolution scanning microscopy and laser mass spectrometry. The rock is believed to have originated underneath the Martian surface between 3.6 and 4 billion years ago, a time when it is generally thought that the planet was warmer and wetter. In fact, water is believed to have penetrated fractures in the subsurface rock, possibly forming an underground water system. Then, 15 million years ago, a huge comet or asteroid struck Mars, ejecting a piece of the rock from its subsurface location with enough force to escape the planet's gravity. For millions of years, the chunk of rock floated through space. It encountered earth's atmosphere 13,000 years ago and fell in Antarctica as a meteorite.

Evidence for subsurface deposits of water on Mars was substantiated by studies published in the journal Science May 2002. Data from the Mars Odyssey spacecraft suggest that large deposits of ice are buried just one to two feet (0.3 m to 0.6 m) below the planet's surface. The recent findings corroborate earlier studies that showed strong evidence that Mars was once covered with water. The planet has valleys, gullies and channels, characteristic evidence of the flow of water. The new studies suggest that the water may have drained downward until it was trapped as ice just below the protective layer of soil. The presence of water on Mars increases the odds that microbial life once existed on the planet.

Evidence of ancient life in the meteorite includes tiny globs of carbonates, organic compounds called polycyclic aromatic hydrocarbons (PAHs) and mineral compounds (iron sulfides and magnetite) that are commonly produced by anaerobic bacteria and other microbes on earth. In addition, these compounds were found in locations associated with fossil-like structures strikingly similar in appearance and size to microscopic fossils of the tiniest bacteria found on earth. The presence of carbonates near these imprints is similar to fossilized limestone deposits on earth. When microorganisms die, their complex molecules often degrade into PAHs. Not only did the NASA scientists find PAHs, but they found these molecules concentrated in the vicinity of the carbonate globules. For more information and photographs about this remarkable discovery, please refer to the following NASA web sites:

In addition to the archaebacteria, there are also species of eubacteria that live and multiply under some of the most extreme environmental conditions on earth. For example, bacteria in the gram-positive genus Deinococcus are resistant to levels of radiation that would kill many life forms. A decade ago, scientists considered it impossible for life to exist deep in the earth's surface, in furnace-like rock heated by the earth's interior to temperatures of 131 to 140 degrees Fahrenheit (55-60 degrees Celsius). Biologists also believed that organisms needed a steady supply of nutrients, oxygen and a constant source of energy from the surface just to survive. But during the past ten years, modified oil drills have penetrated supposedly sterile rock two miles below the surface and actually retrieved living bacteria. The microbes had apparently been living for millions of years on little more than water, hydrogen, sulfur, and iron-rich minerals in the rock. Scientists have also found strange bacteria living deep within a South African gold mine near Johannesburg. The bacteria were collected in organically-rich, blackened veins in the rock containing gold, uranium and iron pyrite. Some researchers think the bacteria may actually be deriving energy from natural radioactivity in these rocks. They may also be living on the iron pyrite, oxidizing it to ferric iron and sulfate, while precipitating the gold.

There are also thermophilic eubacteria, including the genus Thermus, that live in the boiling water of hot springs. In fact, a new species of Thermus was recently isolated from hot water trapped in rocks deep in the South African gold mine discussed above. These recent discoveries of subterranean bacteria isolated from the surface for millions of years offer compelling evidence to suggest that life could exist below the surface of Mars, a planet that once had water flowing freely on its now-barren surface. For a nice summary of these recent discoveries, refer to The Chronicle of Higher Education, August 6, 1999 (http://chronicle.com).

One of the driest places on earth is the Atacama desert of Chile that occurs in the vast rain shadow of the Andes. In fact, the bleak and extreme arid conditions of this harsh environment have been compared with Mars. Although no visible life forms occur on soil or rock surfaces, researchers have found endolithic bacteria associated with desert varnish (Kuhlman, et al. 2008). Wierzchos, et al. 2006 also found photosynthetic cyanobacteria (Chroococcidiopsis) living inside halite rocks that absorb trace amounts of moisture from the atmosphere. These discoveries of endolithic life may have implications for the discovery of life below the Martian surface.

So the bottom line here is: Could archaebacteria (or perhaps another type of eubacteria) possibly survive on Mars? Some of these bacteria need no organic nutrients to live, only inorganic ions and light. Some don't even need light--they simply oxidize inorganic minerals to get their energy (ATP). Some can thrive in the crushing pressures of the deepest oceans and in rocks below the earth's surface; and others at a pH as low as 1 or as high as 12. Some can even survive high doses of radiation or starvation for hundreds of years. With the possible exception of extreme subzero temperatures, could any of these remarkable bacteria survive in the extreme environmental conditions on the red planet? And perhaps more importantly, is it necessary to sterilize space exploration vehicles sent to Mars in order to prevent bacterial infection of our neighboring planet? Hopefully, future Mar's probes will provide answers to some of these intriguing questions--answers that could have a profound impact on our perception of the universe.

Life on Earth is mostly composed of the six elements carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. These elements make up the major compounds of living systems, including nucleic acids, proteins and lipds. Felisa Wolfe-Simon and her Nasa astrobiology team (Science DOI: December 2010) discovered a unique bacterium in the saline waters of Mono Lake that utilizes the toxic element arsenic intead of phosphorus. The bacterium is named GFAJ-1, and it belongs to the moderately halophilic (salt-loving) family Halomonadaceae. It is in the domain Bacteria, division (phylum) Proteobacteria, class Gammaproteobacteria.

Arsenic is in the same chemical group as phosphorus (group 15) in the periodic table of elements. Arsenic ions are notoriously deadly to multicellular organisms because of their interaction with protein thiols. A bacterium that can incorporate arsenic into its major organic compounds in the place of phosphorus presents some fascinating speculation in the field of astrobiology. This is another example of a life form that could possibly survive in hostile environments where no organism has previously thought to survive.

Disclaimer: When I originally posted these close-up images in 2013, I neglected to mention rock-inhabiting fungi (RIFs) of the division Ascomycota. They are extremophiles that live on and within exposed rock surfaces in extreme habitats. I originally thought they were cells and spores from a crustose rock lichen. According to lichenologist Kerry Knudsen of UC Riverside, they are rock inhabiting fungi called RIFs.

Black, rock-inhabiting fungi (RIFs) survive in some of the most extreme terrestrial habitats on earth. Some authors have postulated that they may have given rise to lichens. In fact, some authors have suggested that they might be able to survive on Mars. This is a major topic of research by mycologists, lichenologists, and astrobiologists.

Lucia Muggia, Cecile Gueidan, Kerry Knudsen, Gary Perlmutter, and Martin Grube. 2002. The Lichen Connections of Black Fungi Mycopathologia DOI 10.1007/s11046-012-9598-8. Lucia Muggia, et al. 2015. Phylogeny of Lichenothelia Species. Mycologia doi:10.3852/15-021.

Etch pits on the surface of limestone. The explanation was unknown to me until my recent e-mail from lichenologist Kerry Knudsen (see above message).

Microscopic view of black body within an etch pit of limestone. It is composed of fungal cells. Magnification 400x.

Microscopic view of black body within an etch pit of limestone. It is composed of fungal cells. The outer cells are heavily melanized (contain melanin) to protect them from UV radiation. Inner cells are hyaline (translucent). Magnification 400x.

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14. Life On Jupiter's Moon Europa?

The planet Jupiter has a number of named moons, some of which are visible with good binoculars or a good superzoom digital camera on a clear night. The moon Europa is especially interesting to astrobiologists because of the presence of water. In fact, it has a huge ocean below a thick layer of ice. Although Europa is only about one-fouth the diameter of Earth, its ocean may have twice the amount of water as the Earth's oceans. It is a promising area of research during the next decade.

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