Science Fact: The Terrestrial Search for Extraterrestrial Life by Catherine H. Shaffer
We haven't seen any yet, but we can learn quite a bit about the possibilities right here on earth.
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Until recently, astrobiology was primarily a theoretical science. It was impossible to collect data on organisms from other planets. Now remarkable new images from the Mars Opportunity and Spirit rovers have scientists slavering over the possibility of doing some applied astrobiology. What kind of life might be hidden on Mars, Europa, or even beyond our solar system? We can look forward to some solid answers soon.
For now, the best clues lie in organisms adapted to some of Earth's most extreme environments, such as deep ocean vents, deserts, and the Arctic Circle. However, it's not possible to understand the bizarre metabolic processes of these organisms without a solid grounding in the metabolic processes that have achieved evolutionary dominance on Earth.
Life on Earth consists of two basic ingredients: energy and stuff. For almost every ordinary purpose, the energy comes from oxygen or sunlight and the stuff is carbon. The energy is used to perform the mechanical work necessary to life, such as the transport and biosynthesis of molecules--mostly carbon molecules.
The most basic of these life processes is photosynthesis. Oxygen consuming life forms such as us cannot exist without the carbon fixing power of photosynthesis. The basic reaction of photosynthesis is the following:
CO2+H2O+light (right-arrow) (CH2O)+O2
The "light" in this reaction is a photon captured by a large, organic molecule called chlorophyll. Chlorophyll contains multiple five--and six--member rings coordinated around a magnesium atom. The resonance created by the double bonds in combination with the metal allows the molecule to become excited when struck by a photon. Chlorophyll absorbs light at around 425 nm (blue) and 680 nm (red), leaving the range in between, mostly green, free to reflect. Because of the energy relationships between photons and the various excited and unexcited states of intermediary molecules, the color green is non-negotiable. Everywhere there is photosynthesis, there will be green.
To fully understand this reaction, it must be further divided into the "light" and "dark" reactions, respectively:
2H2O+light (right-arrow) 2O+4[H]
and
4[H]+CO2 (right-arrow) (CH2O)+H2O
These reactions should be understood as two, biochemical, "half" reactions, and not as complete, independent reactions such as you see in chemistry class. Although they are correct, many more steps are involved than what is shown above. The half reactions are a way of representing the net change in the system. Therefore, (CH2O) is not a whole carbohydrate, but one carbohydrate unit, the exact nature of which need not be specified to understand that the carbon originates from carbon dioxide. (The construction of carbohydrates is catalyzed by numerous enzymes that string the individual units into the various starches, sugars, and cellulose found in plants.) Likewise, the hydrogen is shown in brackets because it never exists independently as a single hydrogen atom throughout the process.
In the light reaction, an electron is released and passed along through a chain of reactions until it is ultimately used to convert nicotinamide adenine dinucleotide phosphate (NADP+) to its reduced form (NADPH). NADPH is then used in various other metabolic reactions.
Meanwhile, at every step in the electron transport chain, a series of oxidation and reduction reactions generates H+ ions. When an atom or molecule receives an electron, it's said to be reduced. When a molecule gives up an electron, it is oxidized. The most familiar oxidation and reduction reaction is the one that can be observed on your car, especially if you have an older model. The change in color of the metal, and the resulting weakness and crumbling of the structure are due to the oxidation of Fe2+ to Fe3+.
In photosynthesis, the entire process occurs inside the chloroplast on a structure called the thylakoid lumen. This lumen is a membrane, much like a long balloon folded up on itself many times. During the reduction reactions, the H+ ions travel to the inside of the balloon. This creates a concentration gradient, which chemically is something akin to pressure. If you add salt to a cup of water, the salt will slowly diffuse through the water until the concentration is equal everywhere. If you create a membrane, and have a high concentration of salt on one side, and a low concentration of salt on the other, the transport of salt from the high side to the low side will be energetically favorable until the salt is equal on both sides. This is dictated by the laws of thermodynamics.
Thus, a bank of potential energy is created by the buildup of H+ ions (protons). This proton gradient is then used for the final reaction:
ADP+Pi (right-arrow) ATP
...where ADP is adenosine diphosphate, Pi is one phosphate group (or PO3), and ATP is adenosine triphosphate. The resultant molecule, with its three phosphate groups, is loaded with energy, and the reverse reaction is used elsewhere in the body for other metabolic reactions. Approximately 1.25 ATPs are ultimately produced per photon absorbed, and the overall process is sometimes referred to as photophosphorylation.
The proton gradient (which, remember, ultimately derives from the chain of oxidation/reduction reactions) powers this reaction mechanically, by physically causing a change in the shape of the gigantic ATP synthase molecule as the protons pass through a tunnel and out the other side of the membrane. Imagine a mushroom-shaped molecule with a tunnel going lengthwise through the top of the cap, and out the bottom of the stem.
The animal equivalent of photosynthesis is oxidative phosphorylation, which is the "opposite" reaction:
C6H12O6+6O2 (right-arrow) 6CO2+6H2O
...or the two half reactions:
C6H12O6+6H2O (right-arrow) 6CO2+24H++24e-and
6O2+24H++24e- (right-arrow) 12H2O
...where C6H12O6 is glucose--in other words, a carbohydrate produced by plants. So there is a complementarity between photosynthesis and its opposite, oxidative phosphorylation, which in many ways explains our dependence and interrelationship with plants in the environment. Like photosynthesis, oxidative phosphorylation produces a proto gradient across a membrane, the mitochondrial matrix, which is used to produce ATP and nicotinamide adenine dinucleotide (similar to the plant's NADPH).[Footnote 1]
[Footnote 1: A more detailed explanation of photosynthesis and oxidative phosphorylation can be found in Donald Voet and Judith G. Voet, Biochemistry, (John Wiley and Sons, Inc., 1990).]
In addition to these basic reactions of life, there are a host of others that mediate the breaking down of carbohydrates, proteins, and fats, and also the synthesis thereof. Many of these reactions make use of ATP or NADH or NADPH, depending on the organism and the reaction. Spent ADP, NAD+, and NADP+ are returned to the mitochondria or the chloroplast to be replenished by more energy harvested from the sun or from the cracking open of O2.
These are the chemical processes of life in an environment rich in sunlight, water, carbon, and oxygen. However, nature has made amazing accommodations for environments where one or more of these elements are missing. By studying the biochemistry of extremophiles, scientists can guess at the types of organism that can be found on other planets.
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Sulfur 
Probably the most famous extremophiles are microorganisms that dwell at the bottom of the ocean near thermal vents. These deep-sea vent organisms were first observed in 1977 by scientists John B. Corliss and John M. Edmond traveling on a deep-sea submersible named Alvin[Footnote 2]. Alvin a is national oceanographic facility owned by the Navy and operated by the Woods Hole Oceanographic Institution.
[Footnote 2: pubs.usgs.gov/publications/text/exploring.html]
Vents--basically underwater volcanoes--heat the surrounding water to as much as 400 degrees Celsius. Because they are so deep beneath the surface, there is no sunlight. These organisms still use carbon as their stuff, but harness sulfur for their energy, thusly:
2CO2 + 2H2O + H2S (right-arrow)
2CH2O + SO4 -2 + 2H+
Notice that oxygen is not absent from this equation; it is simply reduced to the role of supporting actor. The water is saturated with the gas H2S instead of O2.
These organisms were so unusual, in fact, that scientists realized that the existing system of nomenclature was inadequate to classify them. A new kingdom of life on Earth was created, and named Archea. Archea superficially resemble bacteria, but genetically and evolutionarily they are as far removed from bacteria as we are. Archea tend to thrive in extreme environments and have evolved weird chemistries to adapt to these environments. Near ocean vents, other organisms, such as worms, crabs, mussels, crustaceans, and even octopuses form a community around the sulfur-loving Archea, deriving all of their energy from these bacteria-like organisms, which ultimately comes from heat released by the vent. Because the temperature of the water drops to ambient within inches of the vent, neighboring life forms need not adapt to the extreme heat to benefit from its energy, harvested by Archea.
As an example, vent worms lack a digestive system. These are not a single species, but an entire family of creatures ranging from microscopic in size to several feet in length. They can be found wriggling through the mud or sprouting up from the sea floor like a bouquet of brightly colored soda straws. But they don't eat. Instead, their insides are stuffed with Archea. Blood transports hydrogen sulfide (H2S) to the bacteria, which oxidize it and convert carbon compounds into food for the worm. It's not completely understood how some of the higher organisms have adapted to the weird and poisonous chemistry near these ocean vents, but it provides an example of how a biological community can adapt to a lack of oxygen and sunlight.
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Hydrogen 
Vent communities are only one example of the many chemistries adapted to life in strange environments. In a deliberate search for microbial communities that might mimic those on Mars or Europa, a team of researchers headed by Francis H. Chapelle of the US Geological Survey, South Carolina, has discovered communities of methanogenic microorganisms living deep beneath the surface of the Earth. [Footnote 3]
[Footnote 3: Francis H. Chappelle, et al. "A Hydrogen-Based Subsurface Microbial Community Dominated by Methanogens," Nature 415 (2004): 312.]
They postulated that life on Mars would dwell far below the surface, where liquid water might be found, but no light could penetrate. Their search for a similar environment on Earth led them to Lidy Hot Springs, Idaho. The springs yielded up a vigorous community of methanogenic microorganisms, more than 99% of them Archea. The chemical reaction from which they derive the energy for life is as follows:
4H2 + CO2 (right-arrow) CH4 + 2H2O
In this case, molecular hydrogen is the electron donor and carbon dioxide the electron acceptor. The organism is completely free from the need for sunlight or molecular oxygen.
Living organisms similar to these have been found as deep as three kilometers beneath the surface of the Earth. Early discoveries of deep dwelling bacteria-like organisms were dismissed as contamination from the drilling process. However, scientists have since been vindicated and it seems likely that life not only exists in the deep places of the Earth, but evolved there independently of life on the surface. Some even go so far as to speculate that life could have begun deep below the surface of the Earth, before conditions on the surface were conducive to life. This leads to the obvious conclusion that life could exist below the surface of Mars without having ever successfully dwelt on the surface, which we already know to be quite inhospitable.
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Nitrogen 
Another class of microorganism is so common and so familiar that it would be easy to overlook it as a candidate for life on another planet. In fact, this category is also a prime candidate for the first life on our planet, at a time when atmospheric conditions were completely alien.
A scientific consensus is emerging that life on Earth began in an atmosphere roughly half carbon dioxide and half nitrogen. The atmospheric nitrogen was converted to nitric oxide, and nitric oxide combines with oxygen to form nitrogen dioxide, which readily dissolves in water and releases hydrogen to form nitrate and nitrite ions:
N2 + O2 + lightning (right-arrow) 2 NO
2NO + O2--2NO2
2NO2 + H2O--HNO3 + HNO2
Then,
HNO3 (right-arrow) H+ + NO3 - (Nitrate ions) and HNO2 (right-arrow) H+ + NO2 - (Nitrite ions)
The nitrite and nitrate then become available to nitrifying bacteria such as Nitrobacter and some species of Archea. These reactions are part of the nitrogen cycle, so dear to the hearts of farmers and aquarists, which converts ammonia to nitrite, a valuable fertilizer.
The astrobiological implications of nitrogen-fixing bacteria are for the moment not well researched, probably because the atmosphere on Mars does not resemble the 50/50, CO2/N2 mix of the primal Earth atmosphere. But there are some indications that the lightning reaction could occur on Mars, especially near volcanoes, so it is not impossible that nitrogen-loving microorganisms could be part of an ecology on Mars. We would be more likely to find these organisms on planets with a significant nitrogen presence, such as Pluto. (Remember, with nitrogen as an energy source, we don't need sunlight or a lot of heat.)
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Iron 
There is another, more exotic, form of chemical energy used by life on Earth. That is the oxidation of ferrous iron to ferric:
4Fe2+ + 4H+ + O2 (right-arrow) 4Fe3+ + 2H2O
You'll recall that this is the same reaction that is the bane of your 1980 Chevy pickup truck. As a source of energy for life, it's only advantageous under conditions of high acidity (note that four hydrogen ions are required for each iron). This makes it a desirable companion reaction to sulfur metabolizing bacteria, since sulfur occurs in acidic conditions. In fact, there are some strains of bacteria that can live happily on iron or sulfur. [Footnote 4]
[Footnote 4: P. Margolith, et al. "Sulfur Oxidation by the Iron Bacterium Ferrobacillus ferroxidans," Journal of Bacteriology, 92 (1996) 1706.]
The existence of microorganisms on Earth that derive energy for life from alternate oxidation and reduction reactions implies that there may be even more possibilities for life-giving energy than we suspect. The Earth is rich in oxygen and sunlight, so exotic chemotrophs will be present at best in marginal environments. But in environments where oxygen and sunlight are rare, the chemotrophs (organisms deriving energy from chemicals) would have the upper hand, and highly unusual reactions might form the basis of life. It might be possible to find life based on the chemistry of copper, magnesium, zinc, or something even more exotic and unlikely.
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Carbon Alternatives 
Thus far we have only looked at the possibilities for alternate energy sources for life, but there are many other ingredients in the recipe for life, which could be varied. One of the most famous is the silicon-based life form. My first encounter with this concept came from Star Trek. I can clearly recall Leonard Nimoy's agonized expression as he mindmelded with a sentient rock.
But for all its pathos and drama, silicon-based life is one of the most farfetched concepts ever spawned by science fiction (at least for Earthlike planets). Although the producers of Star Trek probably should have known better, the idea originated with the nineteenth-century astronomer Julius Scheiner. In his time, this was a not- unreasonable proposal, given what little was known of the physical sciences. Silicon has some of the same chemical properties as carbon, is able to form analogous compounds and polymers, and is relatively plentiful. Unfortunately, there are some crucial chemical differences. For example, when we breathe, we inhale oxygen and exhale carbon dioxide. Silicon dioxide, however, does not exist as a gas. Rather, it is found in the terribly inconvenient form of sand. It's likely that many others of the complex molecules that keep us alive and functioning might inconveniently turn out to be sand in a silicon-based system. However, some writers, such as Isaac Asimov and Poul Anderson, have postulated that life could exist based on a framework of silicones (a backbone of alternating silicon and oxygen with branching methyl groups) or fluorosilicones. This would only work at much higher temperatures than ours, but might provide a basis for life on planets too hot for our kind of chemistry.
But carbon offers surprising variability. Our human metabolism centers around glucose, a molecule containing six carbon atoms. Microorganisms have discovered ways of breaking down and harvesting energy from molecules so far removed from glucose that we normally consider them poisonous. There is, in fact, an industry built around such microorganisms: bioremediation. In large part, this is the science of helping soil bacteria consume industrial byproducts, oil and gasoline spills, herbicides, and more.
The University of Minnesota Biocatalysis/Biodegradation Database (umbbd. ahc.umn.edu/index.html) lists 332 microorganisms that consume 865 compounds via 141 biochemical pathways, and the list is only growing because researchers are constantly discovering new critters and new pathways. Examples of chemicals consumed by these bugs include benzoate, toluene, acetone, carbon tetrachloride, cyanamide, naphthalene, nitroglycerin, n-octane, styrene, trinitrotoluene (yes, TNT), and many more.
The mechanisms of degradation of all of these chemicals are not completely characterized. There are many mysteries remaining. However, some of the "tricks" bacteria use include harnessing the redox potential of metals such as copper and manganese and using long chains of enzymatic reactions.
As an example, consider the degradation of phthalate by Pseudomonas cepacia [Footnote 5]. Phthalates are a family of compounds used widely in industry and consequently have become a widespread pollutant. The bacteria convert phthalate first to phthalate 4,5-cis-dihydrodiol. The next step is 4,5-dihydroxyphthalate. Thence to 3,4-dihydoxybenzoate, then 2,4-dicholorobenzoate, and so on. The bacteria must accomplish a great many more enzymatic conversions before they can enjoy the sweet rewards of their phthalate snack.
[Footnote 5: R. P. Aftring and B. F. Taylor, "Aerobic and anaerobic catabolism of phthalic acid by a nitrate respiring bacteria," Arch. of Microbiol, 130 (1981): 101-104.]
The implications for astrobiology are that the building blocks for life can be had from just about any carbon source. For all the synthetic organic chemical innovations of man, nature has consistently matched or exceeded our accomplishments. Nature, of course, has a many- million-year lead on us.
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Living at Extremes 
So the basic chemistries of life are flexible, but what about the terrible extremes of cold, heat, dryness, and radiation encountered on other planets? Might not these be insurmountable obstacles to life?
Yet again, answers to these questions can be found in life forms existing on Earth. We have already met heat-tolerant bacteria, the so-called thermophilic organisms found in and near ocean vents. These bacteria have specialized cellular apparatus that is naturally heat-tolerant. In fact, the DNA polymerase from Thermus aquaticus, discovered in Yellowstone National Park, has become a mainstay of modern biotechnology. The polymerase chain reaction (PCR) relies on the heat stability of this enzyme. Without it, we would hardly have made progress past Watson and Crick in the biological sciences, the human genome could not have been sequenced, hundreds of innocent people would be in jail, and the prospects for cancer cures would be very bleak.
Cold may be one of the easiest hurdles to overcome, given the amazing adaptations of plants and animals in the arctic and Antarctic regions of the Earth. The bacteria Polaromonas vacuolata is found in Antarctic ice at temperatures near freezing. In fact, it cannot grow and reproduce at temperatures warmer than 12 degrees Celsius. And in 1997, in a mirror of the 1978 journey of the Alvin, scientists discovered worms living in methane ice on the floor of the Gulf of Mexico [Footnote 6]. They are thought to feed on chemosynthetic bacteria within the ice. These revelations about cold, wet places have exciting implications for the cold, wet moon Europa.
[Footnote 6: C. R. Fisher, et al. "Methane ice worms: Hesiocaeca methanicola colonizing fossil fuel reserves," Naturwissenschaften, 87.4 (2000): 184-7.]
Dry conditions present a challenge to life because all of the functions of life are carried out in solution. If there's no solution, then the molecules can't rub up against each other and react. There's probably no chance of life in a completely dry environment, but there are many mostly dry environments that harbor life. Bacteria of the genera Metallogenium and Pedomicrobium form a crust of clay, iron, and manganese known in the southwest as "desert varnish." These bacteria use iron and manganese as their food, and thrive in extremely dry conditions, encapsulating themselves with the clay to conserve trace moisture.[Footnote 7]
[Footnote 7: R. I. Dorn and T. M. Oberlander, "Microbial origin of rock varnish," Science 213 (1981): 1245-1247]
Some organisms make the rocks themselves their home, such as hypolithic algae, found living on rocks in Death Valley and the Atacama Desert. Also of note, there's abundant fossil evidence of bacteria that lived inside rocks in wet conditions, burrowing tunnels for themselves. It's possible that Mars expeditions could find fossils of similar endolithic bacteria on Mars.
Another possibility is an entirely different solvent than water. Unlike silicone-for-carbon, ammonia-for-water has not been so easily dismissed since its introduction in 1954 by J. B. S. Haldane [Footnote 8]. Ammonia is a liquid in conditions of low temperature or high pressure. Its chemistry is compatible with the formation of protein and nucleic-acid analogs, and it is plentiful on planets like Jupiter. We have already discussed nitrogen chemotrophs, so we know that organisms could derive energy from nitrites or ammonia directly. Unfortunately, one flaw in the ammonia theory is the lack of hydrogen bonds. These bonds are weak interactions between hydrogen and oxygen atoms on adjacent water molecules. Combined with the natural polarity of water, hydrogen bonds allow for the unique physical properties of water solutions. For example, without water, it may not be possible to form a membrane, which is based on hydrophobic ("water-fearing") interactions between molecules.
[Footnote 8: J. B. S. Haldane, "The Origin of Life," New Biology, 16 (1954): 12-27.]
One of the most hostile conditions for life anywhere is radiation. Nearly all types of radiation (such as sunlight, microwaves, gamma rays, x-rays, and alpha particles) destroy DNA. A constant war is waged within our cells against the effects of radiation, with a small army of enzymes constantly searching for breaks in the nucleic acid chain and repairing them. Left alone, these breaks could cause cells to die (a quick death), or worse, to grow out of control and become cancerous tumors (a not-so-quick death).
Against all odds, however, there is a microorganism that thrives in conditions of high radioactivity. Deinococcus radiodurans routinely survives radiation doses as high as 500,000 rads. For comparison, a dose of 1000 rads would be fatal to a human being. The secret of D. radiodurans's survival seems to lie in vigorous cellular DNA repair machinery and free-radical-scavenging enzymes. No one knows why or how this bacterium evolved its radiation resistance, since conditions of 500,000 rads don't exist anywhere on Earth. However, this bacterium serves as evidence that highly radioactive environments do not, in themselves, pose a barrier to the existence of life. Furthermore, by delving into its biochemistry and molecular biology, we may be able to learn some valuable lessons for keeping less sturdy organisms alive in radioactive environments.
A more tantalizing and exotic possibility is the question of whether energy for life could be derived from ionizing radiation. Although astrobiologists have so far not ventured to ask this question, it has been broached in the field of science fiction. James Tiptree, Jr. wrote about radiation-based life in her story, "With Mad Delicate Hands."
If true, this means that life is not even restricted to planets near suns--the famous "life zone" of so many science fiction stories. However, it is well to remember that we are adhering to a minimal definition of life, and that the organisms described are very small. Large organisms that move and think, such as human beings, require a great deal of energy. Most of these alternate chemistries for life yield energy only in small amounts. Even photosynthesis is so inefficient that Earth has not managed to evolve any mobile or intelligent plants. Thus, it would not be realistic to expect to find an ammonia-based sentient squid civilization floating around in the gas clouds of Jupiter. Nor would one think to discover fossils of giant rock eating, radiation-resistant spiders on Mars.
But the prospects for finding microscopic life on other planets seem excellent, based on the diversity of life on Earth. In fact, given the variety of possible alternate chemistries and extreme habitat adaptations, it seems inevitable that we will be inventing a new taxonomy soon, one that will necessarily include domains for Mars, Saturn, Jupiter, Europa, or even Pluto.
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Copyright © 2006 Catherine H. Shaffer