Astrobiology Extreme organisms on Earth show us just how weird life elsewhere could be. by Chris Impey How life could thrive on hostile worlds Humans have left their mark all over Earth. We’re proud of our role as nature’s generalists — perhaps not as swift as the gazelle or as strong as the gorilla, but still pretty good at most things. Alone among all species, technology has given us dominion over the planet. Humans are endlessly plucky and adaptable; it seems we can do anything. Strain 121 Yet in truth, we’re frail. From our safe living rooms, we may admire the people who conquer Everest or cross deserts. But without technology, we couldn’t live beyond Earth’s temperate zones. We cannot survive for long in temperatures below freezing or above 104° Fahrenheit (40° Celsius). We can stay underwater only as long as we can hold our breath. Without water to drink we’d die in 3 days. Microbes, on the other hand, are hardy. And within the microbial world lies a band of extremists, organisms that thrive in conditions that would cook, crush, smother, and dissolve most other forms of life. Collectively, they are known as extremophiles, which means, literally, “lovers of extremes.” Extremophiles are found at temperatures above the boiling point and below the freezing point of water, in high salinity, and in strongly acidic conditions. Some can live deep inside rock, and others can go into a freeze-dried “wait state” for tens of thousands of years. Some of these microbes harvest energy from meth- ane, sulfur, and even iron. The study of extremophiles has taught scientists that life has evolved on Earth to colonize even the most inhospitable environments. And if they can do it here, why can’t they do it out there — in the extremes of cold, In the steaming hydrothermal waters of Yellowstone National Park’s Grand Pris- pressure, and radiation beyond Earth? matic Spring, scientists found Strain 121 As NASA prepares to spend bil- (inset), the first known extremophile. This lions on the search for life in the solar heat-tolerant bacterium can survive at system, it’s worth recalling the exotic temperatures up to 250° Fahrenheit (121° and weird life right under our noses. Celsius). The orange fringes around the pool are mats of algae and bacteria living We have much to learn from extremo- on energy and minerals in the water. Silica minerals coat bacterial filaments in a philes that could assist and inform the Jim Peaco (NPS)/Inset: Derek R. Lovley sample from the Grand Prismatic Spring in search for life elsewhere. Yellowstone National Park. Scientists hunt- ing for martian microorganisms could search for similar structures in fossilized hot Extreme origins spring deposits. Arizona State University/Jack Farmer Earth is a microbial planet, and the common ancestor of all life on Earth Background montage Astronomy: Kellie Jaeger © 2010 Kalmbach Publishing Co. This material may not be reproduced in any form • 54 Astronomy Decemberwithout 08 permission from the publisher. www.Astronomy.com www.Astronomy.com 55 chlorophyll breaks down, making pho- tosynthesis impossible. Above 212° F (100° C) — the boiling point of water at sea level — cells normally can’t control the flow of molecules in and out. At the boiling point, DNA and proteins unfold and no longer function. With certain extremophiles, high temperatures are less of a challenge. The reigning champion thermophile, dubbed Strain 121, continues to grow when it’s as hot as 250° F (121° C). Extremophiles near deep-sea hydro- thermal vents might even withstand superheated water over 390° F (200° C), although this is still unconfirmed. The deep-sea tube worms in this image live on hydrothermal vents along the Pacific Ocean’s Thermophiles survive by reaching Flat pinkish “ice worms” (inset) colonize mounds of yellow-orange methane ice on the floor of Galápagos Rift. The worms live symbiotically with bacteria in their guts: The bacteria use hydro- deep into the chemical toolbox for the Gulf of Mexico. Methane seeping from deep sources forms ice-like “hydrate” deposits under gen sulfide gas spewing from the vents as a source of energy and, in the process, make carbohy- adaptive strategies. For example, they high pressure. Ice worms do not consume the hydrate themselves. Instead, they appear to graze drates for the worms. Woods Hole Oceanographic Institution on bacteria living off the methane. Pennsylvania State University/Inset: Ian MacDonald (TAMU) use only the sturdiest enzymes, they ingest salts that shield the DNA double was probably a heat-loving extremo- The names of extremophile catego- helix, and they incorporate saturated We think water is essential for life, In 1977, Alvin’s crew discovered hydro- But salt-loving microbes make their phile. When scientists resurrected pro- ries convey their particular characteris- fats to bolster their cell membranes. yet some organisms survive with sur- thermal vents off the Galápagos Islands homes in salt flats, inland seas, and teins from these ancestral bacteria for tics, so they’re not quite as catchy as the Heat tolerance is a legacy of the prisingly little of it. Chile’s Atacama at a depth of 1.6 miles (2.5 kilometers). briny pools on the seafloor. They use study in the lab, the organisms per- names of comic superheroes. There’s conditions on early Earth. These deep- Desert and Antarctica’s high valleys are The crew was amazed to find a charged molecules (ions) and simple formed best at a sizzling temperature of Thermophile, who emerges from the sea survivors may have ridden out the so dry they’re a perfect lab for scientists thriving ecosystem that lived without sugars to protect their cell functions. 150° F (65° C). boiling inferno unscathed. There’s Psy- heavy meteorite bombardment that who test equipment and experiments solar energy. Three hundred species Algae called Dunaliella salina handle Early Earth was as hostile and inhos- chrophile, who shrugs off extreme cold. wreaked havoc with surface dwellers intended for Mars. Yet a small variety of congregated near superheated water salinity 10 times that of seawater. pitable as Dante’s hell. Visualize light- There’s Endolith, who does his best early in Earth’s history. lichens and insects survive in these arid from “black smoker” vents. The crea- Beyond this concentration, salt actually ning, steam, a rain of meteorites, and a work encased in rock. There’s Acido- environments, where it may not rain for tures ranged from bacteria and irides- precipitates out of solution. surface covered with magma and sulfur. phile, who energizes by bathing in bat- The big chill decades at a stretch. cent shrimp to giant clams and Extremophiles also push the edges of This doesn’t exactly bring to mind Dar- tery acid. And there’s Barophile, who At low temperatures, chemical reac- Making do with little is one thing, 6-foot-long (2 meters) red-tipped the 14-point pH scale, which describes win’s “warm little pond.” withstands pressures that would bring tions slow down and life gets sluggish. but there’s another strategy when the worms. The base of this ecosystem con- the concentration of hydrogen ions in a Think of extremophiles as stout- lesser superheroes to their knees. When water freezes, it expands by 10 drying is total. Rather than just giving sists of bacteria living off hydrogen sul- solution. This is important for biologi- hearted little biological superheroes, Extremophiles were first discovered percent and causes cells to rupture. Yet up, organisms maintain a breezy opti- fide gas spewing from the vents. cal processes because hydrogen ions’ doing heroic jobs under nearly impos- in the hot springs of Yellowstone there is a type of nematode — tiny, mism by going into hibernation. Some Recent discoveries add the impor- electrical charge drives chemical reac- sible conditions. If it helps, imagine National Park just over 40 years ago. wormlike creatures — that can survive spores and cysts appear to be able to tant information that not all life on the tions. The pH of pure water is 7, and them clad in tiny masks and capes and Yellowstone is still the prototypical site even being frozen. survive for a million years in suspended deep seafloor requires hydrothermal our cells can’t stray far from this with- booties, though in practice most of for the study of microbes with resis- Organisms ranging from microbial animation, and bacteria manage the heat. For example, pink “ice worms” out serious dysfunction. A substance them are nearly spherical and unex- tance to high temperature and acidity. colonies to the tiny insect called a same trick when they’re trapped in salt burrow into mounds of ice-like meth- with a pH below 7 is an acid; a sub- ceptional in appearance. As the temperature of water rises, Himalayan midge remain active down crystals. To reanimate, just add water! ane hydrate on the floor of the Gulf of stance with a pH above 7 is a base. most life gets into trouble. At 104° F to –0.4 F° (–18° C). Some organisms do Mexico, possibly feeding on methane- (40° C), oxygen won’t dissolve well it by insulating themselves from the Living under pressure metabolizing bacteria. Acid test in water, so organisms like fish external environment with fat-soluble Pressure is also no problem for crea- Miners first worked the The Iron will die. Above 167° F (75° C), molecules called lipids. Others take in tures that have adapted to it. Finding Titans of toxicity Mountain mine near Mount Shasta in salts that act like antifreeze. Many cell life in the deep sea has strongly influ- Extremophiles can also tolerate lethal Northern California in the 1860s gold Tardigrades, a group of tiny lines remain viable but inactive in liq- enced thinking about extremophiles chemical conditions.