Also see: Exobiology, Habitable solar systems

Recent discoveries of living organisms in conditions where once life wasn't thought to be possible has greatly expanded our scope when looking for life in the Universe, as environments that should be sterile and dead could instead reveal to be teeming with life. It's important to keep in mind that, most probably, life on Earth didn't appear in such conditions, but just adapted to them after arising in a more comfortable environment; still, this leaves the possibility of life on worlds that once were more hospitable (such as Venus or Mars) or subjected to panspermia.

Acidity and alkalinity are measured by the abundance of ions, respectively protons/hydrogen ions (H+) and hydroxide ions (OH). Such ions are dangerous for organisms because they're highly reactive and they disrupt chemical processes.

Organisms that live in a medium with a pH lower than 2 (that is, more acid than lemon juice) are called acidophiles. Thiobacillus, a chemoautotrophic bacterium, oxides sulfur inside sulfuric acid, at a pH of 1.3, while Acetobacter oxides ethanol producing acetic acid (vinegar); the archaea Picrophilus, the most extreme known acidophile, lives in boiling pools of sulfuric acid with a pH close to 0; in fact, it cannot live with a pH above 4. Many yeasts and bacteria thrive in the gastric juice of animals (pH close of roughly 1.4). Acidophilus constantly pumps protons out of their cells.

On the contrary, alkaliphiles live in a pH above 9, where they survive with acidic polymers that keep out hydroxide while allowing in positively charged ions such as hydronium (H3O+) and sodium (Na+). Lake Nakuru, in Kenya, is a highly alkaline soda lake (pH around 10) that hosts a great number of alkaliphile bacteria, protists and rotifers, that in turn feed colonies of flamingos.

Variety of extremophiles


Grand prismatic spring

The Grand Prismatic Spring in Yellowstone, a pool of boiling, mineral-rich water brighlty coloured by bacterial mats.

The biochemical processes of life as we know it can only occur in a precise range of temperatures. For example, most eukaryotes cannot live above 45°C, as mitochondria stop performing cellular respiration at that temperature: when cells become too hot, the membrane becomes permeable and proteins are deformed or break down, though some organisms can survive using only proteins better able to withstand heat. On the contrary, when water freezes the exchanges of ions are impossible, and the expanding crystals of ice can physically damage cell parts; however, it's possible to keep water liquid at lower temperatures when it's mixed with salts or organic solvents.
  • Organisms able to survive between 45°C and 80°C, most of them archaea, are thus considered thermophiles. The alga Cyanidium caldarium is among the very few thermophile eukaryotes (it lives at 57°C in sulfuric acid); note that, while many animals and plants live at higher external temperatures, they're not thermophiles because the inner temperature of their cells is within the norm.
  • Hyperthermophiles are organisms that live above 80°C. About seventy species are known, discovered in Yellowstone's hot springs and in hydrothermal vents, where pressure keeps water liquid well above its ebollition temperature. Besides some bacteria, that mostly live between 80°C and 90°C, they're almost always archaea: Pyrococcus survives at 100°C, Pyrolobus at 113°C and Methanopyrus, the current record holder, at 122°C. The temperature at which the integrity of DNA is lost is about 150°C.
  • Psychrophiles live, on the contrary, below -15°C (as with thermophiles, only the internal temperature of the organism is taken into account). As liquid water is strictly necessary for the survival of every terran lifeform (but see here), they live in salty pools (see "halophiles" below) where salt lowers the freezing point of water. The one-celled alga Chlamydomonas nivalis lives in the snow, which it dyes green.


Acidity and alkalinity are measured by the abundance of ions, respectively protons/hydrogen ions (H+) and hydroxide ions (OH). Such ions are dangerous for organisms because they're highly reactive and they disrupt chemical processes.


  • Metallotolerant organisms survive at high concentrations of heavy metals such as copper, cadmium, arsenic and zinc, which, being both highly soluble and extremely dangerous for the stability of compounds, are usually toxic for most lifeforms. Ferroplasma (also an acidophile, as it lives at a pH of 1.7) lives in pyrite (iron sulfide), where it oxidizes iron producing sulfuric acid. Halomonas titanicae has been discovered as it consumed the iron in the Titanic 's wreck.
    The bacterium Ralstonia is known (see here, page 32) to survive at a molarity of 2.5 mM of nickel, 2.5 mM of cadmium, 12 mM of zinc and 20 mM of cobalt; unicellular algae such as Euglena and Chlorella can grow in cadmium, zinc and cobalt at similar concentrations.
  • Halophiles can survive highly saline environments, such as the Dead Sea. By definition, they are anything that can survive in an area with a salt content greater than five times the ocean (more than 12 g/L) and requires salt to grow. Examples include Halobacterium (which can grow in water containing 35% of salt and survive in pure salt) and Dunaliella salina (an alga that dyes water red). As salty water produces a strong osmotic pressure that would otherwise crush the cells, halophiles survive producing glycerol with a internal concentration similar to that of salt outside.
  • Osmophiles live in highly concentrated solutions. As in the case of halophiles, these solution exert osmotic pressure counteracted by the internal proudction of alcohol and aminoacids. They're most commonly found in sugar solutions: among the most resilient there is Saccharomyces cerevisiae, the common beer yeast.
  • On the contrary, oligotrophs live in environments very poor in nutrients. These include caves, oceanic sediment, glaciers, deep soil, aquifers and open sea water. Oligotrophs grow quickly but have a very slow metabolic rate, and a very sparse population. Lichens are oligotrophs, and so is Pelagibacter ubique, which is estimated to exist with 1027-28 individuals in the world, and to be the most common organism on Earth.

Other conditions

  • Pitch Lake

    Trinidad's Pitch Lake, poor in water and lacking oxygen, hosts a community of bacteria, archaea and fungi that breathe metals and eat hydrocarbons. On average, each gram of pitch contains over ten millions of living cells.

    Hypoliths live under rock fragments in extremely poor polar environments such as the Canadian Arctic Archipelago; these rocks shield them from wind and UV light, and trap moisture. Such communities (hypolithon) are dominated by cyanobacteria, and they're usually found in translucent rock such as quartz. However, in the Devon and Cornwallis islands they've been found in opaque fragments of dolomite. The ecological productivity of hypolitha is roughly equal to that of surface moss and lichens.
  • Endoliths colonize interstices in rock, or pores inside the mineral grains that make them up, or mineralized animal bodyparts such as coral or mollusk shells. They divide in chasmoendoliths (living in crevices and fissures), cryptoendoliths (living inside porous rocks) and euendoliths (that burrow their own cavities in stone). Usually, they're hypoliths, psychrophiles or thermophiles too; they're found in basalt on the ocean floor, inside permafrost and in Antarctic Dry Valleys, and down to 3 km deep in Earth's crust (below 4-5 km, temperature is too high even for hyperthermophiles). Photosynthetic endoliths are known, but most of them eat mineral traces of iron, potassium and sulfur.
  • Worms, crustaceans and bivalves live on ocean floor where water's pressure is above 300 atmosphere (the weight of 300 kg of water on each square cm); they're barophiles or piezophiles. Such pressure can be counteracted by the pressure of internal fluids (so that these organisms explode when they're brought in surface). Flatworms in the Gulf of Mexico live at 500 m deep among leaks of ethane, propane, sulfides and oil; the bacterium Halomonas salaria not only survives up to 1000 atm, but it cannot live under 102 atm. Most barophiles live in darkness, and they're very vulnerable to UV light.
    A variety of bacteria (among which Escherichia coli) and the yeast Saccharomyces cerevisiae have a remarkable resistance to acceleration, like the gravity of an extremely dense body: they're known to grow inside a centrifuge spinning with an acceleration above 400,000 g.
  • Radioresistant organisms are, of course, those who survive at very high levels of radiations. To humans, the average lethal dose of radiations is 4.5 gray (Gy; each gray is equal to a J of energy absorbed in a kg of matter), whil a cockroach can survive up to 64 Gy and a fruit fly to 640 Gy. The most extreme radioresistant is the archaea Thermococcus gammatollerans, discovered in 2003 in a hydrothermal vent 2 km deep in the Pacific Ocean, can survive up to 30,000 Gy. Such lifeforms could be radiotrophs.
  • Aridophiles or xerophiles survive in arid environments. In the Negev Desert, an empty expanse of sand in Israel, rocks are covered in lichens. the algae Dunaliella grow on spider webs to collect droplets of dew in the Atacama Desert in Chile; various bacteria inhabit the sandy Dry Valleys in Antarctica, where it hasn't rained once for two million years. Endoliths and halophiles are often xerophiles, too.
  • Scotophiles live in darkness. While the lack of light isn't an issue for heterotrophs, the basis of the foodchains have to find another energy source (heat, radiations, energetic chemicals...). Relatively large-sized animals (spiders, water scorpions, woodlice, millipedes, snails, etc.) have been discovered in the Movile cave, in Romania, in 1986, where light didn't enter for five million years; there, autotrophs extract energy from seeping methane and hydrogen sulfide. Abyssal hydrothermal vents (which also require barophiles and mostly thermophiles) feed their ecosystems with sulfide emissions.

Notable extremophiles

Deinococcus radiodurans

Deinococcus radiodurans.


Spinoloricus nov. sp. The black bar is 50 micrometres long.

Among bacteria and archaea, there are several examples of organisms that live on iron, sulfur hydrogen; that absorb nutrients from rocks and concrete; that survive  incysted as spores to freezing temperatures and absolute lack of food or water for extremely long spans of time; bacterial spores, found in the body of a bee preserved in amber, have been revived after 80 million years simply by immersing them in a nourishing solution.

The bacterium Deinococcus radiodurans is a polyextremophile, as it's able to survive in a wide variety of extreme conditions: it can survive to 5000 Gy of radiations reconnecting the parts of damaged DNA, to freezing, to dehydration, to acids (it's at the same time radioresistant, psychrophile, xerophile and acidophile) and in the complete vacuum; GMO strands are known to consume toluene and mercury in nuclear waste sites.

Among animals, the family Alvinellidae - great worms hidden in mucus tubes, from which they extend long gills - live in hot, sulfurous water around hydrothermal vents in the Pacific Ocean, up to 80°C (they're both thermophiles and slightly acidophiles), while their head lives at a temperature of 22°C: it's still unknown how they survive to such temperature differences.

Tardigrades, tiny eight-limbed invertebrates, usually less than half a mm long, can survive for a few seconds up to 151°C and down to -272°C (only  a degree above absolute zero), and live at -200°C for several days; they survive in vacuum for ten days, at a pressures above 1200 atm; they survive after ten years of dehydration, and to the exposition to 5000 Gy of gamma rays and 6200 Gy of heavy ions, to high concentrations of salt or different toxins, and after being boiled in alcohol. They survive to most of these conditions by retreating in cryptobiosis, a suspension of metabolic activity where their water content drops from 85% to 3%, and counteract freezing by producing glycerol and trealose.

Other examples of animal adaptability are found in three yet nameless species of loriciferans in the genera Pliciloricus, Spinoloricus and Rugiloricus, discovered in the Mediterranean: they're the only known animals that don't breathe oxygen (see here and here). Their cells lack mitochondria, but have instead hydrogenosomes, which produce energy by oxidising hydrogen.

Implications for exobiology

Also see: Planetary models, Panspermia, Exotic life

While the abilities of extremophiles are doubtlessly extraordinary, we do not know if they in fact represent the limits of what life can do, especially with a different chemical makeup, or even more exotic forms of life based, say, on nuclear rather than chemical reactions. However, this sort of life is extremely speculative, and there isn't much we can say about it, if not that it'd be utterly different from anything we know.

As far as Earth-like life is concerned, there simply are some limits that cannot be overcome, such as the enormous gravity of neutron stars, the extreme temperature of stellar plasma, the powerful irradiation of the hottest stars or the dearth of energy and matter in interstellar space. Anyway, some celestial bodies have similar conditions to those life is known to survive in; the dry and frozen surface of Mars is very similar to the Antarctic Dry Valleys, while Titan is covered in hydrocarbon lakes that could resemble Trinidad Pitch Lake. Thermo-acidophiles similar to Picrophilus might inhabit the sulfuric acid clouds of Venus, while hydrothermal vents below the frozen oceans of Europa could host communities of scotophiles and thermophiles.

It's important, though, to remember that all this resilience was developed subsequently, and that the origin of life probably occurred in much more restrictive circumstances. Still, this allows for the possibility of life on worlds that were once more hospitable before turning hostile, such as Mars and Venus, or the transfer of life from planet to planet through panspermia: they would evolve on a relatively comfortable world, adapt as extremophiles, and then be hurled in space by an asteroid impact to find themselves stranded on a distant, and more hostile, planet.

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