As part of the Astrobiology course I’m taking, we looked recently at some of the extreme environments on Earth1 where living things can be found (and in many cases, in fact, thrive). The organisms that live in these extreme environments are called extremophiles The reason we study extremophiles on Earth is to better understand the wide range of conditions under which life can evolve and survive, as well as help us understand some the mechanisms organisms can employ to survive different environmental extremes.
Ultimately when we then look at an extreme environment on some other planet or moon, we’ll have a much better idea of whether organisms (like the extremophiles we’re already familiar with on Earth) could survive there.
There are nine categories of extremophiles found on Earth:
- Psychrophiles (extreme cold) — Organisms with the ability to survive at temperatures below -4° F.
Many psychrophiles produce special proteins that act as antifreeze agents and prevent them from freezing solid. Some have evolved cell layers that resist hardening in extreme cold. An example of a psychrophile is chryseobacterium greenlandensis, which for the last 120,000 years has survived nearly two miles deep within the ice of a Greenland glacier.
- Thermophiles (extreme heat) — Organisms with the ability to survive at temperatures of 140° F or even higher.
Similar to psychrophiles, thermophiles have developed special proteins that allow them to tolerate a broad range of temperatures, in some cases including temperatures above the boiling point of water. One example of a thermophile is cyanidium, an algae that lives in the hot water springs in Yellowstone National Park. Other thermophiles can be found in crater lakes, peat bogs and even in superheated hydrothermal vents on the deep ocean floor.
- Radioresistant Microbes (extreme radiation) — Organisms that can consistently survive doses of radiation that are 500 times greater than the lethal dose for humans.
Radioresistant microbes often channel the energy from radioactivity to purposes such as producing food for themselves, and some have evolved aggressive DNA repair mechanisms to reverse any genetic damage caused by radiation. One example, Deinococcus radiodurans, is listed by the Guinness Book of World Records as “the world’s toughest bacterium.” Another type of radioactive fungi was found growing in the remains of the Chernobyl nuclear reactor following its “meltdown“2 in 1986.
- Alkaliphiles (extreme bases; high pH levels) — Organisms with the ability to survive and thrive in substances capable of neutralizing strong acids (environments with pH values ranging from 9 to 11).
Alkaliphiles have evolved unique enzymes, specialized cytoplasm and efficient cell membranes to protect their cells from damage. One example are the colonies of the alkaliphile Microcystis that flourish in the extremely alkaline Mono Lake in California.
- Acidophiles (extreme acids; low pH levels) — Organisms that survive in highly acidic environments (where the pH value rarely rises about 2).
Acidophiles are the opposites of alkaliphiles, thriving at the opposite end of the pH spectrum. Ferroplasma acidiphilum (found in mine drainage, waste treatment plants and acidic caves) is an acidophile that extracts energy from iron, essentially “eating” the metal and leaving behind rust.
- Halophiles (extreme saltiness) — Organisms that can survive in extremely salty environments (5 to 10 times saltier3 than ocean water).
Halophiles coat themselves with a special protein layer that blocks excessive salt from entering its cells. Dunaliella salina is a halophile algae that lives in salt ponds and concentrates beta‐carotene in its cell walls (resulting in an orange or pinkish color). Other halophiles have been found in Utah’s Great Salt Lake, the Dead Sea4 between Israel and Jordan, and even growing on saltine crackers.
- Xerophiles (extreme dryness; lack of water) — Organisms that can grow and reproduce in conditions with very little water available.
Xerophiles have evolved means to store and conserve any water they encounter, so it is available when needed (even if there is no water left in the surrounding environment). Wallemia sebi is a xerophile mold that grows in dried fruit, salted meats and even the evaporation beds where sea salt is produced. Mold growth on bread is an example of food spoilage caused by xerophilic organisms.
- Barophiles (extreme pressure) — Organisms that live in highly pressurized environments, such as the bottom of the ocean.
Barophiles have evolved a waxy cell layer which protects against both crushing pressures and frigid temperatures. Just to stay alive, the barophile Halomonas salaria requires pressure 1,000 times that found at Earth’s surface. Most barophiles are found on the ocean floor, where pressures are at least 400 times higher than Earth’s surface.
- Endoliths (extreme rockiness) — Organisms with the ability to survive within solid rock, or deep within the Earth’s crust.
Endoliths can survive for hundreds of years by feeding on trace amounts of iron, potassium and sulfur found in the rocks they inhabit. Some endoliths have been found as deep as two miles in the Earth’s crust, while others are found in desert rocks and on mountain slopes. Many scientists think that endoliths are the type of life most likely to be found on Mars (either living there today, or having existed there some time in the past).
Out of all of the extremophiles that have been discovered on Earth, which one did our class co‐instructor Liz Percak‐Dennett single out as “the most extreme living thing on Earth”? That credit goes to a less‐than‐attractive little organism called tardigrada. Tardigrada have been shown to survive:
- Temperatures ranging from -200 °C (-328 F) to 151 °C (304 F)
- Pressure ranging from near vacuum (like that found in outer space) to 1,200 times atmospheric pressure
- Dehydration and lack of water (one documented specimen survived in a dehydrate state for nearly ten years5 )
- Massive doses of radiation, as high as 5,000 Gy (Gray units; 5–10 Gy are typically lethal to humans)
You might expect an extremophile like tardigrada to be found exclusively in Earth’s more hostile environments, but Percak‐Dannett pointed out they are found all over Earth, including among lakeside moss6 in Madison, WI.
Given the many extreme environments on Earth where we’ve found life thriving,7 it’s clear that — an least in theory — certain extremophiles could survive in the conditions found elsewhere in our solar system8 or perhaps even on some of the many exoplanets we are discovering around other stars.
For those who want to learn more about extremophiles, the NASA Astrobiology Institute and the Wisconsin Astrobiology Research Consortium created a great set of extremophile trading cards. A PDF of “Life in the Extremes” is available on the Wisconsin Astrobiology Research Consortium website. [This Link has been fixed — sorry about that!]
- For our purposes, “extreme environments” means environmental conditions that are extreme — to the point of being fatal — for human beings. [↩]
- Yes, I realize that technically the Chernobyl reactor didn’t melt down, but rather suffered a catastrophic steam explosion. What do you expect from a guy would learned his “facts” about Chernobyl from a particularly bad episode of Airwolf? [↩]
- To provide some perspective, one gallon of water from the salt ponds where halophiles often are found contains 2.5 pounds of dissolved salt. [↩]
- Apparently the Dead Sea isn’t so dead if halophiles thrive in it. [↩]
- There is one less‐well‐documented report of a tardigrada having survived after 120 years in a dehydrated state. [↩]
- Tardigrada eat moss, as well as other plants and bacteria. [↩]
- In the case of Earth, pretty much everywhere we look we find life. [↩]
- The most likely candidates are Europa, Ganymede or Callisto (three of Jupiter’s moons); Titan or Enceladus (two moons of Saturn); Triton (one of Neptune’s moons); or Mars. [↩]