Extremophiles: How Do These Organisms Push the Limits of Life?

Extremophiles are organisms that inhabit "extreme" environments. The name, first used in 1974 in a paper by a scientist named R.D. MacElroy, literally means "extreme-loving" [source: NASA].

These hardy creatures are remarkable not only because of the environments in which they live, but also because many of them couldn't survive in supposedly normal, moderate environments. For example, the microorganism Ferroplasma aci­diphilum ­needs a large amount of iron to survive, quantities that would kill most other life forms.

Like other extremophiles, F. acidiphilum may recall an ancient time on Earth when most organisms lived in harsh conditions similar to those now favored by some extremophiles, whether in deep-sea vents, geysers or nuclear waste.

­However, extremophiles aren't just bacteria [source: Science Resource Education Center]. They come from all three branches of the three-domain system: Archaea, Eubacteria and Eukarya. (We'll explore taxonomy more next.)

So extremophiles are a diverse group, and some surprising candidates — yeast, for example — qualify for membership. They're also not always referred to strictly as extremophiles. For instance, a halophile is so named because it thrives in a very salty environment.

The adaptation of extremophiles to these extreme conditions involves various biological processes that enable them to survive and reproduce in challenging environments like contaminated sites. For example, some extremophilic bacteria found in hydrothermal vents can use carbon dioxide as a carbon source for chemosynthesis, which is essential for the food chain in these deep-sea environments.

Underground Discovery

In the 1960s, researchers began studying extremophiles in places like the Great Salt Lake, leading scientists to reassess how life began on Earth. Numerous types of bacteria have since been found deep underground, an area previously considered a dead zone (because of its lack of sunlight) but now seen as a clue to life's origins. In fact, the majority of the planet's bacteria live underground [source: BBC News].

These specialized, rock-dwelling extremophiles are called endoliths (all underground bacteria are endoliths, but some endoliths are nonbacterial organisms). Scientists speculate that endoliths may absorb nutrients moving through rock veins or subsist on inorganic rock matter.

Some endoliths may be genetically similar to the earliest forms of life that developed around 3.8 billion years ago. For comparison, Earth is about 4.5 billion years old, and multicellular organisms developed relatively recently compared to unicellular, microbial life [source: Nature].

Every year, researchers discover and name thousands of new species. In recent years, microorganisms have formed an important part of this enormous growth in species discovery. More than 2 million species have been identified around the planet, but some experts speculate that 8.7 million or more may exist [source: National Geographic].

The Kingdoms

But there's more to finding new species than naming and cataloging them. And for comparing living creatures, nothing beats a good classification system. The two most popular methods in use are the five-kingdom and the three-domain systems.

Created in the late 1960s, the five kingdoms separate life into Monera, the kingdom of prokaryotes (cells lacking membrane-bound nuclei and organelles) that includes bacteria, as well as four eukaryotic (cells with membrane-bound nuclei and organelles) kingdoms: Protista, Fungi, Plantae and Animalia.

For a short while, the five kingdoms seemed to serve scientists well. But in the 1970s, a scientist named Carl Woese decided to classify organisms based on genetic differences rather than differences in visual appearance.

When Woese began his classification efforts, he noticed that there were distinctions between some types of organisms that had been previously lumped together as bacteria because they were all prokaryotes.

Uncovering the Third Domain

Woese's groundbreaking research reshaped our understanding of the diversity of life. He uncovered evidence suggesting that bacteria and another unidentified group of organisms diverged from a common ancestor billions of years ago.

Recognizing the uniqueness of these other organisms, Woese restructured the Monera kingdom of prokaryotes into two domains: Archaea (formerly archaebacteria) and Eubacteria. He designated a third domain, Eukarya, for organisms with a nucleus.

Archaea, many of which are extremophiles (meaning they thrive in extreme conditions), have distinct ribosomal RNA (rRNA) and specialized cell membrane adaptations for survival. In contrast, Eubacteria, or "true bacteria," are more recent in origin and include pathogens.

The broad Eukarya domain encompasses organisms with nuclei and further divides into kingdoms like Protista, Fungi, Plantae and Animalia. Some eukaryotes — organisms that contain a true nucleus and membrane-bound organelles — also thrive in extreme environments. (Humans fall under this domain.)

These classification methods, while occasionally sparking debate, offer valuable insights into the distinctions between extremophiles and other life forms.

Sample of Extremophile Classifications

Before we look at a few of the environments extremophiles favor, here's a list of some additional names used to classify specific types of extremophiles:

An environment is called extreme only in relation to what's normal for humans, but for an extremophile, their favored environments are "normal." And beyond Earth, conditions that make life possible for humans are likely rare. In turn, so-called extreme environments and the extremophile­s that populate them may be more commonplace.

Here on Earth, a number of factors might earn a place the label "extreme," including the following:

­Remember that these factors can sometimes be extreme in one of two ways: very hot or very cold, highly acidic or highly alkaline. Most organisms that we see or encounter subsist in temperatures ranging from 41 degrees Fahrenheit (5 degrees Celsius) to 104 degrees Fahrenheit (40 degrees Celsius), but extreme life has been found in nuclear reactors, penguin guano, volcanoes, practically oxygen-free zones, incredibly salty areas like Utah's Great Salt Lake and in the digestive systems of many animals, including insects [source: Science Education Resource Center].

In one case of thriving in extreme conditions, bacteria were found entombed in Alaskan ice. When the ice melted, bacteria that had been dormant for tens of thousands of years resumed activity, as if nothing had happened.

Antarctica's Lake Untersee is a great example of an extreme environment. The water is brimming with methane and has a highly alkaline pH, comparable to laundry detergent [source: Science Daily]. Scientists at NASA are particularly interested in the lake because its distinct environment — lots of methane and low temperatures — may be similar to those of other planetary bodies, such as Jupiter's moon Europa [source: NASA].

Acidic Environments

As we've mentioned, some of these organisms love a good acidic environment. But just how acidic? First, let's take a look at how acidity is measured. "Potential of hydrogen," or pH, is a measure of the acidity or alkalinity of a substance, indicating the concentration of hydrogen ions (H+) in a solution. In terms of pH: 0 is most acidic, while 14 is most basic or alkaline.

Humans prefer a pH of 6.5 to 7.5, but acidophiles thrive in places with pH levels ranging from 0 to 5. The human stomach actually falls into this category, and we have some extremophiles living in our bodies.

In general, acidophiles survive in acidic environments by strengthening their cell membranes. Some produce biofilms (colonies of microorganisms that aggregate, creating slimy, extracellular protective films) or fatty acids that protect their cell membranes. Others can regulate their internal pH to keep it at a more moderate level of around 6.5.

Extremophiles in highly alkaline environments also manage to regulate internal pH and have enzymes that can withstand the effects of high alkalinity.

One such extremophile is Spirochaeta americana, a bacteria that lives in the mud deposits of California's Mono Lake and whose discovery was announced in May 2003. One of 14 known spirochetes, this extremophile needs an alkaline pH from 8.0 to 10.5. And it's anaerobic, which means it's incapable of living in environments with oxygen.

Spirochetes like sulfurous mud deposits and don't rely on oxygen. For example, Spirochaeta thermophila lives near deep-sea hydrothermal vents. Mono Lake's mud is alkaline with a pH of 10, very salty and filled with sulfides. The lake became this way because it's a terminal lake — water flows in but not out. As water evaporates, chemicals and minerals stay, becoming highly concentrated.

Geysers and Rivers

­Many other notable extreme environments also play host to extremophiles. Numerous geysers around the world, including some in Siberia, have extremophiles living in their hot pools and vents.

In the United States, Yellowstone National Park has thousands of geysers, springs and other geothermal features, with varying levels of temperature, acidity and sulfur — and many types of extremophiles.

Rio Tinto, a river in Spain, is full of heavy metals because the region has been host to mining operations for thousands of years. Similarly, Northern California's Iron Mountain has water so loaded with heavy metals and acids (byproducts of mining) that it can eat through a metal shovel in a day.

But even here, deep in underground mines, microbes from the Archaea and Eubacteria domains manage to survive scrappily, using biofilms for both protection and nutrient absorption.

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­In the 1960s, Dr. Thomas Brock, a biologist, was investigating bacteria in Yellowstone National Park's hot springs when he stumbled upon something unprecedented: Bacteria living in the area were thriving at extraordinarily high temperatures. The newly named Thermus aquaticus lived in water that was nearly 212 degrees Fahrenheit (100 degrees Celsius) — practically boiling.

T. aquaticus provided the basis for two groundbreaking discoveries in biology and proved to be the first Archaea. (Remember Archaea are a diverse group of organisms with their own unique type of rRNA, different from bacteria.) Equally significant, this extremophile produced an enzyme known as TAQ polymerase, which found an industrial application in PCRs (polymerase chain reactions).

PCR allows scientists to replicate a piece of DNA billions of times over a span of a few hours. Without the process, nearly all work requiring DNA replication — from forensic science to genetic testing — wouldn't be possible.

Other extremophiles have proved useful in industrial and medical research applications, though likely none so much as T. aquaticus. Scientists have examined at least one extremophile that produces a protein similar to one found in humans. This protein appears to play a role in various autoimmune diseases and conditions like arthritis.

Enzymes from alkaliphiles are used for making laundry and dishwashing detergents. They are also used for removing hair from animal hides. Another alkaliphile from Yellowstone is used in making paper and treating waste because it produces a protein that breaks down hydrogen peroxide.

Radiation Resistance

­NASA is studying an extremophile, Deinococcus radiodurans, which is extremely resistant to radiation. This microbe can withstand doses of radiation 500 percent higher than would be lethal to humans [source: NASA]. Interestingly, the radiation actually does break the microbe's DNA into pieces. But in many cases, the DNA can reassemble and work normally again.

It accomplishes this by shedding broken parts of DNA, using a special enzyme to attach good DNA to other still-healthy pieces of DNA, and then creating complementary pieces to­ bond to these newly formed long DNA strands.

Understanding how D. radiodurans does this could allow scientists to bring dead cells back to life. For NASA, harnessing this DNA-resistance could offer clues for building better spacesuits or spacecraft.

Panspermia is the idea that primitive life forms could travel between planets and survive the journey. For some, panspermia represents a possible origin of life on Earth, as microbes from other planets — yes, we're talking extraterrestrial life — could have arrived here and acted as the forebears of all subsequent developing species.

The concept is often ridiculed as unrealistic and speculative, but several recent studies have lent panspermia more credibility.

One study found that some tardigrades, microscopic eight-legged invertebrates, were able to survive after spending 10 days exposed to space and solar radiation. Between various other research efforts, scientists have found that organisms classified as bacteria, lichens and invertebrate animals have survived at least some time spent in the vacuum of space. Some protection from radiation, such as being on a rock, seems to help organisms survive the journey.

But wherever they land, these space travelers need an environment that will allow them to live and grow.

So with these ideas in mind, is it fair to say that we humans might be aliens? One popular panspermia theory holds that earthly life originated on Mars, which, about 4.5 billion years ago, was far more hospitable to life than our planet [source: USA Today].

In addition, the Late Heavy Bombardment, a period of numerous asteroid impacts on Earth and Mars, might have brought life to Earth around 4 billion years ago.

But if this is true — and many scientists don't think it is — life almost certainly didn't come from another solar system or stars. The distances are still widely considered too great for life to have survived.

Astrobiology and Microbial Life Forms

Instead of a rather farfetched theory like panspermia, the answers to our origins may come through astrobiology, the study of life throughout the universe. Astrobiology draws heavily on the study of extremophiles because of the belief that life forms beyond Earth may be residing in extreme environments.

But astrobiology isn't just a quest for life in other parts of the universe. It also examines basic questions about the origins of life, environments that are conducive to optimal growth for life, how life develops and the limits of what life can tolerate.

Central to astrobiology is the search for the original ancestor of all living things on Earth — variously referred to as the Last Universal Common Ancestor (LUCA), the Last Common Ancestor (LCA) or the Cenancestor. Scientists believe that LUCA was an extremophile that lived more than 3 billion years ago in a harsh, anaerobic environment.

Even so, scientists are also debating what came before that, going back in time from DNA-based organisms (like humans and LUCA), to RNA-based ones and finally to the First Living Organism (FLO).

But this quest points us toward even more basic questions: Namely, what is life? Is life just a bundle of amino acids? Similarly, when, exactly, did Earth shift from a chemical world to a biological one? Is life something that can replicate itself? Something that can evolve?

In probing these questions of where we come from, extremophiles, those strange survivors from our past, will surely be part of biology's exciting future.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.