Our perception of the nature of life on Earth has changed over time. Before the invention of lenses, and the subsequent development of microscopes, there was a depth to our world that we could not grasp. As we have developed more and more powerful microscopes and discovered a seemingly infinite amount of microbial diversity, we have been forced to re-evaluate how we think about the nature of life. The world of microorganisms has spurred our desire to define a set of concrete boundaries that encapsulate the limits for life on Earth. It seems like the harder we try to establish the constraints; the more that life surprises us. Not only are we learning more about the diversity of life on Earth, but we are also learning that life thrives in some very shocking conditions. The environments in which we are discovering life are indeed extreme. And the organisms that we find living in those environments are called extremophiles.
We must answer one question before proceeding any further: what is extreme? Surely if we could travel back in time, snatch one of Earth's earliest living organisms, and bring it to modern Earth, that organism would consider our environment very extreme. After all, early Earth's environment would have been nicely suited for that early organism; little oxygen would have been in the atmosphere, the Sun would have been dimmer, and the climate on Earth would have been completely different. Our time-traveling microbe might find it very difficult to survive in its new oxygen-rich environment. To it, modern Earth would represent a very extreme environment. Yet our environment seems rather temperate to us. Using this logic, then, a definition of extreme can't be reached unless we define our system more concisely. It is most common to take a more anthropocentric perspective when defining extreme. Almost any environment that would make it difficult for humans to survive could be considered an extreme environment. However, after we explore the range of known extremophiles, a better feel for what constitutes an extreme environment will likely be gained.
We should pause here to make a general note about extremophiles. These are not organisms that merely tolerate life in the extreme, these are organisms that are known to thrive in the extreme and perish in the mediocre. Most extremophiles are microorganisms, and in particular they generally belong to the archaea domain of life. However, there are multi-cellular, eukaryotic organisms that have been noted to survive in harsh environments and may be considered extremophiles. Some of the most amazing photos of extreme environments come from the deep sea near hydrothermal vents and include images of eukaryotic extremophiles. This is important to remember as we examine the extreme environments of Earth and the creatures that inhabit them.
As humans, we may perceive our planet Earth as a moderate and comfortable place to live. However, there are a multitude of environments which would prove far too harsh for our survival, especially if we prohibited the use of technology. Environmental extremes on Earth include niches that are very hot or very cold, very acidic or very basic, extremely arid or dry, represent areas of extreme salinity, exposed to high amounts of radiation, under great amounts of pressure, and/or contain little to no oxygen. This is not meant to represent a comprehensive list of extreme environments, but rather is meant to provoke the reader to consider the wide range of environments that are represented on Earth.
On a hot summer day, we may exclaim that we are going to "die of the heat", or on a cold winter's evening we make think to ourselves, "I am freezing to death!" but the truth of the matter is that humans don't experience temperature in the same way as extremophiles. Can you imagine a hot summer's day that reached temperatures of 235 ° F (113 ° C)? No? Well, this is currently the highest temperature recorded in which life has been found living. The creatures that live there are a type of extremophile known as thermophiles (thermo- meaning "heat"and -phile meaning "loving"). On the other end of the temperature spectrum we find psychrophiles, organisms that thrive in very cold environments, down to -18 ° C (less than 0 ° F)! The environments in which these two extremophiles live couldn't be more opposite. Thermophiles grow best at temperatures greater than 176 ° F (80 ° C) which can be found in places such as boiling hot springs or deep in the ocean near hydrothermal vents. On the other hand, psychrophiles prefer temperatures much less than 15 ° C for optimum growth and have been discovered living in snow, ice, and glaciers.
We've already examined cases of extreme temperature. These cases are perhaps the easiest to think about because as humans we encounter temperature extremes in our daily lives. However, it is far less intuitive to think about some of the extreme environments on Earth. For example, what might an environment with extreme pH be like? The entire pH scale only goes from 0 to 14. A neutral value for pH is 7. Values less than 7 indicate acidic conditions while values greater than 7 are basic. Let's begin by considering extremely low values of pH. Acidophiles (acid-loving) prefer environments that are acidic, generally with a pH value of less than 4. This would be like living in battery acid. They are commonly found in acidic hot springs in places like Yellowstone National Park. Other extremophiles, known as alkaliphiles, are found living in more basic environments, generally in areas with a pH of 10 or greater. Alkaliphiles like to live in places like soda lakes. In chemistry class, you may remember being warned about acids such as sulfuric acid or bases such as sodium hydroxide. These substances are equally harmful to living tissue as they are both highly caustic and can damage living cells. Acidophiles and alkaliphiles have developed unique cellular mechanisms to deal with these pH extremes.
It has been proposed by astrobiologists that one of the environmental requirements for life on Earth is the presence of liquid water. As a result, we can identify organisms that thrive under conditions with limited access to water as another type of extremophile known as xerophiles. For centuries, human have been using the knowledge that life doesn't respond well to a lack of water to preserve our food. Drying out fruits and meats decreases the availability of water in those foods and prevents harmful microorganisms from growing on them, thus preserving it for human consumption. Examples of natural environments that possess extremely small amounts of liquid water would be the dry valleys in Antarctica, the surfaces of rocks, and organic fluids (like oil). It is important to note that although xerophiles exist in regions with little water, they must have access to liquid water for at least short intervals of time in small amounts in order to live.
Environments that have a high salinity (high concentrations of salt) represent environments similar to that occupied by xerophiles. After all, if there is a high concentration of salt, it is likely that the availability of water is decreased. The main problem with high salinity is an issue of osmotic pressure. If a cell were to be placed in a solution containing a great deal of salt, water would leave the cell due to osmosis. This would cause the cell to shrivel up and eventually die. Organisms that live in environments of high salt concentration, like the Great Salt Flats or the Dead Sea, are called halophiles. To provide a frame of reference, ocean water has a salinity of approximately 3.5%. Some halophiles have been found in environments with 10 times as much salt up to 35%!
Another extreme environment on Earth is that consisting of high levels of radiation. The word radiation can mean many things. In this case, we are considering radiation that is known to be detrimental to life. For example, if our Earth did not have a protective ozone layer to shield the surface from large amounts of UV radiation emitted from the Sun, it is unlikely that life would have ever evolved to live on land. In addition to UV radiation, exposure to ionizing radiation can also be damaging. A very surprising environment on Earth where we have found life is in nuclear waste dumps and nuclear reactor water cores. The extremophile that has been identified living in this environment, Deinococcus radiodurans, is considered a radiation-tolerant extremophile.
Now let's consider an environment not often visited by humans — the deep ocean. Although the technology has existed for divers to explore the ocean up to a hundred meters or so, it has only been recently that we have been able to investigate deep sea hydrothermal vents a few thousand meters deep. As we plummet deeper and deeper into the water the cumulative weight of the water above increases and so does the pressure. Organisms that thrive at high pressures are called barophiles or piezophiles. Whereas the atmospheric pressure at sea level is 1 atmosphere (atm) of pressure, barophiles will thrive at pressures up to 700 atmospheres. In fact, if you were to place a barophile in an environment with only 1 atm of pressure, it wouldn't be able to live.
The last extreme environment that we will discuss in detail is a good example of how anthropocentrically we define extreme environments. This extreme environment is one that lacks oxygen, otherwise known as anaerobic. As we have already discussed, early organisms on Earth would not consider an environment with little to no oxygen as extreme. Furthermore, there are many organisms currently on Earth that do not require oxygen for survival. The types of organisms that live in anaerobic environments can be split into two categories: facultative anaerobes and strict anaerobes. Facultative anaerobes are organisms that function well without oxygen but that can tolerate oxygen if it is present in the environment. Strict anaerobes abhor oxygen; if it is present in the environment it will kill them. There are many examples of anaerobic environments, ranging from lake sediments to your digestive tract.
Although we have not discussed all of the possible extreme environments or the creatures that inhabit them, it would only take a little imagination to come up with other types of extreme environments like those with excessive electric currents or inside of rocks. The goal is to identify extreme environments on Earth, study the nature of life within these environments, and extend that knowledge to our search for life in the Universe.