At a chemical level, life consists of many types of molecules that interact with one another to carry out the processes of life. Life also needs an environment in which those complicated molecules are stable (don’t break down before they can do their jobs) and their interactions are possible. Your own biochemistry works properly only within a very narrow range of about 10°C in body temperature and two-tenths of a unit in blood pH (pH is a numerical measure of acidity, or the amount of free hydrogen ions). Life overall must also have limits to the conditions in which it can properly work but, as we will see, they are much broader than human limits. Let’s have a look at some of the conditions that can challenge life and the organisms that have managed to carve out a niche at the far reaches of possibility.

Extreme Conditions

Extremophiles are organisms that live in habitats that seem extreme to humans (the suffix -phile means “lover of”). The ability of organisms on Earth to live in a wide range of environments is the secret to the survival of life on this planet and extremophiles help us to understand the limits to life. On the early Earth, the large swings in climate would have cycled between hot and freezing conditions. Before microbes invented photosynthesis, life adapted to take advantage of many different metabolic pathways. There is evidence in the geologic record that oxygen levels in the atmosphere fluctuated wildly before a precipitous rise 2.4 billion years ago. Through every mass extinction, there were some niches of life that survived. Even our flammable, oxygen-rich atmosphere constitutes an extreme environment.

Being “extreme” is in the eye of the beholder. Most of the organisms that humans identify as extremophiles are small prokaryotes in the archaea or bacteria domain on the Tree of Life. They are known to survive in a wide range of extraordinary environments.

Temperature

Both high and low temperatures can cause a problem for life. As a large organism, you are able to maintain an almost constant body temperature whether it is colder or warmer in the environment around you. But this is not possible at the tiny size of microorganisms; whatever the temperature in the outside world is also the temperature of the microbe, and its biochemistry must be able to function at that temperature. High temperatures are the enemy of complexity—increasing thermal energy tends to break apart big molecules into smaller and smaller bits, and life needs to stabilize the molecules with stronger bonds and special proteins. But this approach has its limits.

Thermophiles

Thermophiles (temperature-loving organisms) live in high temperature environments, some at temperatures of 235°F (113°C). High-temperature environments like hydrothermal vents on the deep seafloor (Figure 1) and hot springs on land surfaces (Figure 2) often offer abundant sources of chemical energy and therefore drive the evolution of organisms that can tolerate high temperatures. Bacteria feeding on this chemical energy form the base of a food chain that can support thriving communities of animals—in the case shown in Figure 1, a dense patch of red and white tubeworms growing around the base of the vent. What appears to be black smoke is actually superheated water filled with minerals of metal sulfide.

High temperatures are challenging for life because the energy causes proteins to denature, or unfold. Chemical reactions proceed more quickly, and temperatures above 100°C can denature nucleic acids, causing DNA to lose its helical shape or break apart the structure of biomolecules. The fluidity and permeability of cell membranes are affected by heat. High temperatures decrease the solubility of carbon dioxide and oxygen in water, a problem for aquatic aerobic life. Thermophiles have metabolisms that take advantage of the higher chemical reactivity enabled by high temperatures. They make use of special temperature-resistant proteins and protective mechanisms for shielding DNA. Thermophile coping mechanisms include an altered ratio of saturated hydrocarbons in cell membranes.

The young Earth, with a tremendous amount of internal energy from accretion and differentiation, may have harbored many hot springs and ocean floor hydrothermal vents. Young Earth would have been a paradise for thermophiles, and the archea and bacteria that are most deeply rooted in the phylogenetic tree of life are thermophiles. The range of metabolisms for thermophiles may be the result of the diversity of chemistry at deep sea hydrothermal vents.

Currently, the high temperature record holder is a methane-producing microorganism that can grow at 122°C, where the pressure also is so high that water still does not boil. That’s amazing when you think about it. We cook our food—meaning, we alter the chemistry and structure of its biomolecules—by boiling it at a temperature of 100°C. In fact, food begins to cook at much lower temperatures than this. And yet, there are organisms whose biochemistry remains intact and operates just fine at temperatures 20 degrees higher.

Psychrophiles

Cold can also be a problem, in part because it slows down metabolism to very low levels, but also because it can cause physical changes in biomolecules. Psychrophiles are organisms that can withstand extreme cold. They are found in liquid brine inclusions in ice cores, or living under rocks in the extreme deserts of the world. This class of extremophiles includes bacteria and eukaryotes.

Cells must be resilient to freezing, which can form ice shards that would pierce and destroy ordinary cells. Cell membranes—the molecular envelopes that surround cells and allow their exchange of chemicals with the world outside—are basically made of fatlike molecules. And just as fat congeals when it cools, membranes crystallize, changing how they function in the exchange of materials in and out of the cell. One defense of a psychrophile is to lower the freezing point with solutes in cytoplasm that essentially act as an antifreeze. To increase fluidity of cell membranes, psychrophiles have evolved a different ratio of unsaturated to saturated fats. At cold temperatures, proteins are more rigid, so enzymes are used to lower the activation energy for biochemical reactions. Thus far, the coldest temperature at which any microbe has been shown to reproduce is about –25 ºC in Arctic permafrost.

pH

Acidophiles and alkaliphiles thrive in environments with very low or high pH, respectively. Conditions that are very acidic or alkaline can be problematic for life because many of our important molecules, like proteins and DNA, are broken down under such conditions. The most acid-tolerant organisms (acidophiles) are capable of living at pH values near zero—about ten million times more acidic than your blood. Acidophiles have been found in conditions as acidic as battery acid. They have been able to adapt mechanisms for keeping the acid out so that the cell cytoplasm can have a neutral pH. Figure 3 shows Rio Tinto in Spain where acidophiles thrive; the rusty red color that gives the river its name comes from high levels of iron dissolved in the waters

At the other extreme, some alkaliphiles can grow at pH levels of about 13, which is almost a million times more alkaline than your blood. This is also comparable to the pH of household drain cleaner, which does its job by breaking down the chemical structure of things like hair clogs.

Salt

High Radiation

Life has even been found next to nuclear waste storage sites in the presence of enough radiation to grant super powers. Radiation can be very damaging to DNA and cause cancerous cells or tumors. These radioresistant organisms have evolved mechanisms to protect their DNA even in the presence of over 1000 times the radiation a typical organism can withstand. There is even an organism, Deinococcus radiodurans, that can tolerate ionizing radiation (such as that released by radioactive elements) a thousand times more intense than you would be able to withstand. It is also very good at surviving extreme desiccation (drying out) and a variety of metals that would be toxic to humans.

Life is everywhere on Earth

Many other adaptions to environmental “extremes” are also known. Endoliths are extremophiles that live inside of rocks. On Earth, endoliths have been found living inside rocks in deserts in Antarctica, Chile, and Namibia, as well as regions deep below the Earth’s surface, including inside a gold mine in South Africa that is more than 2 miles underground. Barophiles thrive in high-pressure environments. Very high pressures can literally squeeze life’s biomolecules, causing them to adopt more compact forms that do not work very well. But we still find life—not just microbial, but even animal life—at the bottoms of our ocean trenches, where pressures are more than 1000 times atmospheric pressure. There also still exist anaerobes that do not require oxygen and live deep underground. Xerophiles require very little water and are found in the soil of the world’s largest deserts.

From many such examples, we can conclude that life is capable of tolerating a wide range of environmental extremes—so much so that we have to work hard to identify places where life can’t exist. A few such places are known—for example, the waters of hydrothermal vents at over 300°C appear too hot to support any life—and finding these places helps define the possibility for life elsewhere. The study of extremophiles over the last few decades has expanded our sense of the range of conditions life can survive and, in doing so, has made many scientists more optimistic about the possibility that life might exist beyond Earth. Extremophiles are a great boon to astrobiology both in demonstrating the wide variety of life that is possible, and by expanding the definition of what is “habitable.”

Earth Analogs in the Solar System

There is no other place in our solar system that today has the same hospitable conditions as Earth, with liquid water oceans on its surface today. However, we can learn from the extreme conditions on Earth that host life and identify similar environments on other planets or moons. Regions similar to frozen lakes and dusty deserts where we find extremophiles on Earth can be pinpointed elsewhere in the solar system, where a search for similar extremophiles in these regions holds promise.

Mars

Today, Mars is a barren world with a thin atmosphere dominated by CO₂. Photos of the surface Mars taken by the rovers show a landscape that looks similar to some deserts on Earth (Figure X). Can you tell which picture is of Earth and which is of Mars (before you read the caption)?

The Atacama desert is the driest place on Earth so represents a reasonable analog for some places on Mars, such as the Jezero Crater. Life has been found within the extreme location of the Atacama desert, including halophiles, endoliths and radioresistant bacteria.

Europa and Enceladus

Jupiter’s moon Europa and Saturn’s moon Enceladus are both believed to harbor liquid water oceans beneath their icy surfaces. Several places on Earth serve as analogs for these cold, dark, high-pressure environments. Lakes buried beneath Earth’s surface, such as Lake Vostok in Antarctica, have similar conditions to what we would expect for the oceans of Europa and Enceladus; searches for life within the overlaying ice or in the water of Lake Vostok and other subglacial lakes provide some clues as to what we could reasonably expect to find there. A rich variety of Hydrothermal vent communities, such as the “Lost City” at the Mid-Atlantic Ridge on the floor of the Atlantic Ocean, including alkaliphiles and microbes that use chemosynthesis to generate energy.

Tardigrades

Tardigrades, also known as water bears or moss piglets, are not technically extremophiles but are nevertheless extreme in their own right as well as strangely adorable. They are a type of micro-animal, only 0.5 mm in length on average, with eight-legs, typically found in water, and among the most resilient creatures known. They are different from extremophiles, which are adapted to live in terrifyingly harsh conditions, because they do best in average conditions. However, they are capable of surviving the most extreme conditions on this world, including low pressure environments in space.

The video below nicely describes how tardigrades can exist in a state known as anhydrobiosis (can you guess what this word means just based on the roots hydro and biosis?).

In 2007, samples of tardigrades were taken into low Earth orbit and exposed to the vacuum and radiation of space for 10 days. After the reanimation of the tardigrades back on Earth, most of the tardigrades began living as normal after 30 minutes (though most of the sample did suffer later health effects). Their incredible resilience allowed water bears to survive through five great extinction events on Earth. Their capability to survive in space gives new life to the theory of panspermia. Though not technically extremophiles, tardigrades certainly win a prize for resilience.

In April 2019, an Israeli spacecraft called Beresheet almost made it to the moon. The privately funded mission was the first stage of a privately funded project to transfer DNA to the moon to build a repository for rebuilding life in the event of catastrophic mass extinction. Among the passengers on the spacecraft: dehydrated tardigrades. It is unlikely that these creatures could survive on the moon without liquid water.

Review Questions

Summary Questions

1. Which environments on Earth represent “extreme” conditions for life to survive in?
2. What types of organisms can thrice in these extreme environments?
3. For each of the following extremophiles, explain what extreme condition they can survive in and where they are found on Earth: thermophiles, psychrophiles, acidophiles, alkaliphiles, halophiles, radioresistant, endoliths, xerophiles and barophiles.
4. What are some other places in our solar system with extreme conditions that could support extremophiles?
5. What are tardigrades? Which extreme conditions can they survive in?

Exercises