What is extreme life, and where do we find it?

Note from Justine: This guest post by Rosie Cane explores the lives of microbes in extreme environments.

Have you ever wondered what kinds of exciting life might survive the extremely hot, dry conditions of deserts or the depths of Earth’s oceans? And how we even discovered this life in the first place? Then buckle up for a thrilling ride, as we’re going to visit some of the harshest environments on Earth…and beyond.

In this post, we will cover some background about these tough environments, examples of where we can find them on Earth, and how they can be used to search for life beyond Earth.

“Extreme” environments from a human perspective

To start our journey into the extremes, it would help to understand what extreme environments are. Once upon a time in the not so distant past, we thought environments like the hot, acidic caverns of volcanoes and the cold, crushing depths of Earth’s oceans were uninhabitable. And to some extent, they are, but only from a human-centric viewpoint. We humans deem any environment too harsh for people to survive in as ‘extreme.’

However, we know that life can exist in these so-called extreme environments from studies as far back as at least the 1890s. One example of this is the findings of Dr. Thomas Brock in the 1960s, who discovered a temperature-loving extremophile, called Thermus aquaticus. This hardy bacterium grows best when under temperatures ranging from 50 °C to 80 °C (122 °F to 176 °F) in a hot spring in Yellowstone National Park, USA.

Extremophiles: the organisms that thrive in extreme environments

The majority of life found in extreme environments are microorganisms (microbes). These are split into two groups: extremophilic life or extreme-tolerant life. Extremophilic (meaning extreme-loving) organisms or “extremophiles” thrive within extreme environments and actually require these extremes to grow. Extreme-tolerant organisms do not thrive in these extremes but simply persist within them. Most extremophilic microbes are archaea (learn more about different types of microbes here) but can also be bacteria, algae, fungi, protozoa, and viruses.

Types of extreme environments

Temperature        

Normal body temperature is generally accepted as 37 °C (98.6 °F). But microbes can adapt to temperatures much higher and lower than this. If an organism grows best at 45 °C to 80 °C (113 °F to 176 °F), it is considered a thermophile (heat-loving). If it grows best at temperatures above 80 °C (176 °F), it is a hyperthermophile (extreme heat-loving). And we do know of life on Earth that not only survives but also thrives at these temperatures. One example is Pyrococcus furiosus, a type of archaea found in hydrothermal vents. This microbe has optimal growth conditions at exactly 100 °C — the boiling point of water. In stark contrast, if an organism grows best at temperatures below 15 °C (59 °F), it is a psychrophile (cold-loving). It is believed that -20 °C is the cut off for any life on Earth, as far as we know.

pH

Another type of extreme is based on pH. The pH scale goes from 0 to 14, with 0 being acidic and 14 being alkaline. pH 7 is neutral, like tap water, yet we find microbes on Earth that can survive in extremes of high or low pH. Any life that thrives below pH 5 is an acidophile (acid-loving) but becomes a hyperacidophile below a pH of 3. It’s the same at higher pHs: organisms that grow best above pH 9 are alkaliphiles (alkaline-loving) and become hyperalkalphiles above pH 11.

Salt

We can also get extreme conditions in terms of salt (yes, the stuff you put on your chips). Earth’s oceans have an average salinity of about 3.5%. Any organism that can live in these salt concentrations is known as halotolerant (able to tolerate salt). However, any life that flourishes above 8.8% salinity is known as a halophile (salt-loving).

And it’s not just extreme temperatures, pHs, and salt concentrations that these extremophiles live in. It’s also extreme pressure, radiation, desiccation (lack of moisture), and much more.

A famous example of an extremophile

The first extremophile (and first archaea) to have its genome sequenced was Methanococcus jannaschii. It was isolated from a hydrothermal vent called a white smoker, a chimney in the seafloor spewing out white-colored elements such as barium, calcium, and silicon. At thousands of meters below sea level, this microbe withstands significant pressure, enough to crush a standard submarine and near-boiling temperatures.

Translating extremes on Earth to extra-terrestrial extremes

However, we don’t just find extreme environments on Earth. One example of an extreme environment beyond Earth is an icy moon in our Solar System called Europa. Europa is one of Jupiter’s 79 moons and is thought to have a liquid water ocean beneath its surface. This moon is four times smaller than the Earth, yet we think its ocean contains about twice the amount of water in Earth’s oceans combined. Hydrothermal vents may even be present in this ocean, just as they can be on Earth. By understanding how life on Earth survives in such an environment, we can better understand how life might survive within Europa’s ocean (or other extra-terrestrial locations). Other icy moons in our Solar System may also have subsurface liquid water oceans, such as Saturns’ moons Enceladus and Titan.

But why do we care about water in extra-terrestrial environments? It helps in the search for life beyond Earth because all life we know requires liquid water as a solvent for chemical reactions. 

We can also use extreme environments on Earth to help us search for life on our neighboring planet, Mars. For example, by studying volcanic environments in Iceland and highly salty lakes in North America, we can get an idea of Mars’s environment in the past and whether it was ever habitable. The NASA Mars 2020 mission rover was tested in the Nevada Desert, a dry, barren environment of dried-up sea beds, to mimic the landing site’s surface on Mars. The rover is named Rosalind Franklin — after the British scientist who contributed to unraveling the double helix structure of our DNA — and will search for signs of past or present life on Mars.

Final thoughts on extreme environments

Overall, we have some weird and wonderful forms of life in the most extreme environments on Earth. A little over 100 years ago, we had no idea they existed, and now we’re discovering new extreme life every day. These environments are fascinating in general. But by studying them, we get the bonus of potentially understanding the habitability of other planetary bodies beyond Earth.

Further reading

• Types of extreme environments
• What are extremophiles?
• Meet the extremophiles
• Microbes and extreme environments
• Astrobiology @ NASA

Rosie Cane is a final year PhD student at The University of Edinburgh and UK Centre for Astrobiology studying life in extreme environments. She is currently Head of Astronomy at Thompson STEM Engagement and loves talking to people about space and astrobiology wherever possible! You can find her at @rosieccane and @ThompsonSpace on Twitter.