Extremophiles are organisms that have evolved to inhabit the harshest environments on Earth. These creatures have adapted to conditions that would be fatal to other animals, such as extreme temperatures, high pressures, acidity, toxicity, and a lack of oxygen. Extremophiles can give worldbuilders insight into what life may resemble in worlds drastically different than our own, and what adaptations, characteristics, and behaviors may be necessary to not only survive but to thrive in these environments.

Most extremophiles are unicellular organisms, typically bacteria or fungi.

These heat-loving organisms inhabit scorching environments, such as hot springs and deep-sea hydrothermal vents. Thermophiles can withstand temperatures that would instantly incapacitate or boil to death most other life forms (up to 122 °C / 252 °F). Worldbuilders designing super-heated regions or alien planets may want to look to thermophiles for inspiration.

Adaptions: Normal animals are powered by protein cellular machinery that falls apart at high temperatures. Frying an egg is an example of this process, called denaturation. The protein content of the egg denatures at high temperatures, turning from clear in color to solid white as it hardens. Thermophiles often use proteins that do not denature at high temperatures, meaning their cells aren’t cooked when the heat rises.

Volcano snails have a biological suit of armor made of iron sulfides. While the purpose of this iron is unknown, it may be an adaptation to their habitat, deep sea vents.

Opposite to thermophiles, psychrophiles thrive in frigid conditions, such as arctic areas or subglacial lakes. They can survive freezing temperatures (down to -39 ºC / -38 ºF).

Adaptations: Many psychrophiles can be frozen solid and can exist in that state for some time without harmful effects. After thawing out they can resume normal behaviors.

Acidophiles are found in highly acidic (pH levels of 3.0 or below) environments, including acid mine drainage, acid lakes, and even battery acid. Alkaliphiles are their opposites, being found in highly alkaline environments (pH levels of 8.5 and above).

Adaptations: The cellular proteins of acidophiles have evolved to be resistant to acid-induced destruction. Like thermophiles and their resistance to high temperatures, the proteins of acidophiles do not denature when exposed to high levels of acidity thanks to useful mutations in their genome. Alkaliphiles, on the other hand, have evolved an acidic coat of amino acids on the membranes of their cells to help protect them against the low pH environment.

Radioresistent organisms can live in areas of high ionizing radiation, either UV radiation from the sun or nuclear radiation from nuclear reactors. Radioresistence may be important for fictional creatures living in out in space or on planets without a protective atmosphere. It may also be an important attribute in post-apocalyptic wastelands.

Adaptations: Some radioresistant organisms are also radiotrophic and can feed off radiation as an energy source. Radiotrophic fungi have been found inside the destroyed Chernobyl nuclear power plant.

• Capnophiles: can inhabit environments with high levels of carbon dioxide, like the stomach of a large herbivore ruminant.
• Halophiles: reside in highly saline environments, such as salt pans, salt lakes, and even within salt crystals.
• Metalotolerant Organisms: capable of living in solutions with high levels of dissolved metals such as copper, cadmium, arsenic, and zinc.

The suffix – phile, means ‘lover of’ in Greek. Thermophiles love heat. Acidophiles love acid.