Extremophiles are lifeforms that exist under conditions / in environments that would not be conducive to human life. In the real world, most extremophiles are single-celled organisms, but not all of them are. There are worms, crustaceans, and insects that can survive in environments where mammals would surely perish.
Extremophile is a bit of a catch-all phrase that includes many different types of life, and when discussing a particular extremophile it may be helpful to further subdivide them into categories based on what environments they thrive in.
- Acidophile - organisms adapted to life in extremely acidic environments
- Alkaliphile - organisms adapted to life in extremely base (opposite of acidic) environments
- Anaerobe - organisms that don't need oxygen to live
- Aridophile or xerophile - organisms adapted for extremely dry desert environments
- Barophile or Piezophile - organisms that thrive in high pressure environments
- Endolith - organisms that live inside rocks or corals
- Halophile - organisms that thrive in extremely salty conditions
- Hypolith - organisms that live beneath frozen deserts
- Lithoautotroph - organisms that eat rocks, requiring only mineral nourishment
- Metallotolerant - organisms that are resistant to (not poisoned by) heavy metals
- Oligotroph - slow-growing life that barely needs any nutrients to survive
- Osmophile - organisms (mostly yeasts) adapted to live in sugar
- Psychrophile or Cryophile - organisms that thrive in extremely cold environments
- Radioresistant - organisms that can survive high levels of radiation
- Scotophile - creatures that don't need light to survive, and prefer the dark. (See also Troglomorphy - the adaptations that are likely to happen when species exist in the dark for generations.)
- Thermophile - organisms that thrive in extremely hot environments
Note that most of the above terms imply a general preference for whatever environment is indicated, but can be further modified for emphasis, by adding "hyper" (meaning excessive or beyond) or "obligate" (meaning required) to the main term. While a thermophile might be happy in a very warm environment, a hyperthermophile considers the boiling point to be a little tepid. An anaerobe doesn't require oxygen to live, but an Obligate Anaerobe will actually die in the presence of oxygen.
Note that the term "extremophile" is very subjective, and showcases our human biases. We term things as being "extreme" if they live in places we wouldn't want to (or wouldn't be able to) call home. Aliens that met us (or even Shadow Life in obscure corners of the globe) might consider us to be the extremophiles. AMesophile is a creature that thrives at mild temperatures, and a neutrophile is an organism that needs a neutral pH to thrive. Aliens might use either of those terms to categorize us, especially if they are more robust or adaptable than we are.
Game and Story Use
- If you want to make an alien (or monster) especially alien and incompatible, have them evolve in an unusual (from our perspective) environment, so they and humans can't live on the same worlds, and don't want the same things out of life.
- Example: A hyperthermophile that lives on the molten surface of a Brown Dwarf might have a hard time recognizing humans as even being a form of life. "Surely nothing so sensitive to heat and pressure could possible be alive" thinks the alien as he sets our fragile forms ablaze for fun.
- On the other hand, such extremes of preference could actually make it easier for aliens to coexist with us, or hide right under our noses. If they live at the molten core, or beneath the frozen ice, or high in the upper atmosphere, our paths won't cross all that often.
- There could still be some conflicts, though. If they live in the polar regions, they may take efforts to stop global warming whether we want them to or not.
- Microbial extremophiles could be a potential hazard.
- Not only do you have to worry about whether or not your exploration vehicle can survive the crushing depths and intense heat, you now also have to go through a rigorous decontamination process afterwards.
- After passing through the weird nebula or warp rift, the starship picked up a bad infestation of cryo-vacu-barnacles. Everything's fine till you attempt atmospheric re-entry at some planet, then the barnacles burrow in to your hull to escape the heat. That can't be good.
- Extremophiles might be a resource of some sort. Perhaps in important ingredient in a potion, medicine or technology is a byproduct of the chemical processes going on inside an extremophile colony. You just have to figure out how to "milk" the endoliths to get them to excrete more monofilament fullerne. Not to mention the exciting adventure to the heart of a volcano to gather the microbes needed to start the process.
- The fun isn't just restricted to science fiction. A fantasy game with elemental plane may use a few of the concepts here (or on the alien biochemistry page to figure out what sort of critters could exist on the demiplane of ash.