Moderate and extreme are relative terms. What many assume to be the normal conditions for life do not apply equally across all species. Habitability can be thought of in terms of the Goldilocks principle. For example, to survive people need an environment that is not too cold and not too hot. Ranges like this exist for other factors as well, such as pressure. However, what is just right for us and most other creatures can be too much or too little for some. Animals that live on the sea floor provide an illustrative example in this regard. Where people need submersibles to maintain their equilibria, creatures that natural selection has adapted to thrive in the ocean’s depths are right at home. Cold, highly pressurized environments are the norm for them. When deep sea creatures are brought to the surface through accident or scientific curiosity they often either melt, as with glacial ice worms, or fatally decompress, like with blobfish. In this way the term extremophile is a relative one and is used to describe organisms that live in environments that are extreme to us. Beyond their novelty, extremophiles are important objects of study as they challenge and expand our notions of life and its necessities.
The majority of extremophiles are prokaryotes, organisms that lack membrane-enclosed nuclei. Prokaryotes consist of two groups: bacteria and archaea. It should be noted that not all prokaryotic life is suited to extremes. Eukaryotes are the other category in this system, and they include all plants, animals, and fungi. Eukaryotic extremophiles are fewer in number, though often more stunning.
The various environmental resistances of extremophiles each have their own name. Thermophilic and hyperthermophilic both describe organisms that do best in extremely high temperatures, barophilic creatures flourish in environments with large amounts of pressure, and acidophilic extremophiles live in habitats with a pH between 1 and 5. Other categories also exist, and most extremophiles have several such resistances since extreme environments tend to have overlapping hazards. The sea floor, for instance, is not only highly pressurized, but cold, dark, and relatively low in nutrients as well.
Extremophiles are capable of succeeding in what to us seem harsh environments thanks to what are called extremozymes. These enzymes allow organisms to maintain homeostasis in otherwise extreme environments. Due to their ability to stand up to various pressures extremozymes are of interest to both the industrial and medical sectors. To get a better sense of why manufacturers and scientists want to study these organisms, here are some of the more fascinating examples.
Measuring out to a millimeter in length, tardigrades are able to withstand several extreme environmental conditions. Counter to their size, lab tests have shown this organism’s livable range to be quite massive. Tardigrades can survive for extended periods without oxygen, water, and with temperatures as high as 304 °F (151 °C) or as low as -328 °F (-200 °C). They can even withstand prolonged exposure to radiation hundreds of times stronger than what we can endure.
One of the main ways tardigrades accomplish these feats is through a trope straight out of science fiction. What novelists imagine cryostasis as, cryptobiosis is. Rather than some outside force freezing a living creature to put it in suspended animation, tardigrades are able to slow their own metabolisms to the point that they almost appear dead. This state of low energy consumption gives them a chance to survive on the resources they already have when in an environment that has too few to sustain them otherwise. And to top it off, this fancy trick can go on for decades at a time.
Neither a eukaryote nor a bacteria, these microorganisms are preternaturally hearty. Many are both halophilic, able to withstand salinities that would poison most other life forms, and hyperthermophilic. This helps them greatly, as some of the first archaea were found in volcanic hot springs like the ones at Yellowstone National Park (later on non-extremophilic archaea were also discovered). Achaea obtain their energy from a variety of sources. Some rely on sugars like us, though others derive nourishment from ammonia or metal.
Methane Ice Worms
First encountered on the sea floor hunting among piles of frozen methane, this two inch long worm checks several extremophile boxes. While the depth of the ocean floor varies, it is always a high pressure environment. To combat what would be drawbacks to most other creatures, the methane ice worm has adapted to its cold, dark habitat as a predator whose bacterial prey likes to colonize frozen mounds of methane. While this somewhat alien creature calls the depths its home, researching it might also give clues into how other organisms might survive in the other great unknown.
Space and the deep sea exert similar pressures, thus studying how extremophiles thrive on the ocean floor might give scientists clues as to how extraterrestrial life might persist. Most of the bodies in the Solar System are outside of what has been dubbed the Goldilocks zone, a region where a star radiates enough heat to allow water to exist in its liquid state. Scientists have traditionally viewed this as one of the benchmarks for life, though perhaps one day some extraterrestrial extremophile will come along and defy this notion. What we find might be alive, long-dead, or in a tardigrade-like cryptobiosis.
Saving quite possibly the most extreme for last we have brine shrimp, so named for their ability to live in environments that are up to 50% salt. For reference, ocean water is about 3.5% salt by weight on average. Like with tardigrades, brine shrimp can undergo cryptobiosis and persist in environments that lack water for extended periods. This is how some brine shrimp are stored, shipped, and sold as sea monkeys.
Brine shrimps’ cryptobiosis was pushed to its limit in a series of tests carried out during the Apollo missions. Biostack I and II were enclosures of brine shrimp embryos (and other organisms) that were loaded aboard Apollo 16 and 17 along with radiation censors to test the effects of cosmic radiation on living things. While many of brine shrimp hatched and succumbed to genetic mutations, some from both tests were able to mature without any problems. Our inability to do certain things can cloud our judgement of what is possible for others. Space may be extreme to us, but to some life forms it might be relatively cozy.
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Langley, Liz. “5 Extreme Life-Forms That Live on the Edge”. blog.nationalgeographic.org, National Geographic, Aug 2 2013, https://blog.nationalgeographic.org/2013/08/02/5-extreme-life-forms-that-live-on-the-edge/, Accessed March 16 2021
Niederberger, Thomas. “Archaea”. britannica.com, Encyclopedia Britannica, Oct 21 2020, https://www.britannica.com/science/archaea, Accessed March 16 2021
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