With the Society for Applied Microbiology’s Winter Meeting on Psychrophiles and Extremophiles rapidly approaching (19 January 2016), we asked Louisa Preston “How would you define extremophile?”
One of the most astounding realizations of recent years has been just how hardy life on Earth really is. Terrestrial microorganisms have now been found flourishing in environments so hostile that previously they were simply assumed to be sterile. Organisms that make these locations their home are called “extremophiles”, literally meaning “extreme lovers”. They are found to withstand and thrive in physical extremes of temperature, pressure and radiation; and chemical extremes of salinity, acidity and limited availability of liquid water. The extreme ecosystems found on Earth are similar to what we expect to find on other planetary bodies in the Solar System and so provide a glimpse into what life might be like elsewhere in the Universe.
Most of the hardiest extremophiles are microbes – large, multicellular lifeforms like humans and plants are far more limited in the conditions they can tolerate. Some of the best-known examples are thermophiles – or heat-tolerant organisms – and these are commonly found basking around the scalding vents of geothermal hot springs. The current record for the upper temperature limit of life is set at 121°C by the aptly named ‘Strain 121’, a single-celled microbe discovered in 2003 around a deep sea hydrothermal vent. Such deep-sea hydrothermal vents are important environments for astrobiology because they are thought by many researchers to be likely crucibles for the origin of life on Earth. They could also provide crucial energy sources for ecosystems elsewhere in the solar system, such as the watery ocean deep beneath the surface of Europa, one of the icy moons of Jupiter.
At the other end of the thermometer scale are psychrophiles that survive in freezing temperatures, even down to -15°C in ice-bound pockets of very salty water. Acidophiles, as the name suggests, thrive in very acidic solutions at a pH below 3. One of the most acidic environments on Earth is Rio Tinto in Spain, a drainage system with a pH of 2.3 that still supports a large ecosystem of microbes, most of which power themselves by oxidizing the high concentrations of iron in the water. Rio Tinto is thought to closely resemble acidic, iron-rich rivers and lakes that once covered the face of ancient Mars, and so provides a crucial test-bed for the prospects of martian life.
Many organisms do not require oxygen for growth, whilst some tolerate punishingly alkaline solutions, or extremes of pressure, salinity, or desiccation. The most adept lifeforms are the polyextremophiles which can survive several extremes at once, for example thermoacidophiles thrive in water that is both exceedingly hot and acidic, such as in volcanic ponds.
Extreme ecosystems on Earth provide excellent ‘analogue’ sites for environments on other worlds that we now realise may be suitable for harbouring alien life. One well-studied location is the Dry Valleys region in Antarctica, which is one of the coldest and driest places on Earth, and also bathed in very high levels of ultraviolet radiation. This is a very Mars-like environment, but even here life finds a way to survive, and entire microbial ecosystems of ‘cryptoendoliths’ hide themselves inside protected niches within rocks.
We’re continually discovering new bizarre ecosystems on the face of the Earth and an entire extremophilic microbial world remains to be explored deep in the subsurface of the planet. As each of these ecosystems is scrutinized, we take one step closer to finding life on another world.
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