Our planet has some pretty intense environments, ranging from dense ice to molten rock — and they all play host to some form of life. What do extremophiles — creatures that live in unimaginable conditions — tell us about the very nature, and limits, of life?
Extremophiles allow us to quantify what evolution is capable of under circumstances that human scientists might find a bit daunting (say, inside of a nuclear reactor). Often, these ecosystems are assumed to be barren and lifeless, but almost without fail, when scientists start flipping over rocks and poking in ponds, they find life adapted to these circumstances.
These extreme ecosystems give us the best idea of what life may look like and be capable of on planets far different than our own.
Planets that may, for example, have:
Basically No Water
The Atacama Desert, which covers 600 miles from Peru's southern border to Chile's coast, is the driest place on planet earth. On average, the desert receives just a few millimeters of rain per year (measurements range between 1 and 15 millimeters, depending on location) and it is likely that this desert is the oldest desert on planet earth, experiencing continuous extreme aridity for millions of years.
Despite this incredibly dryness, life persists. Because the region has been a desert for such a long time, life has had the opportunity to adapt to these extreme conditions, giving rise to a number of plants that can harvest marine fog. Along with plants that have seeds that can persist for decades, awaiting a rainfall event that will allow them to sprout.
But perhaps most incredibly (and super-duper exciting for astrobiolgists), the desert is teeming with microbial life just a few meters below the surface. The soil of the Atacama is incredibly salty, and these salt crystals can attract water from the air, forming a thin water film for microbes to use as they munch on salt. These life-harboring conditions might mirror those on dry, salty places like Mars, giving astrobiologists hope for future microbial discoveries.
Too Much Water
In terms of being a living thing on planet earth, access to water is critical. Scientists are pretty confident that water is a key element for life, and so planets in and out of our solar system with signs of water are really exciting for astrobiologists. But living in a watery world with deep oceans, like those that are hypothesized to be on Europa, come with their own sets of challenges.
At the bottom of the deep ocean, there is very little energy available. Sunlight doesn't penetrate and waters are typically cold. Plus, water is heavy, and the weight of the ocean creates an extremely cold, high pressure environment. A number of organisms have evolved to live at these great depths, but one, Halomonas salaria, requires these incredible pressures to survive.
Living at pressures that hover around 1,000 atmospheres, this species requires cold temperatures and darkness to survive and reproduce. Unlike most other bacteria, the cell membrane of H. salaria is thick and waxy, rather than fluid, which is a characteristic of many high-pressure loving bacteria.
In 1977, the number of known ecosystems on planet earth grew, when scientists discovered deep-sea hydrothermal vents. These vents spew superheated water and minerals out into cold ocean environments, creating warm oases for life.
These habitats are unique because, unlike nearly all other ecosystems, this system requires no input from the sun to fuel life. Instead, extremophile bacteria munch on chemicals coming out of the vents, providing the basis for the vent's food chain.
A unique assemblage of species typically occupies the vents, including giant tube worms, crabs, urchins and shellfish. These species are so dependent on the chemical-eating bacterial that the entire vent community collapses the vents wink off (as they sometimes do). Vents like these are hypothesized to exist on Jupiter's moon Europa, and may be one source of life on a planet covered by ice and very distant from the Sun.
Radiation, up to a certain point, is pretty awesome. Most creatures on Earth like solar radiation (sunlight, if you are feeling benign) just fine…up to a point. Too much solar radiation and you start to acquire some DNA damage. On humans, this manifests as sunburn. Cells essentially die from radiation poisoning and are shed by the body (or mutate and become skin cancer). Some planets may have such high radiation that RNA and DNA can't form, and any organisms would die quickly as their DNA is damaged and unravelled. Or so the thinking goes.
But some bacteria here on Earth have adapted to live in very high radiation environments indeed. Take "the toughest bacterium on planet earth", Deinococcus radiodurans. This little bug can survive lesser terrors like freezing and acid, but D. radiodurans can withstand a good dose of radiation. It was discovered while food products were being bombarded with gamma radiation in an attempt to sanitize them of all living organisms.
When that supposedly sterile food began to spoil, scientists discovered colonies of D. radiodurans laughing and throwing a party. Turns out, these little buggers can rapidly repair their DNA with essentially no ill effects and can survive in extremely high radiation conditions.
Extreme Acidity and Alkalinity
Acidic and very alkaline environments are problematic. Most life has evolved to be at a fairly neutral pH, and high concentrations of acids or bases can cause cell membranes to erode and pH to change, which can lead to cell death (in humans, this would be a chemical burn). However, planet earth has provided a number of extremely acidic and basic ecosystems, and life has learned to thrive there. For example Natronomonas pharaonis are bacteria that have been isolated in soda lakes in Africa with measured pH of around 11 (for reference, neutral pH is 7). And humans have actually harnessed the power of one group of organisms that takes advantage of acid ecosystems: lactobacillus. Not only do these bacteria live inside our gut, they also make food magic, surviving in high-salt environments while converting sugar to acid, killing harmful bacteria and fungus and turning cabbage into sauerkraut, milk into yogurt, and cucumbers into pickles.
Heavy metals (like lead) and metalloids (like arsenic) are sprinkled throughout the environment like deadly little seasonings. These metals tend to cause damage by accumulating in cells and messing with basic metabolic processes, and can kill you rapidly or slowly, depending on the dose. This is the primary reason why we have, for the most part, decided to stop eating lead paint chips.
But some organisms have adapted to life in ecosystems that are dominated by these deadly metals, thriving in mine runoff, cadmium enriched soils, and toxic waters. For example, Mono Lake in California is extreme on a lot of levels. It is highly saline, high pH, and has a hefty dose of arsenic in its water (for added flavor, one assumes). Despite sounding awful, life actually thrives here, as specialized bacteria and plants feed specialized brine shrimp that support millions of migrating birds. This lake was also the site of the now overturned discovery of "arsenic-based life", making it an important example of the scientific method at work.
Other, more complex lifeforms
Fairly recently, science decided to turn the microscope around, and take a good, hard look at itself. Specifically, the microbes that live in and on scientists and other humans. As it turns out, humans are composed of approximately 90% non-human organisms, and 10% human cells. That's right, your "microbiome", or the microorganisms that live in and on you, outnumber your own cells by about 10 to 1.
Go find a mirror. Look at yourself. Think about your microbiome. Because, you are the ecosystem. And it turns out, like any ecosystem, we are only as good as the collection of organisms that define us. Scientists are learning that dysfunctional or non-diverse microbiomes may explain autoimmune conditions, cancer, and mood-disorders in human beings and other mammals. One recent study found that baby mice raised with no gut microbiome moved and explored more and had less anxiety than those mice with gut microbes, and that they had fundamentally different brain development than those with gut microbes. In other words, the presence or absence of a microbiome fundamentally changed the behavior and development of these mice.
If there is one thing that the extreme environments of earth have taught us, it is that evolution will, with enough time, find a way to exploit any available niche. Although these environments may seem inhospitable and down-right deadly to humans, they can be hidden oases for microbes, and sometimes even more complex life. And they may represent our best shot at understanding life on other planets.