Our planet is home to a diverse array of habitats. These can range from cozy, nutrient-rich, temperature-controlled havens to deadly, gruesome battlegrounds where only the fittest survive. Each habitat, no matter how extreme, serves as home to millions of microbes. For instance, the microbes in our bodies are only happy at a balmy 98.6 °F. They live a cozy life, feasting on food scraps and dead cells in and on our bodies. However, not all microbes live in such luxury.

In a previous article, I described tardigrades, micro-animals that can freeze, dry out, and travel in space without dying. But these animals are not alone in their adaptive abilities. Microbes that inhabit the soil deal with large temperature fluctuations and compete for more scarce nutrients. Even more so, these taxing conditions are a cakewalk compared to boiling, acidic waters or freezing, salty waters, both of which are also home to microbes. Previously, we’ve discussed how some marine animals survive in some harsh environments, but how do these extreme microbes adapt to such a diverse range of conditions?

One of the factors that limits where life can prosper is pH. All living cells need to maintain a constant pH, usually around 7 (the neutral pH of water). If they don’t, essential molecules get damaged, and life cannot survive. pH can range from 0 (highly acidic) to 14 (highly alkaline), and is based on the concentration of protons, the positively-charged subatomic particles you learned about in school. The higher the proton concentration, the lower the pH, and the more acidic. Even though no single microbe can grow across the entire pH range, some specialized microbes can grow in extreme pH conditions.

The stomach contains digestive juices with a pH of 1.5-2.5 (the pH of vinegar), which is too acidic for most microbes. However, this environment is perfect for Helicobacter pylori, the bacteria that causes stomach ulcers. It survives by releasing enzymes into its surroundings that increase the pH of the stomach.

One can also find these acid-loving bacteria in areas that experience volcanic activity. The hot springs in Yellowstone National Park, in fact, provide a double whammy – scalding temperatures  and low pH. Which daredevil bacteria lives here? Sulfolobus solfataricus, which thrives under these conditions by constantly pumping protons out of itself to raise its internal pH and keep it around 7. (Their adaptations to heat are also impressive and will be discussed later.)

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Nymph Lake hot spring in Yellowstone National Park. A combination of acidic water, harsh sunlight, and hot temperatures only allow a small number of microbes to flourish.

On the other side of the pH scale, microbes that live in alkaline environments need to acidify their cells in order to maintain pH 7. To this end, they maintain a matrix of acidic components in their little bodies and bring in protons. For example, Natronomonas pharaonis grows best at pH 8.5.

Natronomonas is also great at growing in high salt concentrations. In fact, it can grow in waters that contain 7 times more salt than sea water! Where are these salty conditions found? There are several hyper-salty lakes across the world including, the Don Juan Pond in Antarctica, the Dead Sea, and the Great Salt Lake in Utah. Additionally, the way we prepare food such as soy sauce and salted anchovies requires microbes that thrive in salty conditions. The problem with living in high saltiness, though, is that the excess salt from the environment makes its way into bacterial cells. As a result, water also enters the cells to dilute out the salts, which can cause the cells to burst. To prevent this, these microbes adapt by either creating specialized molecules that keep critical operations running in cells despite high salt concentrations or by pumping salt out from their cells.

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Colored scanning electron micrograph of Pyrococcus furiosus cells (Photograph: ANP). The name Pyrococcus is derived from the Greek word for “fireball” and furiosus means “rushing” in Latin, a reference to its fast swimming ability.

Extreme environments can also be caused by extreme temperatures. For example, some bacteria live in deep sea vents, the fissures on the Earth’s surface from which superheated water arises. In fact, this is where scientists think life began. Temperatures in these vents can range between  140 °F to 868 °F, temperatures that aren’t friendly to life because such extreme heat damages most organic molecules, like the enzymes that microbes need to function. By this point, you might not be surprised that several microbes enjoy these scalding conditions: Pyrococcus furiosus grows the best at 212 °F (the boiling point of water!) and Pyrolobus fumarii feels at home at 235 °F. How do they do it? They make heat-stable proteins as well as collect salts that stabilize their DNA and prevent it from denaturing.

On the other end of the spectrum are cold-loving microbes. These are found in permafrost, or permanently frozen ground, where the temperature ranges from 23 °F to -4 °F. Humans would be able to survive these temperatures for a maximum of 10 hours. In 2012, scientists found the most extreme example of such microbes. Planococcus halocryophilus Or1 was discovered in the salty waters of melted permafrost, and it is happiest at a temperature of 5 °F. This microbe’s proteins are adapted to the cold temperatures, and these microbes also carry high levels of a type of antifreeze inside its body.

Many other factors can push environments to the extreme, but even so, there are organisms that have adapted to them, including high water pressure, radiation, and dehydration.

Why do scientists know all of this stuff?

Besides scientific curiosity, scientists study so-called “extremophiles” for two main reasons. First, these microbes contain unusual proteins that can be employed for various research purposes. Second, the way they adapt to radiation and low moisture are of special interest to astronomers because of their extraterrestrial implications. Chroococcidiopsis, for example, can adapt to radiation and dehydration, and is capable of producing its own oxygen. This ability means it can be potentially used to seed the soil on Mars and make it more suitable for producing crops. Furthermore, the microbes that inhabit icy seas and thermal vents at the bottom of the ocean are of interest because these environments, in fact, are not unique to Earth. Astronomers believe that Europa, one of Jupiter’s moons, and Enceladus, one of Saturn’s moons, contain hydrothermal vents. Many of these moons also have ice-covered oceans that could serve as home for resilient microbes. By studying the adaptations employed by our microbes here on Earth, we can predict the sort of adaptations extraterrestrial microbes would need to have to survive out in space. After all, just because an environment seems uninhabitable at first glance, that doesn’t mean that it cannot serve as a home to the right species.  


  • Ananya Sen

    Ananya Sen is a graduate student in Microbiology at the University of Illinois at Urbana-Champaign.  When she’s not studying oxidative stress, she is busy pursuing her passion for scientific writing.

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