By: Priya Amin
Imagine diving into the Gulf of California and reaching a 120 °C hydrothermal vent located deep on the seafloor. Rich in hot hydrogen and carbon dioxide gas, these vents seem to spell death for any creature that dares to swim by. But if you look closely, you’ll notice an organism that not only survives but also thrives in this seemingly toxic environment. How could anything live in such adverse conditions?
HYPERTHERMOPHILES: DON’T SWEAT THE HEAT
Meet Methanopyrus kandleri, Earth’s record-holder for hot temperature growth. Capable of reproducing at 122 °C, M. kandleri is a hyperthermophile, an organism that likes intense heat (1). Hyperthermophiles comprise some of the world’s most extreme life forms and were only discovered a few decades ago in 1965 when Thomas D. Brock isolated them from hot springs at Yellowstone National Park (2). Since then, we’ve been able to learn much more about these resilient organisms.
How have hyperthermophiles adapted to live in such extreme conditions? Normally, under extraordinary heat, a cell membrane disintegrates, allowing toxic chemicals into the cell. However, hyperthermophiles combat this by using high levels of saturated fatty acids to line their membranes (3). This type of structure is quite strong and stable, and it helps the cell stay intact. Another issue is that at such high temperatures, ordinary proteins denature, lose their shape, and cease to function. Hyperthermophiles have evolved to have hyperthermostable proteins that are compact and wound up in spiral-like helixes (4). These proteins can maintain their structure and function even in harsh environments. In fact, they use the high temperature to their advantage: the abundant heat energy makes chemical reactions proceed faster than usual, spurring on the processes that allow cells to proliferate and grow.
To survive, M. kandleri must use unique metabolic pathways tailored to the molecules in its environment. Interestingly, in addition to being a hyperthermophile, M. kandleri is also a methanogen, which means it gets its energy by producing methane in environments where oxygen is absent.5 Its metabolic process looks a little like this: CO2 + 4 H2 → CH4 + 2H2O. M. kandleri is remarkably resourceful. It consumes hydrogen and carbon dioxide, making it a perfect match for deep-sea hydrothermal vents! In addition, the metabolic process is anaerobic, with no need of oxygen! The result is energy in the form of ATP, a special molecule that can be later used to fuel the cell’s basic functions (5). Most hyperthermophiles have similar chemical reactions that use carbon dioxide, iron, or sulfur to anaerobically produce energy. This allows them to live in a vast array of hot environments, like deep-sea vents, hot springs, and terrestrial volcanoes.
M. kandleri’s habitat is characteristic of Earth’s early conditions. Therefore, many scientists claim that the Last Universal Common Ancestor (LUCA), the most recent ancestor of all organisms on Earth, was closely related to M. kandleri (6). Extraordinarily, by studying hyperthermophiles, we’ve been able to open a window into our evolutionary past.
CRYOPHILES: PLAYING IN THE COLD
Now, after taking a dive to visit the scorching hot hydrothermal vents in the Gulf of California, why don’t we cool down a bit? Let’s travel 3800 miles to Ellesmere Island in Antarctica to meet another extreme organism, Planococcus halocryophilus.
If you take a step into the permafrost on Ellesmere Island, you would probably get frostbite pretty quickly. The permafrost contains a lot of salt, which keeps it freezing over even at temperatures as cold as -25 °C. This is the environment in which the cold temperature growth record holder P. halocryophilus lives. At -16 °C it can reproduce, and at -25 °C it is still able to remain active (7). Discovered a century earlier than hyperthermophiles, cryophiles were first described by J. Forster in 1887 upon examining a sample of cold-preserved fish (8).
To adapt to below-freezing temperatures, cryophiles have developed unique structures and molecules. Unlike hyperthermophiles, cryophiles have high levels of polyunsaturated fatty acids, which form malleable and fluid structures that keep their cell membranes from freezing (9). They also have two special types of cold-active proteins: cold shock proteins and antifreeze proteins. Cold shock proteins ‘turn on’ once the temperature falls below a certain threshold. Because they’re designed for flexibility, they are able to help other proteins, such as those needed for DNA replication, function at less ideal temperatures (9). Antifreeze proteins help the cell avoid the harmful effects of thawing and freezing. They release chemicals outside the cell that effectively lower the freezing point of water (5). In this way, cryophiles manipulate the environment around them to survive.
While the exact metabolic pathway for P. halocryophilus is still being researched, it likely derives ATP energy from molecules in its environment in a process similar to that of M. kandleri. Given the low temperatures and thus low amount of energy in the environment, ATP’s role as an energy source becomes even more important than usual. Incredibly, the microbe can still synthesize and break down the molecules it needs at temperatures as low as -25 °C!
This cold temperature organism holds insights about the possibility of similar microbial life on other planets in our solar system. For instance, cryophiles have the ability to live in between ice crystals on Earth; similar environments are found on other celestial bodies, such as on Mars or on Saturn’s moon, Enceladus (10). Scientists are currently racing to discover the secrets these organisms hold about our world and the universe.
M. kandleri and P. halocryophilus are just two examples of countless extremophiles. Thriving in harsh, adverse environments, extremophiles are like the daredevils of biology—wherever life has an opportunity to develop, extremophiles make a home. This group of organisms consists of life from two broad classifications: Archaea, like M. kandleri, and Bacteria, like P. halocryophilus. Extremophilic creatures from these domains can live in seemingly impossible environments, such as acidic pools, salty lakes, freezing brine water, or extremely hot hydrothermal vents! What makes archaea and bacteria best suited to handle extreme conditions?
The answer is quite simple: they are single-celled organisms. Archaea and Bacteria are the two great branches of life that encompass all prokaryotes. (Eukarya is the third domain, consisting of plants and animals.) When you think of single-celled prokaryotes, which do not have a nucleus, you might only picture a bacterium, like the E. coli that live in your gut. Scientists used to think in the same way, classifying all prokaryotic organisms as bacteria. That is until 1977, when RNA analysis revealed that many single-celled organisms were actually in a separate domain, namely Archaea (11).
But what does being a prokaryote have to do with being more likely to be an extremophile? How does being single-celled allow the possibility for an organism to survive harsh environments? The first reason is that these organisms are able to reproduce much more quickly than complex, multicellular organisms such as plants and animals. They are time and energy efficient. This proves especially advantageous for hyperthermophiles that need to rapidly proliferate across a large surface to capture more nutrients from a deep-sea vent before it collapses. Another reason is that archaea and bacteria do not have a membrane-bounded nucleus, which means that they are able to replicate DNA and create proteins in less time. In cold environments, for example, the cell would be able to efficiently make antifreeze proteins, which are crucial to the survival of cryophiles. It’s also important to note that the ability to quickly reproduce creates opportunities for rapid adaptations to environmental changes.
By studying these amazingly simple yet resourceful organisms, we’ve found the most disparate forms of life. Hyperthermophiles withstand the blistering heat of hydrothermal vents, similar to the environment when life was developing on Earth. Cryophiles survive the freezing cold of Antarctica, similar to environments out in our solar system. We have much to learn from extremophiles both as we seek to understand our evolutionary past and as we look to the future for life beyond Earth.
Priya Amin ’19 is a sophomore in Pforzheimer House concentrating in Integrative Biology.
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