Lovers of the Extreme

The environments around the deep-ocean volcanic rifts can be described with a single word: extreme. Extreme heat, extreme cold, extreme pressure, darkness and toxic-waste waters are conditions seemingly inhospitable to every living thing. Yet over the past two decades, oceanographers and biologists who have braved the perils of the long trip to this depth in their small submarines have made stunning discoveries. The finding of bizarre tubeworms and clams was completely unexpected, but even this life is conceivable to us, for it exists in the warmed waters around the volcanic vents. What was not expected, however, was that life could live not only around, but also amid, the vents. Within these scalding cauldrons of superheated water, a rich diversity of microbial entities grow and thrive at temperatures far too hot for any animal. Yet here, indisputably, is life, in a region previously thought as sterile as Mars.

It is just such environments on Earth that may hold the most important clues to the possibility of extraterrestrial life on a place such as Mars. If the harsh hydrothermal vents can harbor life, why not the inhospitable habitats of Mars, or Europa (a moon of Jupiter), or unnumbered planets farther away as well? Life does exist in the hydrothermal vents of the deep sea, just as it does in other seemingly sterile habitats where organisms have recently been discovered, such as deep underground in cold basalt, in sea ice, in hot springs, and in highly acidic pools of water. Because of where they live, the microorganisms in these uninviting places have been dubbed extremophiles, "creatures that love the extreme."

The discovery that life is abundant and diverse in extreme environments is one of the most important of the Astrobiological Revolution. It gives us hope that microbial life may be present and even common elsewhere in the solar system and in our galaxy, for many environments on Earth that are now known to bear extremophile life are duplicated on other planets and moons of the solar system.

The majority of research on extremophiles has centered on two types of habitats: the undersea hydrothermal vents described above and the terrestrial equivalents of the hydrothermal vents: geysers and hot pools on land. Volcanic processes create both of these habitats, and accordingly, they provide windows into the deep Earth. Life is tougher than we thought. If bacteria-like organisms can inhabit high-temperature geysers, they can live deep in Earth's crust in the subterranean blackness and heat of the underworld. The deep-ocean hydrothermal vents, and the hot springs and geysers of volcanic regions on land, are places where these previously unknown, deep-Earth assemblages of microbes can be observed and sampled. And they may also offer windows into regions where extraterrestrial life may exist on other planets and moons.

The first extremophiles were discovered not in deep-sea settings but in the geysers of Yellowstone National Park. There, in the early 1970s, microbiologist Thomas Brock and his colleagues discovered "thermophilic" extremophiles, microbes capable of tolerating temperatures in excess of 60°C, and they soon thereafter recovered microbes that could live at 80°C. Since then a variety of such extreme-heat-loving microbes have been isolated from hot springs at many localities around the world. Until that time it was believed that no life of any sort could live at temperatures much above 60°C, just as it is still believed that no multicellular organisms (such as animals or complex plants) can tolerate temperatures above 50°C. Yet, many hot springs extremophiles thrive in temperatures above 80°C, and some can live in temperatures above that of boiling water, 100°C. In contrast, the majority of bacteria grow best at 20-40°C. Discovery of these hot springs extremophiles inspired the search for similar microbes in the deep-ocean hydrothermal settings.

The deep-sea vents are characterized by three conditions previously considered deleterious to life: high pressure, high heat, and lack of light. Be cause of the great pressures encountered deep in the sea, water can be heated well past its boiling point at Earth's surface. The highest temperatures encountered in these environments can exceed 400°C. When this superheated, mineral-rich water hits the near-freezing sea water surrounding the vents, it is rapidly cooled, although extensive zones of water well above 80°C are found around the vents.

The submarine hydrothermal vent systems cover enormous lengths of the sea floor and may be one of the most unique habitats on Earth. However, they were virtually unknown before the 1970s because of their remoteness and depth. Since the advent of deep-diving submarines such as Alvin, these habitats have been intensively studied. The water near the vents, once thought too hot for life, is now known to be inhabited by a diversity of microbial life, which appears to provide food for a whole host of larger organisms living around the vents. The abundant microbes thus form the base of a deep-sea food chain that requires neither light nor photosynthesisers such as plants. Most ecosystems we are familiar with have, at the base of their food chain, organisms that take carbon dioxide and light and produce living cells through photosynthesis. Light is thus the energy source that allows growth. Many of the extremophile bacteria have no need for light. They derive their energy from the breakdown of compounds such as hydrogen sulfide and methane, which fuels their metabolism. Furthermore, these organisms evolved early in earth history, and this suggests that the earliest life on our planet— and by inference on other planets as well—may be chemically fueled rather than powered by light. The implication is that light may not be a prerequisite for life.

Perhaps the most unexpected aspect of these discoveries was that many of the bacteria in these regions not only support, but also demand and thrive on, temperatures above 80°C. One species discovered in the deep-sea hydrothermal vents reproduces best in water at temperatures above 105°C and remains able to reproduce in water as hot as 112°C.

Even more startling lovers of extreme heat have recently been found in these environments. In 1993, John Baross and Jody Deming of the University of Washington published a paper entitled "Deep-sea smokers: Windows to a subsurface biosphere?" In this paper, the two oceanographers advanced the idea that the interior of Earth is home to microbes capable of living, under high pressure, at temperatures above that of boiling water—as much as 150°C. They called these organisms "super thermophilic." This bold prediction was supported when John Parkes of Bristol, England, discovered intact microbes at 169°C in a deep-sea drill core. What is the upper temperature limit for life? Microbiologists now theorize that life may be able to withstand 200°C in high-pressure environments.

Although some of them fall within the taxonomic group formally called Bacteria, the majority of these extremophilic microbes belong to the taxonomic group known as Archaea. The archaea are biological stalwarts indeed. They thrive in boiling water and live on elements toxic to other life, such as sulfur and hydrogen. The discovery of this major group of living organisms itself precipitated one of the great revolutions in biology, for their existence required a substantial reconfiguring of the time-honored model we can call the "Tree of Life," the theorized evolutionary pathway leading from the earliest life to the most complex.

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