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Life in the Dark - Deep Sea Ecosystems

Biologists always thought life required the Sun's energy, until they found an ecosystem that thrives in complete darkness.

by Patrick L.Barry

Dr. Cindy Van Dover manoeuvres her robotic craft closer to the strange, rocky landscape below. It's totally dark, except for lonely circles of light where she points her flood lamps. Back on the mother ship her monitor reveals tall, thin towers of craggy rock billowing black smoke from their peaks. Very strange!

All around the towers stand dozens of red-and-white, tube-like organisms. These bizarre, 3-foot-long, wormish creatures have no mouth, no intestines, and no eyes. Stranger still, they derive their energy from the planet itself, not from the light of the nearby star -- a feat most biologists didn't believe possible until these creatures were found.

She steers toward the worms and uses the robotic arm to reach out and take a sample for later examination.

Is this a science fiction tale? No. Is the intrepid Dr. Van Dover truly exploring another world? Yes!

Van Dover is as real as is the alien world she's discovering. And both are right here are Earth!

Cindy Van Dover, a marine biology professor at the College of William and Mary in Williamsburg, Virginia, is one of some 60 scientists, technicians and sailors who sailed aboard the research vessel Knorr from the Woods Hole Oceanographic Institution between March 27th and May 5th 2001. This 40-day expedition sent a 1-ton robotic submarine named JASON 2,000 metres down to explore the peculiar sunless world of deep-sea hydrothermal vents.

Images courtesy Woods Hole Oceanographic Institution

A view of a "chimney" vent (top photo) captured by the deep-sea submersible JASON. The superheated black water pouring from the vent provides high-energy chemicals that sustain the tubeworms (bottom photo) and other organisms that thrive in this unlikely habitat.

"I really never thought that one could be an explorer in this day and age," said Van Dover, chief scientist for the expedition and a member of NASA's Astrobiology Institute. "But in the ocean, it's absolutely true," she added. "You're going places that nobody's ever been before!"

The hydrothermal vents - which are essentially geysers on the sea floor - support exotic chemical-based ecosystems. Some scientists think the vents are modern-day examples of environments where life began on Earth billions of years ago. And the vents might also hold clues to life on other planets.

The thriving communities of life that surround these hydrothermal vents shocked the scientific world when the first vent was discovered in 1977. 

Before 1977, scientists believed that all forms of life ultimately depended on the Sun for energy. For all ecosystems then known to exist, plants or photosynthetic microbes constituted the base of the food chain.

In contrast, these vent ecosystems depend on microbes that tap into the chemical energy in the geyser water that billows out from the sea floor -- energy that originates within the Earth itself.

Because they offer an alternative way for life to meet its fundamental need for energy, these vent ecosystems have piqued the interest of astrobiologists - scientists who study the plausibility of life starting elsewhere in the universe. 

"It's the only system we know of on Earth where life can thrive in the complete absence of sunlight," said Bob Vrijenhoek, senior scientist at the Monterey Bay Aquarium Research Institute in Moss Landing, California. Vrijenhoek will conduct DNA analysis on the samples gathered by the expedition.

One chore that astrobiologists have struggled with for years is to define the range of conditions (temperature, salinity, irradiation, chemical composition, etc.) in which "life as we know it" could exist. The discovery of hydrothermal vent ecosystems expanded that range.

Images courtesy Woods Hole Oceanographic Institution

The Study Area

"It (the life around the vents) was the first discovery of 'life as we don't know it,'" Vrijenhoek said. 

Hydrothermal vents form along mid-ocean ridges, in places where the sea floor moves apart very slowly (6 to 18 cm per year) as magma wells up from below. (This is the engine that drives Earth's tectonic plates apart, moving continents and causing volcanic eruptions and earthquakes.) When cold ocean water seeps through cracks in the sea floor to hot spots below, hydrothermal vents belch a mineral-rich broth of scalding water. Sometimes, in very hot vents, the emerging fluid turns black -- creating a "black smoker" -- because dissolved sulphides of metals (iron, copper, and several heavy metals) instantaneously precipitate out of solution when they mix with the cold surrounding seawater.

Deep-sea bacteria form the base of a varied food chain that includes shrimp, tubeworms, clams, fish, crabs, and octopi. All of these animals must be adapted to endure the extreme environment of the vents -- complete darkness; water temperatures ranging from 2°C (in ambient seawater) to about 400°C (at the vent openings); pressures hundreds of times that at sea level; and high concentrations of sulphides and other noxious chemicals. 

Images courtesy Woods Hole Oceanographic Institution

A variety of animals live near these hydrothermal vents, including the shrimps, crab, and anemone in this picture taken at the Indian Ocean vent. So far, the hallmark red and white tubeworms have not been spotted at this vent.

The ability of life to tap such geothermal energy raises interesting possibilities for other worlds like Jupiter's moon Europa, which probably harbour liquid water beneath its icy surface. Europa is squeezed and stretched by gravitational forces from Jupiter and the other Galilean satellites. Tidal friction heats the interior of Europa possibly enough to maintain the solar system's biggest ocean. Could similar hydrothermal vents in Europa's dark seas fuel vent ecosystems like those found on Earth? The only way to know is to go there and check.

Astrobiologists are increasingly convinced that life on Earth itself might have started in the sulphurous cauldron around hydrothermal vents. Vent environments minimise oxygen and radiation, which can damage primitive molecules. Indeed, many of the primordial molecules needed to jump-start life could have formed in the subsurface from the interaction of rock and circulating hot water driven by hydrothermal systems.

If this idea proves true, then the next time Van Dover gazes through the submarine's camera at the vents on the floor of the Indian Ocean, she may be seeing both a portrait of life's genesis in Earth's distant past - and a glimpse of alien life yet to be discovered.

The images in this story are courtesy of Dive and Discover. Funded by Woods Hole Oceanographic Institution, National Science Foundation, Ohio's Centre of Science and Industry (COSI). Image courtesy of shipboard scientists from College of William and Mary, Harvard University, Monterey Bay Aquarium Research Institute, University of New Hampshire, Oregon State University, Portland State University, University of Washington, Woods Hole Oceanographic Institution.

For more information about this topic visit Dive and Discover here.

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First Science 2014