Biologists always thought life required the Sun’s energy, until they found
an ecosystem that thrives in complete darkness.
Dr. Cindy Van Dover maneuvers 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 currently sailing the Indian Ocean aboard the research vessel
Knorr from the Woods Hole Oceanographic Institution. The 40-day expedition,
from March 27th through May 5th, is sending a 1-ton robotic submarine named
JASON 2,000 meters down to explore the peculiar sunless world of deep-sea
hydrothermal vents.
“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.
“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 sulfides of metals (iron, copper, and several heavy
metals) instantaneously precipitate out of solution when they mix with the
cold surrounding seawater.
Unlike plants that rely on sunlight, bacteria living in and around the dark
vents extract their energy from hydrogen sulfide (HS) and other molecules
that billow out of the seafloor. Just like plants, the bacteria use their
energy to build sugars out of carbon dioxide and water. Sugars then provide
fuel and raw material for the rest of the microbe’s activities.
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 sulfides and other noxious chemicals.
The ability of life to tap such geothermal energy raises interesting
possibilities for other worlds like Jupiter’s moon Europa, which probably
harbors 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 sulfurous cauldron around hydrothermal vents. Vent
environments minimize 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 as 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.
