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NASA  | | Formation
of a black smoker: After sea water seeps into the crust (1), oxygen and potassium
(2) and then calcium, sulfate, and magnesium (3) are removed from the water. As
the water begins to heat up (4), sodium, potassium, and calcium dissolve from
the crust. Magma superheats the water, dissolving iron, zinc, copper, and sulfur
(5). The water then rises back to the surface (6), where it mixes with the cold
seawater, forming black metal-sulfide compounds (7). Image courtesy Woods Hole
Oceanographic Institution. |
Miles
below the ocean surface exist some of the most fascinating habitats for life on
Earth. Here, where sunlight never reaches, live complex ecosystems that can appear
and disappear within a matter of decades. What provides the thermal and chemical
energy that fuels these ecosystems are deep-sea hydrothermal vents, one of the
unofficial wonders of the natural world.
NASA's Astrobiology Institute
and Science@NASA are teaming up to present a live webcast from the research vessel
Knorr, where scientists sailing the Indian Ocean will discuss their latest discoveries
about undersea hydrothermal vents.
These vents occur at oceanic "spreading
centers," mountainous ridges where magma from deep within the Earth's crust forces
its way up to the ocean floor, creating new ocean crust and pushing the old crust
out of the way. This is the engine that drives apart the Earth's tectonic plates,
moving continents about and causing volcanic eruptions and earthquakes.
From
time to time, hydrothermal vents, known as "black smokers," occur along these
ridges. They are underwater geysers. At these vent sites, cold ocean water seeps
down through cracks in the seafloor to hot spots underground. The water gets superheated
to several hundred degrees Celsius and is spit back up in a mineral-rich broth
of scalding fluid. And in this bizarre environment, life flourishes.
Until
a little over 20 years ago, no one knew that deep-sea hydrothermal vents existed,
much less that they were teeming with life. The first such vent was discovered
in 1977 east of the Galapagos Islands. Since then, dozens of vents have been discovered
and explored along ridges in the Atlantic and Pacific.
Active vents are
inhabited by a complex ecosystem of organisms containing both microbial and more
complex animal life. (There is no plant life in the deep ocean, because sunlight
cannot reach down that far to drive the process of photosynthesis on which plants
depend.) The animal life includes tube worms, shrimp, clams, mussels and crabs.
 Left:
A Quicktime video
shows a black smoker spewing scalding water cloudy with metal-sulfide minerals.
Courtesy Woods Hole Oceanographic Institution.
Last
year scientists discovered a vent along a ridge in the Indian Ocean. (It's located
south of the southern tip of India and east of the African island nation of Madagascar.)
An expedition is currently underway to explore this vent. Japanese scientists
visited this vent in August, 2000, but spent only four days there. The new expedition
plans to spend several weeks at the new vent. Cindy Lee Van Dover, of the College
of William and Mary in Williamsburg, VA, is chief scientist on the research cruise.
Van
Dover has been exploring ocean vents for many years. "I really never thought that
one could be an explorer in this day and age. But in the ocean, it's absolutely
true. You're going places that nobody's ever been before."
Van Dover studies
the morphology (body shape) of vent animal life. Mussels are her specialty. Bob
Vrijenhoek, a senior scientist at the Monterey Bay Aquarium Research Institute
in Moss Landing, CA, takes a different approach. He studies vent animals, as well
as the bacteria that inhabit the vents, by analyzing their DNA. Members of Vrijenhoek's
research group are participating in the Indian Ocean expedition.
The study
of how animal populations evolve and disperse geographically is known as "biogeography."
Hydrothermal vents offer a unique opportunity for biogeographers because the underwater
environment is affected by fewer factors than are land environments. "People study
biogeography on land and it's always got superimposed on it the effects of latitude
and climate," says Van Dover. Hydrothermal vents, in contrast, are "largely decoupled
from climate. They are isolated from what goes on above."
 Right:
These tubeworms thrive on the chemical energy expelled from deep-sea
hydrothermal vents. These creatures have no mouth, no gut and no eyes. Instead
of feeding, they host chemosynthetic bacteria that produce sugars for the tubeworms
in a symbiotic relationship. Courtesy Woods Hole Oceanographic Institution.
Most
vent organisms, scientists believe, can exist in their adult form only near an
active vent site (although many of these organisms have swimming larval stages,
which can travel for great distances). Individual vents remain active for anywhere
from a few decades to a few thousand years. When a vent shuts off, the adult animals
living there die. Yet as soon as a new vent emerges, it is rapidly colonized.
Within a few years, a new vent undergoes a complete transformation from uninhabited
to fully populated.
By studying the similarities and differences among
the animals that live at different vents, scientists have begun to piece together
a picture of how organisms move from one vent to another, what are the natural
barriers to such movement, and how the geography of the deep ocean affects the
evolution of the species that inhabit it.
Most of the research into the
fauna (animal life) that inhabit hydrothermal vent systems has been done in the
northern region of the Mid-Atlantic Ridge and along the East Pacific Rise, which
runs roughly parallel to the west coast of South America. Although similar types
of animals can be found at both Atlantic and Pacific vent sites, there is more
similarity among the vent ecosystems along the same ridge than there is between
the two ridges. For example, shrimp are found at both Atlantic and Pacific sites,
but one particular type of shrimp, known as "swarming shrimp," is found only in
the Atlantic.
The Pacific is a very old ocean, while the Atlantic is relatively
young, having fully formed only about 120 million years ago. One question scientists
are interested in is how the animals that inhabited the Pacific ridge system made
their way to the younger Atlantic ridge.
 Right:
The Atlantic Ocean formed around 120 million years ago, as the continents
drifted apart. Courtesy Nova Scotia Museum.
One
theory is that some of the organisms may have arrived by way of the Tethys Sea.
Don't look for it on a map, unless it's a map of what Earth looked like 100 to
200 million years ago. All that's left of it today is the Mediterranean. The Tethys
Sea was a much larger body of water, which once connected the Indian Ocean to
the Atlantic. Scientists theorize that animals could have migrated along ocean
ridges from the Pacific to the Indian Ocean, and from there through the Tethys
Sea to the North Atlantic.
Some vent organisms, for example, vent shrimp,
haven't been around all that long. They are thought to have evolved only 20 million
years ago. So they couldn't have arrived by way of the Tethys Sea, because by
20 million years ago it had closed up. Another possible route for organisms to
have traveled is through the Indian Ocean around the Cape of Good Hope to the
South Atlantic.
 Left:
The Knorr expedition is currently in the Indian Ocean exploring a newly
discovered vent south of India and east of Madagascar. Click on the image for
a map of the known vent sites around the world. Courtesy Woods Hole Oceanographic
Institution.
In
either case, the Indian Ocean vents may provide a "missing link" between Atlantic
and the Pacific vent ecosystems. Early photographs from the Indian Ocean site
taken by Japanese scientists show shrimp and mussels that appear very similar
to those found at Atlantic vents. "If you had shown me one of those pictures and
asked me where that picture came from," says Vrijenhoek, "I'd have told you it
came right from the mid-Atlantic ridge." But, he cautions, "we could get fooled
just by superficial appearance." He is looking forward to the results of the DNA
analysis that his colleagues will perform on these animals.
Recently developed,
highly efficient DNA-based tools have dramatically changed the way scientists
study evolution. Scientists like Vrijenhoek use these tools to determine the similarities
and the slight mutational changes between the genes in organisms found at different
vent sites. Using this information leads to a better understanding how the life
cycle of an organism interacts with the changing typography of the seafloor to
affect both the geographic dispersal and evolution of that organism. "We do the
same thing that a forensic scientist would do," Vrijenhoek explains. "We basically
extract DNA from the organism and then we use that DNA to look at the degree of
relationships within populations, and then between populations."
For example,
Vrijenhoek and his colleagues have found what he calls "genetic discontinuities"
among populations of vent amphipods (small crustaceans) that don't appear among
populations of other vent organisms. This is due, he explains, to the fact that
there is no swimming larval stage in the amphipods' life cycle. As a result, one
population of organisms can easily be cut off from another, causing the two populations
to drift apart genetically.
 Left:
An amphipod. Photo by Chris Clark.
Says
Vrijenhoek, "The amphipods probably just ride up and down these ridge axes like
a corridor. So if there's a disruption in that corridor, through a transform fault
or lack of habitat, or something like that, they simply can't get from point A
to point B." The isolated populations then evolve along separate pathways.
Genetic
isolation is less likely to occur among populations of animals that have do have
a swimming larval stage, because they can more easily cross such physical barriers.
This is just what is seen in mussels, clams and tube worms.
Although there
is plenty of work yet to be done in the Indian Ocean, Van Dover and Vrijenhoek
are also excited about taking a look at other vent sites that remain completely
unexplored. The southern Atlantic is one such region.
But if given a choice
(and funding), Van Dover would head for the Arctic Ocean, because of its isolation.
"The Arctic Basin's been separated from the Atlantic and the Pacific since the
Arctic Ocean was formed by shallow fill. So the deep fauna of the Atlantic and
the Pacific, the ones that occur at the vents, may not have gotten up into the
Arctic. If you wanted to pick the place to go find the most unusual vent organisms,
I'd have to choose the Arctic."
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