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Oceans
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Seminars on Science learn.amnh.org The Ocean System - Integrated Earth, Life, and Physical Sciences
ESSAY 2.4
How can there be life on the ocean floor?
by Dr. Adriana Aquino
The last great frontier
It makes up about 90% of the volume of the ocean. It’s
the most extensive habitat on planet Earth. Yet the deep
sea — a vast realm of frigid temperatures, intense
pressure, and near-total darkness — is almost unknown
to humans. Since only 1% of all sunlight penetrates more
than about 100 m below the surface, and since most life
on Earth is fueled by the Sun, people in centuries past
presumed that the deep sea was devoid of life and that
An illustration of the HMS Challenger landing the bottom was barren. They reasoned that in order to
live there, animals would have to be extremely large —
at St. Paul’s Rocks, an island group in the
insulated enough to withstand the cold and pressure and
Atlantic Ocean off the coast of South
able to travel quickly to the surface for food or oxygen.
America. ©NOAA
Medieval seafarers told tales of giant squid and colossal
whales, glimpsed in rare moments at the ocean surface. (It wasn’t until the advent of SCUBA
technology that we saw the blue whale underwater, and the giant squid has yet to be observed.) But the
ocean’s sunless depths and unseen floor could hold no life, right?
Hardly. By the 1870s, the British HMS Challenger — a series of scientific expeditions that laid the
foundation of modern oceanography — was already collecting deep sea fish. Still, the understanding
was that any life in the deep would have to depend on a “rain” of nutritious sea snow (particles of dead
plankton and fecal matter) and carcasses of dead animals from the photic, or sunlit, regions of the sea.
No one could imagine a self-sustaining ecosystem on the seafloor.
Life in the deep!
A century later, after the theory of plate tectonics was confirmed in the 1960s, scientists strongly
suspected the existence of deep-sea hot springs. In the late 60s and early 70s, they measured slight
temperature anomalies around an undersea ridge south of the Galápagos Rift’s volcanic valley. In 1976,
an expedition returned to the site with a towed camera system that photographed a gaping crack in the
seafloor, rocks that seemed to be “frosted” with white and bright yellow deposits, and a pile of big,
empty, white clamshells. Sensors also detected a narrow zone of water about 0.2°C warmer than the
surrounding seawater — all of which was exciting evidence, but too circumstantial to prove the
existence of hydrothermal vents. A year later a team of geologists, geochemists, and geophysicists
returned (no one imagined that a biologist would be useful), descending 2.5 km (8,200 ft) in the Alvin
submersible. Alvin’s sensors measured water temperatures of 8°C (46°F) at the bottom of the sea,
confirming the first discovery of a hydrothermal vent. Even more astonishing was the discovery that
strange new animal species — clusters of giant white clams, this time clearly thriving, thickets of redtipped tube worms, tiny limpets and worms — clustered around the chimneys of the hot springs. The
turbid, milky water spewing from these deep sea vents contained a unique community of microbes. The
geologists had stumbled upon an oasis of life on the near-barren deep sea floor. What on earth were
these animals eating?
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Oceans
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A comparison of photosynthesis to chemosynthesis shows the similarities in the two
processes at the most fundamental level. ©AMNH
Tapping energy from the inner Earth
Terrestrial life relies on photosynthesis. Primary producers — like plants, algae, and some bacteria —
trap sunlight and use its electromagnetic energy to convert carbon dioxide and water into sugar and
oxygen. This process is the basis of the food chain on land and also in the upper layers of the sea.
At deep sea vents the primary producers of the food web are Archaea and other microorganisms. These
microbes use energy from chemicals instead of sunlight to synthesize sugar, in a process called
chemosynthesis. (“Chemo” means chemical, while the “photo” in photosynthesis means light.) In
general, the source of energy for chemosynthesis can be varied, depending on the evolution of the
microbe and the chemical that is available. At deep sea vents, one of the energy sources for
chemosynthesis is hydrogen sulfide, which abounds in the hot, mineral-rich waters spewing from the
vents (and is toxic to almost all other organisms). Microbes in this environment get energy by breaking
down hydrogen sulfide. They use this energy and oxygen to convert carbon dioxide into sugar, sulfur,
and water. In anaerobic species, the chemosynthetic process can even take place without oxygen. Just as
plants do for land organisms, the microbes make “usable energy” readily available to the rest of the vent
community.
The vent ecosystem
Seafloor: An Alien World
Vents are home to an astonishing variety from the Hall of Ocean Life...
of animals. In the Pacific Ocean, species Some of the animals found
include giant tube worms (see this week’s at hydrothermal vent springs
Case Study), huge clams, and mussels,
are more than merely odd —
many species of worms, and shrimp-like they are truly bizarre. Before
crustaceans. Eyeless shrimp are perhaps their discovery in 1977, their
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the most distinctive inhabitants at vents in very existence was
the Atlantic Ocean. The composition of considered impossible.
Deriving no energy from the
these animal communities varies
Sun, they survive on
significantly, even between the East
chemicals that would be
Pacific Rise off the western coast of
Mexico and the Juan de Fuca Ridge off toxic to most other animals
on Earth.
Vancouver, Canada. And if those
organisms are sisters in the family tree,
the vent animals of the Mid-Atlantic
Ridge are cousins several times removed,
sharing some genera, but few species.
Why the differences, since the basic
components (basaltic rock and seawater)
and chemistry of hydrothermal vent
©WHOI
systems are relatively uniform around the
world? The major reason is likely the geographic isolation of the two vent systems; small chemical
differences also probably play a part.
A new way to look at life
Other groups of chemosynthetic microbes have been found, each metabolizing the source of chemical
energy most readily available. (Sulfur compounds are the chemicals most commonly used by microbes,
followed by hydrogen, methane, and nitrogen.) For example, in areas known as "cold seeps," where
methane, sulfides, and hydrocarbons seep out of the ocean floor, scientists have found microbes that
consume these chemicals and in turn feed vast fields of tube worms and mussels. The floor of the Gulf
of Mexico turns out to be home to a community of clams and worms that consume methane-“eating”
microorganisms. It’s also where a brine seep feeds a strange underwater salt lake almost 200 feet wide
and less than a foot deep. This super-salty lake contains no oxygen, but sulfide-oxidizing Archaea
inhabit the border between the lake and the seawater and help feed a specialized community of
organisms.
Scientists have known about the existence of chemosynthesis for a century, but they had no idea that it
had the power to drive whole ecosystems like these. Deep sea vents are still being intensively studied,
and there is much we don’t understand about the geological and biological processes that govern these
ecosystems. In the late 1980s, for example, scientists documented an infrared light near certain
hydrothermal vents. This natural light suggests that photosynthetic organisms might be part of the deep
sea food chain as well!
Why is the Earth habitable?
from the Hall of Ocean Life...
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©NASA
Billions of years ago, Earth was bombarded by cosmic rays that would have made life on its surface
nearly impossible. But since we now know that abundant life can flourish without sunlight, it seems
possible that the first life on Earth may have originated on the deep sea floor. There could even be life
elsewhere in the universe—at hydrothermal vents on the seafloor of Jupiter's moon Europa, for example,
or in warm, wet places beneath the surface of Mars.
Here, we see two possible models for the interior of Europa, based on current scientific understanding
from the Galileo spacecraft. One suggests an icy subsurface under a 15 km surface ice crust, the other
model shows a 100 km deep liquid ocean also under a thick ice crust. A Europan ocean this deep would
contain twice as much water as Earth’s oceans and rivers combined.
Online Resources
Required Links
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NOAA: New Millennium Observatory – Dive!
Virtual dive experience using video and animation from research expeditions.
http://www.pmel.noaa.gov/vents/nemo/dive.html
WHOI: Dive and Discover
Go on board the research cruises of Woods Hole Oceanographic Institution through this exciting
interactive distance learning program. Join a current expedition or find curriculum resources for
the classroom.
http://www.divediscover.whoi.edu/
AMNH: Black Smokers Expedition
The Museum’s expedition to the Juan de Fuca ridge off the Pacific Northwest coast in 1997-98.
http://www.amnh.org/nationalcenter/expeditions/blacksmokers/
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