Source: NOVA: "Volcanoes of the Deep"
Less than a century ago, people expected the ocean floor to be as featureless as the surface of the water above. However, underwater explorations revealed quite the opposite: towering mid-oceanic ridges and deep ocean trenches. This video segment adapted from NOVA reveals the unexpectedly rich ecosystems researchers found when they looked more closely at the seafloor near mid-ocean ridges.
Seafloor spreading generates new ocean crust, but it may also have created conditions necessary for the beginning of life on Earth. At mid-ocean ridges, molten lava pushes through breaks in the crust during amazing undersea volcanic eruptions. This molten rock quickly cools, and as it does, it contracts, forming long cracks and fissures. Ocean water seeps into these cracks to a depth of several kilometers, where it is heated by magma in Earth's mantle to as much as 350 degrees Celsius (660 degrees Fahrenheit). This superheated water then rises and eventually emerges from hydrothermal vents in the ocean floor as an underwater geyser. When the hot vent water meets cold ocean water, dissolved minerals in the vent water precipitate. In some places, compounds of iron and sulfides form "chimneys" on top of the vents.
Such an extreme environment seems unlikely to support life, given its high temperature, crushing pressure, and absence of light for photosynthesis. In 1977, however, Alvin, a manned submersible vessel built to withstand extreme pressure, dove to the deep ocean floor where its crew made a surprising discovery -- a vibrant ecosystem surrounding the hydrothermal vents of the mid-ocean ridges. Since that time, more than 300 new species have been identified at various vents around the world.
Bacteria called chemoautotrophs serve as the primary producers at the base of the hydrothermal vent food chain. These bacteria are functionally equivalent to plants, which serve as primary producers in terrestrial ecosystems by fixing carbon and creating sugars through the process of photosynthesis. In the absence of sunlight, these deep-sea bacteria must rely on another source of energy. As their name suggests, these chemoautotrophs break the chemical bonds of the mineral compounds found in abundance around hydrothermal vents and use this energy to fix the carbon necessary to sustain life. Other species in this unusual ecosystem, such as tube worms, clams, mussels, crabs, shrimp, and octopuses, use the energy captured by the bacteria to fuel their own activities.
The appearance of bacteria in these extremely hot, high-pressure, and dark environments has caused scientists to speculate that hydrothermal vents or places like them may have been sites of the earliest appearance of life on Earth -- fueled by water, energy from chemical reactions, and a rich supply of resources.
Classify symbiotic relationships between individual organisms of different species in this NOVA classroom activity.
NARRATOR: When life took hold on the planet around four billion years ago, there was a vast network of hydrothermal vents perhaps providing the geochemical energy to spawn and support life. There is now evidence that the surface of this early Earth was bombarded by meteors and asteroids. The safest harbor for life may have been in the deep—a deep-sea mountain range over 46,000 miles long known as the mid-ocean ridge system. It stretches around the planet like the seams on a baseball and marks where the great plates of the Earth's crust are spreading apart. All along this vast network, volcanic eruptions give birth to new ocean floor. And molten rock, deep under the seabed, creates the scalding black smokers that stream from vents and chimneys. The geology and chemistry of the vents have changed little. Some of these chimneys tower as high as 15-story buildings. And though sunlight never reaches them, they are blanketed with life.
VERONIQUE ROBIGOU, Marine Geologist, University of Washington: It's part of this mystery about this environment is that you go through the really dark water where you don't expect to find much, and then suddenly you start seeing life and it's life that's very beautiful because it is very bright and luminescent and also very striking, because it doesn't look like anything that we're used to.
NARRATOR: These worms harbor inside their bodies a remarkable source of food: tiny, single-celled bacteria. These microbes are able to produce food using hydrogen sulfide and other chemicals that flow around the chimneys. Each microbe is invisible to the naked eye, but when billions clump together, they appear like cottony webs draping the sea floor. As plants at the surface use the energy of light these microbes use energy stored in chemicals to grow and multiply. The larger creatures in this world either live off microbes within their bodies or prey on one another. Giant spider crabs...snails and sea stars...the fish and the octopus—all are ultimately dependent on tiny, single-celled organisms and the volcanic fluids that flow from the rocks. This is a world where the energy for life springs not from the Sun but from the geothermal forces of the Earth itself.
CHUCK FISHER, Biologist, Penn State University: As a biologist, I have no doubt in my mind whatsoever that the geology is the driver for the biology and, in fact, even the structure of the rocks that are in the chimneys are going to determine the type of fauna you find on top of the chimneys. And by delving inside a chimney, they may find clues to how life itself originated.
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