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Deep Seas, Dark Worlds
Christy Brownlee 

There's barely enough room for three people to fit. But marine scientists Richard Lutz and Peter Rona of Rutgers University crammed themselves into Alvin, a tiny submersible vehicle made for exploring the deep seafloor. Unlike scientists who boarded Alvin before them, Lutz and Rona weren't going on a research expedition to gather data on ocean life. Rather, they were guiding a camera crew in shooting the IMAX movie Volcanoes of the Deep Sea. This is the first film ever to take an up-close look at deep-sea vents and their ecosystems (all of the living and nonliving things that interact in an environment).

Five minutes after the sub began its long, chilly descent into the Pacific Ocean, its passengers were steeped in total darkness — no sunlight penetrates the water's opaque depths. The pilot and crew pulled on thick sweaters and warm socks as the temperature dipped to match that of the frigid water outside the vehicle — only 1.1C (34F), just above freezing.

Besides the occasional glimmer of luminescent (glowing) sea creatures splashing against Alvin's portholes, there was no life to be seen. But two hours later — and 3,600 meters (12,000 feet) deeper — Lutz and Rona found a surprising treasure among the otherwise barren waters. Alvin's blue-green headlights panned across a bizarre scene of red-tipped tube worms, shrimp, and fish crowded around a rocky column spewing smoky water. "It's like a little oasis in the middle of the desert," says Lutz.

What was this towering plume? Peering through Alvin's 10-centimeter (4-inch) porthole, the scientists had spied a hydrothermal vent, a hot spring on the seafloor that pumps out water superheated by volcanic activity from inside the earth.

Monumental Mountain

Most hydrothermal vents sit on top of the mid-ocean ridge, the longest continuous mountain range on the planet. "It's the largest geographic feature on Earth," says Rona. But that's not the only thing that sets this mountain range apart: The ridge snakes over more than 64,000 kilometers (35,000 miles)— along the bottom of each of the world's oceans.

The ridge, which is made of solidified lava, is like a giant zipper being pulled open as two tectonic plates (slowly moving, giant rock slabs that make up Earth's outer shell) spread apart. Molten material erupts through an opening in the ridge, pushing older rock to both sides of the mountain range. As the hot liquid lava meets cold seawater, it instantly solidifies into new crust. "It's like an engine pumping up new material from the earth. As it pumps lava up, it forces each wall of the mid-ocean ridge to move aside," explains Lutz. Because parts of the ridge are continually spreading and forming new sections of seafloor, scientists call these places seafloor spreading centers.

Birth of a Vent

Sometimes the scalding lava cracks as it solidifies. Cold, heavy seawater seeps into the cracks. As the water sinks several kilometers into Earth's interior, metals, minerals, and gases dissolve in the water. When this water comes into contact with molten rock under Earth's crust, it gets heated to temperatures as high as 400C (750F)— hot enough to melt lead!

The water's newfound heat makes it light and buoyant, causing it to rise. As it exits through another set of cracks, the seawater creates a new hydrothermal vent. The minerals and metals that had dissolved in the water crystallize around the vent, forming a tall tube called a chimney. The hydrothermal vent will continue to discharge hot, metal-rich water for decades. Eventually, it becomes clogged with minerals, topples over, or uses up the heat, Rona explains.

Prime Real Estate

Along with minerals and metals, hydrothermal vents also gush forth dissolved gases like hydrogen sulfide — toxic to humans, with a potent stench like rotten eggs. Not a great place to set up house — unless you're a hyperthermophile. These heat-loving microbes live on and around vents and use this nasty gas as an energy source to manufacture sugars and starches to nourish themselves. What's even stranger: The microscopic members of these unique underwater cities breathe the iron that was once locked beneath Earth's oceans but is now dissolved in the watery plumes. These chemosynthetic (chemical-using) microbes give rise to other life that normally couldn't live in the pitch-black deep sea. "These bacteria are the bottom of the food chain at the vents," says Lutz.

After bacteria spreads into a thick mat around a new vent, other creatures gradually set up colonies in a process called succession. These animals slowly crowd onto the tiny section of livable space, braving the surrounding sea's frigid temperatures and water pressure, which is strong enough to crush an army tank. Of the life that Lutz and Rona spotted on their underwater tour, the shrimp-like amphipods (AM-fih-pods) and cope-pods (COH-puh-pods) were likely among the first to drift down to graze on the vent's bacteria. The biologists suspect the snail-like limpets and shrimp arrived months later to dine on the amphipods and copepods. Next to follow? Tube worms with tops as red as lipstick. Lobsters, octopi, mussels, and clams are some of the last creatures to set up house at a vent.

Alien Life

Although a few deep-sea creatures are familiar, more than 95 percent of vent animals are completely unknown to science. Researchers find weird new species on almost every dive, including snails covered with plates of iron armor and shrimp with infrared light detectors on their backs instead of eyes.

Because these strange creatures can survive super-hot temperatures, high pressure, and darkness — conditions too extreme for humans — Lutz and Rona say the vent's unique ecosystems may hold the key to finding life elsewhere in our solar system. For example, many scientists believe that hydrothermal vents may exist under the icy surface of Europa, one of Jupiter's moons, and may harbor vent microbes similar to those on Earth. "These bacteria are the most primitive kinds of organisms that we know on Earth," says Lutz. "They're strong evidence that life might have started in the vents and that it could exist someplace else."

How Hydrothermal Vents Form

At the mid-ocean ridge, molten lava rises to the top of the oceanic crust, Earth's outermost layer of rock in the oceans. Cracks in the oceanic crust allow dense seawater to sink beneath the rock surface. During the descent, minerals dissolve into the water. The mineral-rich water then warms, rising back to the seafloor to form deep-sea vents.
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