lunes, 13 de agosto de 2012

Deep Sea Vents: Science at the Extreme

Deep sea vents in the Atlantic Ocean’s Mid-Atlantic Ridge support tubeworms, eelpout fish, and crabs. The Mid-Atlantic Ridge is an underwater mountain chain that extends about 10,000 miles (16,000 kilometers) from the Arctic Ocean to the southern tip of Africa. Photograph by Emory Kristof
Republished from the pages of National Geographic magazine

With a mile and a half (two and a half kilometers) of Pacific Ocean sitting on their shoulders, ghost-pale crabs and fish forage among blood-red tube worms. Such communities flourish where super-heated water gushes from seafloor springs. Advances in the tools that scientists use to investigate deep-sea ecosystems are expanding knowledge of these creatures and their hostile environment.

Strange Life Clearly Seen
Water heated as high as 760°F (404°C) by magma from Earth's interior billows from a seafloor chimney. The surrounding ocean is just a few degrees above freezing. When the two fluids meet, iron sulfide precipitates, giving the "black smoker" its color. In these dark depths, chemosynthesis—based on thermal and chemical energy from the vents—is the primary mechanism sustaining life.

A living cloud flecks the water around a clump of limpet-encrusted tube worms and mustard-yellow mussels in the high-definition image at far right. With a magnifying lens on another camera the cloud resolves into a crowd of flea like crustaceans called amphipods. Amphipod swarms like this one—observed at 9° N on the East Pacific Rise—may be the densest concentrations of invertebrate life on Earth.

High-intensity lighting and high-resolution imaging technologies provide researchers with the equivalent of a microscope to examine life in the deep sea. These tools can reveal organisms that have always been part of vent communities but have been hidden until now.

Timothy Shank, a marine ecologist at Woods Hole Oceanographic Institution, calls the array of previously unknown species found at vents "mind-boggling." He has calculated that, on average, a new species has been described every week and a half since biologists first visited the Galápagos Rift vents in 1979. "More than 20 years later," he says, "we're still on the tip of the iceberg. We're trying to understand relationships among vent animals—and we're still discovering new species!"

Evaluating an Arctic Oasis

Launch and recovery of submersibles can become a daily drama in Arctic waters, where storms packing 45-mile-an-hour (74-kilometers-an-hour) winds and 20-foot (6-meters) waves blow up rapidly. Undaunted by grueling conditions, scientists from Russia, Germany, Norway, and the U.S. Naval Research Lab (NRL) used Russia's twin Mir submersibles to investigate the Haakon-Mosby mud volcano 4,100 feet (1,250 meters) below the surface.

The team discovered "a chemosynthetic oasis," says NRL geophysicist Peter Vogt. It is populated by small worms, numerous eelpout fish, and nearly 20 previously unknown species of bottom-dwelling organisms. The team also found white bacterial mats growing on frozen methane hydrate that coated much of the seafloor around the volcano. If the Arctic Ocean warms by just a few degrees, as some climate-change models predict, massive amounts of methane could be released into the water column and then into the atmosphere, says City University of New York geologist Kathy Crane. "Methane is ten times more effective than carbon dioxide as a greenhouse gas," she notes. "The effect on climate would be powerful."

Hot Spots in the Deep Sea
We've studied a handful of sites on the East Pacific Rise and Mid-Atlantic Ridge in detail, but most of the 46,600-mile (75,000-kilometer) globe-circling mid-ocean ridge system still beckons—unexplored. Off the ridges, researches examining deep-sea mud volcanoes have discovered communities of animals as exotic as those found at hydrothermal vents.

Hydrothermal Vent
The Earth's crustal plates pull apart along the mid-ocean ridges, and lava surges up between them, cracking as it cools into new crust. Seawater penetrates fissures that can be miles deep; then, heated as it nears the magma layer, the water expands and rises rapidly. Heavy with minerals leached from surrounding rocks, it gushes from the bottom in fuming geysers or in lower temperature springs with a gentler flow.

Mud Volcano
Gassy mud, rather than molten rock, erupts from mud volcanoes. Heat from deep in the mantle liquefies miles of overlying sediments, driving them upward and releasing fluids and gases into the water column. High pressure and low temperature transform released methane gas into a stable solid—methane hydrate—that covers much of the seafloor surrounding the volcano, like frozen icing on a warm cake.

Survival Strategies
Armored bodies and leathery tubes help shield some vent creatures from attack. Soft-bellied spaghetti worms are defenseless—but prodigiously fertile. Each worm's body is lined with pill-shaped structures, primed to spawn a new generation.

The concentration of life at vents attracts deep-sea carnivores. A finned octopod uses its powerful beak to devour prey, including bottom-dwelling crustaceans and worms.

Temperatures in a vent field can shift drastically and swiftly, especially near big smokers. Creatures that can't adapt equally quickly don't survive here.

Some of my colleagues have nominated the worm Alvinella pompejana as the most thermally tolerant animal on Earth. Whole galleries of alvinellids live in tubes on the sides of black smokers, at the very seething heart of the vent. University of Delaware marine biologist Craig Cary has gathered data that show alvinellids living in water that's 149°F (65°C) and surviving frequent temperature spikes well above 175°F (75°C). The standard wisdom used to be that no multicellular organisms lived above 130°F (54°C), but Cary thinks that these worms do so routinely.

Alvinellids—on average a half inch (1.25 centimeters) in diameter and about 3 inches (7.6 centimeters) long—also tolerate the steepest temperature gradient on the planet. Specimens have been found in water 140°F (60°C) hotter at one end of the animal than at the other. Superheated fluids from black smokers do not mix well with ambient cold seawater, so transitions between them are abrupt.

"Textbook biology tells us that animals can be psychrophilic [cold loving] or thermophilic [heat loving] but not both," says Cary. "I guess the alvinellids just didn't read the textbook."

We don't yet know how Alvinella worms survive these extremes. The answer may lie in their behavior or in some specialized cellular biochemistry, or both.

Capturing Deep-Sea Detail
Visiting a vent is a dangerous and costly undertaking. Since the earliest dives scientists and image makers have worked with submersible pilots—who act as our eyes and hands on the seafloor—to create a photographic record of these rare experiences. The Woods Hole workhorse Alvin took us to 9°N in fall 1999. Woods Hole research specialist William Lange helped equip the submersible to allow photographer Emory Kristof, documentary filmmaker Mike DeGruy, and IMAX producer Stephen Low to shoot conventional video, high-definition video, and IMAX footage. Each tool has pluses and minuses; all produce pictures that look good on the printed page. But for scientists the critical comparison is resolution. As the comparisons illustrate, the technologies differ in how much detail can be recorded and in how far into an image we can look before clarity breaks down.

IMAX: With ten times the image area of 35-mm motion film, IMAX yields more detail than other media. All images have been adjusted to better illustrate the decrease in resolution.

35-mm Still Film: Less flexible than motion-imaging tools, it offers relatively high resolution.

35-mm Motion Film: Despite its limits, it's the familiar standard for evaluating image quality.

HDTV: Its resolution is equivalent to 35-mm motion film. Don't see it? Try squinting.

Conventional Video: Its resolution is one-sixth that of 35-mm motion film or HDTV.

Vent creatures called dandelions typically disintegrate when they're brought to the surface. Early video images helped scientists identify the species as a colonial siphonophore—a tiny relative of the Portuguese man-of-war.

Conventional video can't match the resolution of its digital descendants, but the technology—compact, relatively simple cameras using inexpensive, easily duplicated tapes—remains important for recording deep-sea observations.

Shooting from about a foot away, a standard video camera with a macro lens fills its screen with a subject less than half an inch square. The bright orange spheres are tiny eyes on the head of a shrimp.

Fan worms use frilled limb like structures to capture minute food particles suspended in the water. High-definition resolution and macro-lens magnification can now be combined to reveal each delicate anatomical detail.

A frame of IMAX film can fill a screen eight stories tall with astonishing detail. The unwieldy camera and thousand-dollar-a-minute film limit deep-ocean IMAX use, but images likes this one come closer than anything seen before to capturing the experience of observing a thriving sea vent community firsthand.

Dynamics of Life and Death
Stripped to the bone by scavengers off California, what was once a 35-ton gray whale still teems with life. "Whale falls" create microhabitats rich in the sulfide compounds vent organisms need to survive. University of Hawaii marine biologist Craig Smith has counted nearly 400 species colonizing such remains, including clams, limpets, and mussels also found at vents 1,000 miles (1,600 kilometers) away. He suggests that larvae drifting over ventless areas may settle on whale fall "stepping stones," mature, and release their own offspring, eventually dispersing populations among distant ridge systems.

Giant Clams—typically among the last animals to colonize an active vent—have arrived in force at 9°N. Nearly a decade of observations suggests that hydrothermal activity is likely to diminish here soon.

Vents are ephemeral. The volcanic fire that gives birth to them eventually cools. Animals perish as the flow of hot, sulfide-bearing water slows, then stops, and only fantastical lava formations remain.

Cataloging the rich life at the vents has taken 20 years. In the next 20 we hope to understand the life histories of the creatures we've discovered. Exploring the origins of chemosynthetic organisms may also point us toward the source of the earliest lift on Earth

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