Feature

Defying Depth

How deep-sea creatures, and close relatives, survive tons of water weight

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10:37am, June 1, 2012
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When you think about life’s pressures weighing down on you, consider the plight of Palaemonetes varians — the Atlantic ditch shrimp.

Smaller than a finger, and covered with only a thin shell, the translucent creature flourishes in the warm, shallow waters off the coast of northern Europe. Recently, though, scientists at the University of Southampton in England have plucked dozens of the critters from their homes and carried them to the lab, placing them in reinforced containers that replicate the crushing pressures found more than three kilometers beneath the sea’s surface. Here, where the strain of the water can squash a human’s rib cage, the shrimps survive quite happily. When the pressure’s on, this animal can sink and swim.

Scientists are subjecting the shrimps to these extreme conditions to better understand the mechanisms that allow some marine animals to adjust to life in the deep sea. Other teams are traveling to the deepest parts of the ocean — where pressure can reach more than 16,000 pounds per square inch — to study the biology of creatures that already thrive when tremendous weight bears down on them.

Exactly how deep-sea animals withstand intense pressures is not completely known, even though scientists have puzzled over this question for decades. For a long time researchers have relied on tissue samples taken from animals pulled up in nets. By comparing related proteins in shallow-water and deep-water species, researchers have found that extreme pressure can inhibit the activity of some proteins. Other studies have shown that small molecules called piezolytes may help protect proteins from the pressure.

While there is still much to be learned from tissues, the ability to bring live animals up from the deep, while maintaining their natural pressure along the way, is allowing scientists to look beyond individual molecules and study whole-body reactions. New tools designed to keep creatures alive at the surface for months will help researchers document a host of changes in the animals over time.

The work may help scientists piece together a more complete picture of how life in the ocean’s high-pressure zone survives, and how it got there in the first place, says Sven Thatje, an evolutionary ecologist at Southampton. He presented findings from his shrimp studies in Vancouver earlier this year at a meeting of the American Association for the Advancement of Science.

Evolutionary scientists believe that climate shifts at various times during Earth’s history wiped out many of the animals living in the deep sea. These extinctions were probably followed by recolonization of the dark depths by shallow-water species, which somehow were able to adapt.

Understanding how creatures could make such a remarkable transition not only sheds light on the basic biology of living under pressure, but may also provide clues to how sea life will respond to new conditions brought by global warming.

“Going to greater depths might allow some species to escape undesirably warm surface waters,” Thatje says.

But greater depths come with much more water weight.

A deep menagerie

Living under high pressures is actually quite common. The deep sea — defined as any part of the ocean more than 200 meters below the surface — makes up about two-thirds of the inhabited biosphere. Here, the dark, frigid waters come with pressures intense enough to mash a Styrofoam cup to the size of a thimble. For years, though, this harsh environment was left largely unexplored.

But in the last half century, advances in deep-sea submersibles and technologies designed to peer into the darkness have allowed researchers to observe and uncover many mysteries of the deep ocean realm. Instead of finding a barren landscape, these explorers have discovered scores of strange-looking animals living in even stranger conditions.

In 1977, scientists stumbled for the first time on communities clustered near hydrothermal vents, roughly 2,500 meters down on the seafloor near the Galápagos Islands. The vents spew superheated, chemical-rich fluids that sustain animals such as giant clams, mussels, tube worms, snails and shrimps. Other researchers, exploring even greater depths, have found a diversity of invertebrates including shrimplike amphipods and sea cucumbers, as well as unusual fish such as the angler.

Still, the number of animal species in the sea drops the deeper you go. Fish, for example, are seldom seen below 8,000 meters, possibly because of high pressures.

On the Earth’s surface, humans experience what is known as one atmosphere of pressure: a mere 14.7 pounds of air per square inch. But water is denser than air. Diving below the sea, the weight of water above causes the pressure to increase about one atmosphere every 10 meters. At around the depth of the Galápagos vents, the pressure is 250 atmospheres, or more than 3,600 pounds per square inch. Bruce Shillito, a biophysicist at the Université Pierre et Marie Curie in Paris, compares that pressure to an elephant sitting on your toenail.

Even in the early days of deep-sea exploration, scientists found themselves puzzling over the ways in which close kin of shallow-water species could thrive at much greater depths. But bringing species up for study in the lab proved difficult. Some animals have gas-filled swim bladders that explode on the way to the surface, killing the animal. Animals that lack swim bladders and make it to the surface alive often don’t live long because of the huge change in pressure. So scientists had to busy themselves with tissue samples taken from creatures pulled from the water.

Some of the first clues to high-pressure adaptations came in the late 1970s. Two biochemists at the Scripps Institution of Oceanography in La Jolla, Calif., compared a common protein in shallow- and deep-water thornyhead fishes, a group related to the rockfishes found off the west coast of North America. Joseph Siebenaller, now at Louisiana State University in Baton Rouge, and George Somero of Stanford University discovered a key difference in an enzyme known as lactate dehydrogenase, or LDH, which plays a key role in the generation of energy needed to power swimming movement.

Their studies showed that LDH of the shallow-water relatives performed well only at the low pressures associated with depths less than about 500 meters. In contrast, the slower-working LDH of a deep-living fish continued to work well at the elevated pressures of deeper environments. Further studies found that the LDH in pairs of species across four different families exhibited similar pressure dependence.

Siebenaller also teamed up with biologist Paul Yancey of Whitman College in Walla Walla, Wash., to find another potential way in which deep-sea animals adapt: through small molecules called piezolytes that prevent pressure from distorting proteins. Yancey was studying how one such molecule called trimethylamine oxide, or TMAO, helps stabilize shark proteins against the high concentrations of urea stored in shark tissues. He figured if TMAO could help shark proteins perform with urea, it might help with pressure too.

When added to tissue samples of fish, TMAO stabilizes the proteins against the effects of high pressure, allowing them to fold into their proper three-dimensional shapes.

TMAO is also found in bony fish, crabs and shrimps. “It’s what gives fish and shrimp their fishy smell,” Yancey says.

Since these discoveries, scientists have found other molecules that act as protein stabilizers in deep-sea bacteria and animals. Yancey and his group have discovered an as-yet-unidentified stabilizing molecule in clams. And a pressure-related molecule found in sea cucumbers is now being tested to see if it can stabilize the proteins and perhaps prevent the malformed amyloid-beta clumps associated with Alzheimer’s disease.

Still, questions remain about the details of protein stabilizers: Do they protect all deep-living animals or just some? And can they protect animals living in the deepest regions of the seafloor?

Traveling down

Luckily for scientists, new and improved submersibles are granting easier access to the ocean floor. Yancey has recently linked up with an international team to study tissues of animals pulled from the deepest canyons on Earth. Early next year, the group will explore the Kermadec Trench, an area located off the northeastern tip of New Zealand where parts of the seafloor dip more than 10 kilometers below the sea’s surface. To reach the ocean’s bottom, researchers will use an unmanned, remotely operated vehicle called Nereus, developed at Woods Hole Oceanographic Institution.

Nereus is one of the first submarines capable of operating at such crushing pressure, up to 1,000 times the pressure found at the surface, says Andrew Bowen, project manager and principal developer of Nereus at Woods Hole. Weighing nearly three metric tons and measuring more than 4 meters long, the vehicle can operate as a free-swimming robot to survey areas along the ocean floor, or work as a tethered vehicle for close-up views. A hydraulically operated, robotic arm can collect biological samples.

From these depths, Yancey says, he hopes to pull up specimens such as sea cucumbers and arthropods. Once the animals are back in his lab, he will compare their proteins with those found in shallow-water relatives and look at which stabilizing molecules the deep relatives produce and how much.

“We want to see if, down in the trenches, the animals are making a lot more of these protein-stabilizing molecules, such as TMAO,” Yancey says. Such studies could confirm that these molecules help animals tolerate bone-crushing pressures.

Pulling up

While tissue studies have offered a lot of starting information, they don’t provide a firm basis for talking about the effects of pressure for a whole organism, Somero says.

“When you do work at the biochemical level, you think you’re developing a story that applies to the whole living organism, but you often don’t really know that,” he says.

Scientists have been trying for decades to keep animals alive on their journey to the surface. Researchers have brought the creatures up slowly, hoping they will acclimate to the surface-level pressures on the way. Others have tried pulling animals to the surface quickly, then tossing them in a small pressure tank to bring them back to higher pressures. Animals that survived this trauma are placed in decompression chambers (similar to those used by divers) to see if they can adjust to shallow-water pressures for long-term study.

But such processes take a physiological toll on the animals. Somero says that over the years, only a few deep-sea organisms have been kept alive at the surface for more than a few days.

Recently, scientists have found ways to catch, recover and keep marine animals healthy at their natural pressures. Working with Gérard Hamel, an engineer at the Université Pierre et Marie Curie, Shillito developed a chamber to capture deep-sea creatures while keeping them under their natural pressure the whole way up. In the summer of 2008, the pair made headlines when they retrieved a live deep-sea fish from a record depth — more than two kilometers below the surface on vents in the Mid-Atlantic Ridge.

Now the group is testing ways to transfer its catches from the sampling device into a pressurized tank and take them to the lab, with no decompression in the process. Once in the lab, the animals are placed in pressure chambers designed to re-create the home environment. Video cameras, and in some cases a small viewing window, allow scientists to observe the critters.

Such pressurized chambers are being used in a handful of laboratories to keep deep-sea shrimps and crabs alive for long-term studies. At his Southampton lab, Thatje uses one such high-pressure chamber to compare the physiological links between shallow-water and deep-sea animals. He hopes to help answer long-lingering questions about the origins of current deep-sea species.

Scientists believe that the biodiversity seen in the deep sea today evolved from shallow-water creatures that penetrated the depths after major extinction events. Studies suggest that during the warm Mesozoic, 251 million to 65 million years ago, tropical and subtropical conditions extended poleward. Dinosaurs thrived during this time, but life for deep-sea creatures was much different. As waters in the sea stagnated, oxygen levels dropped and life in the deep largely perished. With the onset of glaciation, deepwater circulation was reestablished and the ocean was recolonized by shallow-water species.

Over time, shallow-water animals migrating to the deep would have adapted to the higher pressures. To see how shallow-living animals tolerate deep-sea pressures, Thatje, Shillito and others are subjecting the Atlantic ditch shrimp, native to Western Europe, to pressures similar to those experienced by deep-water cousins living near hydrothermal vents. While applying various pressures and temperatures, the scientists measure the ditch shrimp’s oxygen consumption and behavior.

Findings from the studies, published last year in the Journal of Experimental Biology and presented at the AAAS meeting in Vancouver, show that the hardy crustaceans survive extreme pressures for periods spanning a few days or weeks.

But survival may depend on the temperature of the water. When tested at temperatures between about 10° and 30° Celsius (50° and 86° Fahrenheit) — similar to those in shallow-water habitats — the shrimps tolerated pressures far beyond what they would normally experience. At even lower temperatures, the shrimps became uncoordinated as pressure increased, and they died within hours.
More recently, Thatje’s team studied a group of ditch shrimp living in the pressurized tank for a month. Now, the researchers would like to keep the shrimps under deep-sea–like pressure for a year, to see if the animals can maintain their normal routine on time spans lasting longer than one animal’s life.

“We want to see whether the entire life cycle of growth, reproduction and hatching of larvae is really possible at conditions that species never encounter in nature today,” he says.

During the experiments, scientists will also collect tissue samples from the shrimps to look for any molecular changes that occur. A few mutations in certain proteins, for example, may turn a normal enzyme into a pressure-resistant one in a relatively short period of time. Such a finding, Thatje says, could reveal new insights into evolution itself.

“Evolutionary processes, by definition, are always thought to be long-term and slow, often taking place over tens of millions of years,” he says. “Yet, we know from studies on insects that evolution can be spontaneous. The question is, how much of this holds true for depth-related changes.”

Even if shallow-water shrimps can acclimate to the pressure of the dark depths, there’s little chance that they’ll be heading for the seabed soon. Last year, Thatje and a colleague at Southampton used a deep-sea pressure tank to show that shrimp­like animals called amphipods respond to extreme pressures better at high temperatures, such as those near the thermal vents where the amphipods normally live. The findings, published last December in PLoS ONE, suggest that temperature, pressure and other factors may work together in complex ways to constrain the habitats animals can call home.

Still, some marine animals may be able to make small shifts in their environment due to changing circumstances, such as climate change, says Thatje. The potential for bigger shifts remains unclear.

Despite the recent advances, scientists say that they have only scratched the surface of deep-sea research. Pushing into new realms of the sea will probably yield more clues to how creatures may cope with a changing world. As scientists look deeper, one thing is clear: There’s no escaping the pressure.

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