Sailing the Sea of Life

For centuries the Sargasso was seen as a desert drifting in an ocean. Now scientists are rediscovering it as a nursery of biodiversity

By Jack McClintock, Jenny Gage, and Tom Betterton
Mar 1, 2002 6:00 AMJul 19, 2023 4:03 PM


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September 17, 1492. The crew was restive, their 94-foot caravel Niña becalmed in strange waters. Christopher Columbus, perhaps the first person to describe this place, wrote in his log that the sailors "saw much weed and very often, and it was vegetation from rocks and it came from a westerly direction; they judged themselves to be near land." But the crew's soundings touched no bottom; they were far from shore. We now know the weed as sargassum. It lives nearly all its life in the Sargasso Sea.

A week later the Niña, the Pinta, and the Santa Maria were still drifting aimlessly. Columbus wrote: "Since the sea had been calm and smooth the men complained, saying that since in that region there were no rough seas, it would never blow for a return to Spain. But later the sea rose high and without wind, which astonished them. . . . "

Sunlight pours down through golden fronds of sargassum, the brown algae that gives the Sargasso Sea its name. "It's a mysterious weed," says Fred Lipshultz, senior scientist at the Bermuda Biological Station for Research.

These were seasoned sailors, not easily astonished, trying to get used to a sea unlike any other—clear and calm, with an eerie beauty, but empty and dead. Fishing produced little to eat; the only life seemed to be sea turtles, an occasional whale, and the Sargasso weed itself. They began to see it as nothing more than a desert. In fact, this sea generates little life beyond single-celled diatoms and tiny dinoflagellates. Its water is less than a third as fertile as coastal seawater. Heart-stoppingly blue and a half-mile deep, the 2-million-square-mile body of gin-clear water floats in the middle of a colder, deeper North Atlantic ocean. Across the surface pass great mats of golden sargassum—from a Portuguese word for little grapes, referring to the tiny, air-filled balloons that keep the weed, a form of brown algae, afloat.

For centuries the Sargasso's mysteries have obsessed mythmakers and scientists alike. Its remoteness, its unearthly blue, its slow-moving air and water, and its often thick beds of seaweed gave rise to legends: a place where the weed itself snares ships in its barnacle-encrusted tentacles, holding fast until nothing remains but a rotting hulk with a skeleton crew. Turn-of-the-century paintings depicted steam-driven freighters, ancient Roman triremes, Spanish galleons, and clipper ships trapped in the sea, all draped and shrouded in Sargasso weed. It is only in the past few decades that this strange body of water has yielded its mythology to actual research.

Oceanographers, biochemists, meteorologists, and other scientists from around the world regularly travel to the Bermuda Biological Station for Research in order to study the Sargasso in both its vastness and its microscopic details. The island of Bermuda, perched atop a coral-encrusted volcanic seamount in the northwest quadrant of the Sargasso, about 650 miles east of Cape Hatteras, is the perfect base from which to launch Sargasso investigations. The research vessel Weatherbird II, crowded with scientists, technicians, and their gear, leaves the station and puts to sea at least every two weeks, steaming southeast. There researchers are finding that what had long seemed a dead zone is full of life and serves as a haven for many creatures, from sea turtles to eels. Less a desert than a rain forest, it may, like landlocked rain forests, contain precious biochemical keys to fighting human disease.

Just a few decades ago, a sailor crossing the Sargasso would have passed immense windrows of floating sargassum, sometimes visible from horizon to horizon. But for the past dozen years, it's been surprisingly scarce. Scientists assumed that the disappearance was part of the weed's life cycle. It had waned before and had always come back. Sure enough, two years ago, it began to thicken once again. Few researchers have studied why it waxes and wanes—perhaps because of the difficulty of designing an experiment over so great a time and space. "Its abundance fluctuates dramatically, and we don't know why," says Fred Lipshultz, senior scientist and head of academic affairs at the Bermuda research station.

Every year, some 20 sick or stranded sea turtles are brought to the Bermuda Aquarium for rest and rehabilitation and then returned to the Sargasso. Green sea turtles, like the aquarium resident above, can grow to a length of more than three feet and weigh more than 300 pounds.

Since 1939, when Albert Parr, an oceanographer at Yale University, completed a landmark study of the Sargasso, scientists have believed that the weed grew in the sea itself. But Brian LaPointe, an algal physiologist at the Harbor Branch Oceanographic Institution in Fort Pierce, Florida, believes that the two nearly identical species found in the sea, Sargassum natans and S. fluitans, evolved from similar species that live on rocks near shore. He has proved that most of the sargassum growth takes place in coastal waters. "I think they evolved from an attached species that was torn off and grew," LaPointe says. He found that "the weed was getting enriched in areas where warm, nutrient-rich coastal water meets cooler ocean water. There's lots of it there, and lots of fish that excrete the nutrients ammonia and phosphate. Fish are key, and they're symbiotic: They help provide nutrients, and the sargassum provides the habitat. Most of its growth occurs near shore."

Sargassum and its relatives drift all over the world and find their way into many local cultures. Asians, for example, have used seaweeds similar to sargassum in traditional medicine—and recent studies show there's more to the practice than ancient habit. In the United States, the National Cancer Institute, examining thousands of plant extracts in the search for anti-cancer and anti-HIV properties, has found polysaccharide tannins in many seaweeds, which are promising for their ability to stimulate the immune system. At Gifu University in Gifu, Japan, other researchers have extracted polysaccharides from S. thunbergii. Two of them showed antitumor activity, mostly by boosting immune function. Field experiments in the United States have shown that feed supplements made from sargassum improve the immune responses of farm-raised hogs. Scientists are just beginning to understand the potential of sargassum, and while research has not yet produced an actual cancer therapy, they remain hopeful. As LaPointe says, "There's a lot of biochemistry in this highly evolved plant."

Sargassum may also turn out to be useful in processing industrial waste. "We have discovered that sargassum is by far the best natural sorbent for heavy metals," says Bohumil Volesky, a chemical engineer at McGill University in Montreal. "It functions as a natural ion exchanger, and its dead biomass can accumulate large quantities of otherwise toxic heavy metals: lead, cadmium, copper, chromium, zinc, uranium, et cetera." Volesky has deciphered the mechanism that drives this accumulation. Sargassum contains calcium ions. As heavy metals in solution flow over it, toxic metal ions kick the calcium ions out and remain behind. When the seaweed is saturated with metal, the seaweed can be washed with a mild acid and reused up to 80 times. The highly concentrated extract should be rich in valuable metals. Volesky calls the process electrowinning.

At least 99 percent of the world's microbes have never been cultured or cataloged. Yet from that other 1 percent, says Hank Trapido-Rosenthal, a molecular biologist at the station, humans have already domesticated "a vast array of compounds of immense medical and economic value" such as penicillin. That leaves an untapped reservoir of genetic diversity with enormous potential, and Trapido-Rosenthal, along with his colleague biotechnologist Sandra Zielke, is exploring the Sargasso in search of bioactive molecules that may be useful in medicine or industry. They chose the Sargasso as their point of collection "not because it's particularly high in microbes," Trapido-Rosenthal says, "but because it's the best-studied piece of ocean in the world. We know more about microbes here than elsewhere." In the search for microbes, Zielke collects seawater, coral, soils, and most important, sponges for DNA analysis. She needs to process 200 quarts of seawater to get enough DNA to measure, but a sponge "filters 10,000 quarts per hour, and all those bacteria will settle down in it."

A magnified image reveals the air bladders that keep sargassum afloat. The feathery attachments are hydroids, tiny marine animals that catch a ride on the weed.

The early results of their search are promising. Two years ago, working with a German firm, Trapido-Rosenthal and Zielke found a compound that lowers fibrinogen in the blood, a clotting factor necessary to prevent bleeding during surgery that can also cause dangerous clots afterward. They found an insulin-sensitivity agent that might be useful in the treatment of diabetes. Zielke says that the work is still primitive: "We don't know much about these organisms. You'll see a sponge covering a rock and nothing else living on it, and it makes you assume they cannibalize a substance that has some antibiotic effect on its surroundings. The fish aren't eating it, so it produces something for protection. But you don't know if it was produced by the sponge, or by microorganisms living on the sponge, or if they interact in combinations. It's so tricky to get something out of marine stuff, but as soon as we find the trick, it's going to be exciting."

Related studies conducted by Craig Carlson and Rachel Parsons, microbial ecologists at the station's microbial observatory, are measuring the amounts of bacteria and, recently, viruses in the open ocean. "It'd be nice to know if there's anything dangerous out there," says Parsons, noting that none of the viruses found so far is pathogenic to humans. Researchers are beginning to see how viruses play a critical role in the structure of the oceanic food chain and the carbon cycle. "Say you get an algal bloom. You're going to get a bacterial bloom and a viral bloom too. They can help to ameliorate the red-tide threat." Sargasso nutrient levels are so low that in the top few yards of the water column, the virus level is as low as one-twentieth that of the Chesapeake Bay. But existence of the viruses is further evidence that the Sargasso is far more than a desert.

The European eel, Anguilla anguilla, has long been familiar to seafood-loving continentals both as a delicacy and a curiosity of natural history. Since the 1920s, scientists had believed that the eels, born in the Sargasso, journeyed back there as adults and spawned in a great undiscriminating free-for-all with no regard for the home territory of their partners. Genetic data seemed to support this view since it showed no strong differences between eel populations from different regions—an eel was an eel was an eel.

The research vessel Weatherbird II weighs anchor in the remarkably blue waters of the Sargasso. The less algae and other suspended particles in a body of water, the bluer the water will appear to be.

As DNA science progressed, more sensitive analysis revealed that science itself had been undiscriminating, not the eels. Researchers have discovered that the eels are born in the Sargasso, then ride the Gulf Stream across the Atlantic to coastal European waters, where they grow for two years. Then they swim up a river—possibly the same river their parents inhabited—and dwell there for 10 to 15 years before migrating back to the Atlantic and out to the Sargasso, where they spawn and die. When biologists Thierry Wirth and Louis Bernatchez of Laval University in Quebec subjected 611 eels to refined DNA testing last year, they found the anguilliforms were more likely to mate with eels from the same geographic region.

How do they recognize each other? Perhaps they don't. Wirth and Bernatchez concluded that timing may be the best explanation. Different eel populations appear to have genetic clocks that trigger migration at different times. If so, eels from the same river would arrive back at the Sargasso at the same time. Consequently, when it comes time to reproduce, they mate with eels from the same area. But like many answers to old questions, this one deepens the mystery. How do young eels find their way from the Sargasso wilderness to their parents' native waters, a place they've never been before?

Creatures of the open sea have always been remote, difficult to study, and even hard to imagine. Now some are being lost before they can be studied: All seven species of sea turtles, denizens of the Sargasso, are at risk. Columbus, during his fourth voyage, in 1503, was astonished to find them covering the beaches of the Cayman Islands, where they made their nests and laid their eggs. Today scientists estimate about 6 percent of that turtle population survives.

Loggerheads, the most-studied and best-known sea turtles, nest on beaches from the Carolinas to the Caribbean. Their eggs hatch between July and October, and the hatchlings crawl into the sea. The plucky and lucky ones dodge hungry crabs and seabirds, then disappear and aren't seen again until they return to the same beaches to lay their own eggs.

Turtle specialist Archie Carr declared this hiatus the sea turtles' "lost year" and speculated in 1987, shortly before he died, that the turtles spent it in the Sargasso. Over the last decade, zoologist Karen Bjorndal, director of the University of Florida's Archie Carr Center for Sea Turtle Research, proved Carr's central thesis but found that a year was not long enough. Using DNA tracking, her team discovered where the hatchlings go once they leave the nesting beach. Loggerheads from different beaches display unique mitochondrial DNA patterns, just like the eels from different European rivers. Bjorndal and her colleague marine ecologist Alan Bolten were able to show that the tiny, silver-dollar-size hatchlings migrate thousands of miles from their home beaches to feeding grounds in the Atlantic and the Mediterranean, where they spend not just one year but as many as 11 years feeding on jellyfish, snails, crabs, and shrimps. Their DNA evidence proved that long-line commercial fishermen after swordfish in the eastern Atlantic routinely catch sea turtles born in U.S. waters and protected by the Endangered Species Act. Bolten is working to help fishermen modify their methods to reduce turtle snagging.

Sargassum shelters and feeds the animal life in the Sargasso, such as larval fish, sea turtles, shrimps, and crabs. In turn, the animals' waste products feed the weed.

Jennifer Gray, head aquarist and turtle-project coordinator at the Bermuda Aquarium, Museum and Zoo, has tagged and tracked 2,500 green sea turtles. She says: "The Sargasso Sea is very important, just a crucial developmental habitat for these young animals that take 50 years to mature. Obviously, they have a close tie to sargassum. It's breakfast, lunch, dinner, and home to them." She worries that "any threat to sargassum would be a threat to sea turtle populations." Nonetheless, she believes that they may be safer in the Sargasso than anywhere else. "When they're young and vulnerable, the turtles leave the nutrient-rich areas because their predators are there," says Gray. They take shelter in the Sargasso. "Then [years later] something happens. It's time to go back to a coastal environment—maybe they reach some critical size and need more food."

Once they leave the Sargasso, they're in greater danger, under attack by humans. Fifty tags from the 2,500 green sea turtles Gray and her colleagues had monitored since 1968 have been returned under circumstances suggesting that at least 48 of the turtles were killed for food.

Seventy miles south of Bermuda, aboard the research vessel Weatherbird II, a winch lowers specimen-collecting bottles into the water. They can be seen for hundreds of feet as the cable unreels into the deep blue sea to record temperature, salinity, and other data. On deck a seaman reaches down and scoops a hand-size clump of sargassum from the sea. A visitor takes it below to the galley and studies it. It has a briny smell, a shiny khaki color, a structure of fronds like narrow blades of grass ornamented with tiny seedlike balloons only an eighth of an inch in diameter. Even tinier white barnacles grow on the fronds. There is lots of life here.

When the visitor shakes the golden weed over a white paper plate, a tiny crab, three-eighths of an inch wide, drops out. It is khaki colored as well and has two white dots on its back, like eyes looking up curiously. The crab scuttles a few steps and freezes. "We see amazing things," says Tony Knap, the station's director, with the eager note in his voice that denotes an explorer. "But we're still pretty ignorant about the Sargasso Sea."

A Sea Within an Ocean

The Sargasso Sea is a 2-million-square-mile oval lens of water a half-mile deep that rests in the colder reaches of the Atlantic Ocean. It is defined and contained by currents that swirl around it in a clockwise gyre: the Gulf Stream on the west, the North Atlantic Current on the north, the Canary Current on the east, and the North Equatorial Current on the south. Over the last 47 years, researchers have visited the Sargasso regularly to gather data for what is now the world's longest continuously maintained ocean database—studying not just the behavior of the currents but also the temperature, conductivity, depth, and salinity of the sea and the ocean.

Today they are using that information to understand the ocean's role in global warming, according to Michael Lomas, a phytoplankton ecologist who runs the program. Carbon is key, because carbon dioxide, given off by burning fossil fuels, is an important global-warming gas, trapping sunlight and overheating the atmosphere. Trees and shrubs use up a large portion, while photosynthetic plants in the sea—phytoplankton living in or near the sunlit surface—snare a good deal of what's left. They fix the carbon, removing it from circulation. When the plants die, they drop to the seafloor with the carbon. With increasing levels of CO2 as a result of industrialization, neither plants on land nor in the sea are able to absorb all the carbon. As a result, carbon dioxide in the air has been ramping up rapidly over the past 40 years; data from the Sargasso study show that CO2 levels in the ocean are also increasing, at about one-third the atmospheric rate. The ocean is "a long-term sink for carbon," Lomas says. "And if there's no carbon sink—if we don't protect the ocean—it's going to get warmer." — J. M.

Sargassum shelters and feeds the animal life in the Sargasso, such as larval fish, sea turtles, shrimps, and crabs. In turn, the animals' waste products feed the weed.Illustration by Matt ZangInformation for Illustration courtesy of Rod Johnson/Bermuda Biological Station for Research

Sargasso Sunscreen

Pouring its energy into the sea from dawn to dusk, the sun is the Sargasso's source of life—but it can be a source of death for humans in the form of the radiation that causes melanoma. Now researchers have found that protection against this cancer may be floating quietly in that sea. Hydroids are small creatures related to sea anemones that live attached to sargassum, and filefish love to eat them—with the exception of one species, Tridentata marginata, which manufactures a class of smelly, bad-tasting compounds that operate as an ideal chemical defense against these and other predators.

Marine ecologists Niels Lindquist, Mark Hay, and graduate student John Stachowicz from the University of North Carolina at Chapel Hill identified these chemical compounds while studying predator-prey relations in the Sargasso. An instrument used to isolate the compounds recorded that they absorbed ultraviolet-A and ultraviolet-B radiation. By accident the research team had discovered not just a feeding deterrent but also a natural screen that protects the hydroid against the constant bombardment of the sun. Most sunscreen products on the market protect humans against UV-B rays but give only partial protection against UV-A rays, which are believed to increase the risk of melanoma. — J. M.

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