Researchers cruising the South Pacific between Tonga and Fiji study huge snails that, aided by an abundance of bacteria housed in their gills, feed off plumes of metal-rich compounds at active hydrothermal vents. Scientists working off the California coast use chemical-sniffing probes, robotically driven subs, and seafloor-tethered temperature sensors to watch flows of lava pave over a once-thriving ecosystem at hydrothermal vents several kilometers below the ocean’s surface. And in waters off Papua New Guinea, a mining company analyzes metal deposits around inactive, underwater volcanoes that contain, on average, 10 times as much copper as typical ores on land do.
These studies exemplify the breadth of research under way at one of Earth’s last great frontiers, the geologically active ocean bottom. Sites include hydrothermal vents on the 65,000-km-long ridge that meanders through all the world’s oceans and at deeply submerged volcanoes in the tropical west Pacific. Only 35 years ago, scientists didn’t know that geologically active sites existed underwater. Now, they have direct evidence of some 300 such spots and suspect that another 700 or so await discovery. The researchers are also uncovering signs of past geological activity at many sites.
As recently as 1990, the cost and difficulty of getting to the few then-recognized sites limited visits by scientists. Today, however, submerged volcanoes and other deep hydrothermal vents have become stages for intensive research activity. Every visit to a vent site, even a repeat visit, brings new discoveries, says Cindy Van Dover, director of Duke University’s Marine Laboratory in Beaufort, N.C.
“Some places are so popular that you get ships [above them] stacked up on top of one another,” notes Van Dover. If researchers aren’t careful, she says, they’ll interfere with each other’s work or harm the vent ecosystems.
Such concerns triggered scientists to issue, earlier this year, a code of conduct for hydrothermal-vent research. In July, underwater volcanologist Colin Devey of the University of Kiel in Germany described the code at the Euroscience Open Forum in Munich. His university is the current home of InterRidge, the loosely affiliated group of hydrothermal-vent researchers that developed the code.
Explains Devey, “On a highway with only two vehicles, having rules of the road is fairly unimportant. But every time another vehicle comes along, it becomes increasingly important to know such things as which side of the road to drive on and who has the right of way.” With scientific and mining-exploration visits to vent sites increasing, he says, “we realized we needed rules to deal with that traffic.”
The new code also serves as a self-policing protocol for investigators who conduct their research largely out of sight, usually kilometers below the sea’s surface.
Hot spots
The geologically active sites where these scientists work represent two extremes of the global tectonic system. The best characterized of these hydrothermal vents occur at various intervals along a tectonic plate’s new-forming edge, known as a spreading zone. Earth’s mid-ocean ridge is one near-continuous zone of spreading seafloor.
As two plates pull apart, breaches occasionally permit magma to break through the ridge’s crest, where the molten rock heats zones of rock and water (SN: 4/1/06, p. 202: Uncharted Territory). A spreading zone can be considered a “linear volcano” with vent holes occurring at various points along its meandering crest, explains Edward T. Baker, an oceanographer at the National Oceanic and Atmospheric Administration’s Seattle lab.
Last December, Baker and his colleagues traveled to a previously uncharted 400-km span of the mid-ocean ridge in the Pacific, just north of the Galápagos Islands. Their sophisticated equipment turned up direct evidence of three new hydrothermal vents in that spreading zone and indirect evidence—hydrothermal plumes—of several more.
On the opposite end of a tectonic plate from its spreading zone may be a subduction zone, a span where two plates collide and one plate is forced beneath the other. Weak spots emerge near the edge of the disappearing plate, permitting magma to punch through and form conventional, conical volcanoes that happen to be underwater. These vulnerable areas are known as back-arc basins (SN: 6/10/06, p. 365: Available to subscribers at Deep-sea action).
Both types of hydrothermal vents—at seafloor ridges and at back arcs—spew hot water that’s oxygen-deprived and metal-rich, and they host a wide variety of exotic marine life. Researchers marvel at the diversity of unusual animals populating these dark and seemingly inhospitable zones, some of the organisms surviving near water as hot as molten lead. Because animals at these sites derive their energy solely from the microbial transformation of chemicals spewed by the geological activity, their ecosystems are referred to as chemosynthetic.
In the decade after the first hydrothermal vents were discovered in 1977, notes Baker, the high costs of visiting such areas limited scientists’ choices. Researchers tended to go to ridge sites where they were virtually guaranteed to find vents, so they focused on similar sites and found similar things.
Expeditions gradually began taking more gambles, and over the past decade, they’ve explored a greater variety of locales, including sites in the Arctic Ocean (SN: 4/1/06, p. 202: Uncharted Territory). “What we’ve found is that there’s never been an occasion where we didn’t find evidence of venting within any 200-km section of [the] mid-ocean ridge,” notes Christopher R. German of the Woods Hole (Mass.) Oceanographic Institution.
Vents often differ dramatically from each other in chemical and geological characteristics. However, even vents that are physically similar can host distinctly different communities of animals.
“The classic example” of a vent creature, German notes, “is those giant tube worms, perhaps 8 feet [2.4 meters] long. But you don’t find those outside the Pacific.” First identified on the Galápagos section of the ridge, these worms don’t inhabit the northeastern Pacific. There, smaller tube worms congregate.
Go to the North Atlantic, and the tube worms are replaced by vent communities dominated by shrimp a couple inches long and mussels, German says. Move to vents in the Indian Ocean for an intermediate fauna. Atlantic-type shrimp are common there, as are barnacles, snails, and a host of animals typically found at West Pacific vents.
As part of a 10-year international census of marine life, German and others have begun mapping what they call the “biogeography of chemosynthetic ecosystems.”
Light-touch science
Fifteen or so years ago, studying hydrothermal-vent denizens was crude: “To collect something, people used to simply drag dredges from ships through vent fields,” Baker says. Over time, researchers have become gentler and more selective. Today, they first observe animals from submersibles or in pictures taken by computer-operated, torpedolike devices armed with sophisticated sensors and cameras. Biologists then perform “precision sampling” of a few critters of special interest, Van Dover says.
Using this process, scientists are finding 100 new species each year, Van Dover estimates.
The new code of conduct makes an explicit commitment to such minimally invasive sampling. The InterRidge document, endorsed by organizations representing 2,000 vent researchers, pledges to avoid engaging in research activities that “will have deleterious impacts on the sustainability” of vent populations, lead to long or significant alteration—even in just appearance—of vent sites, or take vent materials “not essential” to research.
The code also prohibits transplanting materials, such as tube worms and other animals, between vents. The researchers want to avoid introducing species that might establish themselves and alter a native vent ecosystem, German explains.
A year ago, Van Dover’s experience during a biogeography study reinforced concerns over such transplants. She and her colleagues had collected vent shellfish at back-arc vents in the Fiji Basin. At least 60 percent of the mussels (Bathymodiolus brevior) there had curious brown or black spots. A few days later, the researchers sampled mussels at back-arc vents in the nearby Lau Basin.
Once home, Van Dover examined the flesh of the Fiji Basin mussels more closely and discovered that their spots were caused by a fungal infection. In some specimens, she says, “there was more fungus than mussel.”
Because her team didn’t wash out the collection boxes between sites, Van Dover worries that her team may have transferred fungus spores from infected mussels of the Fuji Basin to healthy shellfish in the Lau Basin.
“I won’t go back out again without carrying bottles of bleach to do washdowns,” Van Dover says.
Species can spread via other means. Van Dover notes that the submersibles used to survey vent sites contain ballast water that’s jettisoned as the vehicles surface. Animals or microbes can be sucked in with the ballast water at one site and released elsewhere.
Antje Boetius of the Max Planck Institute for Marine Microbiology in Bremen, Germany, finds less reason for concern about transplanting fungi or other microbes. Because all oceans are connected, she notes, “an educated guess would be that we cannot introduce microbes … to new sites because they’re already there.” Nevertheless, she adds, “our code of conduct will be so strict that, in theory, it would prevent even this—transporting microbes from one place to another.”
Commercial appeal
The InterRidge code was drafted and adopted by scientists conducting basic research. However, vent sites also receive attention from investigators engaged in applied research for commercial firms, principally companies interested in mining the copper, gold, silver, zinc, and lead that bubble up at vents and then lie exposed on the ocean floor.
“I don’t expect the mining community is terribly aware of the code of conduct for scientists,” says Toronto-based consulting geologist Steven D. Scott. However, he notes that 5 years ago, the International Marine Minerals Society, based in Honolulu, adopted its own code for environmental management of marine mining.
That code calls for mining companies to “apply best practical procedures” for environmental and resource protection, “consider environmental implications” through all stages of mining starting with exploration, return unused extracted materials to the seabed “in a manner that will facilitate future sustainable use of the area,” and conduct environmental studies as a basis for evaluating risks and responsible postproduction-cleanup strategies.
In other words, Scott argues, the mining community is also trying to exhibit responsible environmental stewardship of vent resources. At the Euroscience Open Forum, he conceded that ocean mining has the potential to adversely affect vent ecosystems. However, he says, “it should present far less of a problem than mining on land.”
With vent water carrying metals directly from Earth’s mantle, 2 million tons of ore from ocean sites should yield as much copper as 80 million tons of material mined on land, according to his company’s recent study in waters off Papua New Guinea, reports David J. Heydon, chief executive officer of Nautilus Minerals in Vancouver, British Columbia. Nautilus Minerals is one of two companies formed so far to mine ocean-bottom minerals.
Mining companies are focusing on extinct-vent sites to avoid the challenges of superheated water and molten rock. Ecosystems at these thermally extinct vents may be less novel than those dominated by the short-lived, quick-growing animals at active vents (SN: 7/14/01, p. 21: New type of hydrothermal vent looms large).
On the other hand, Van Dover points out, there’s evidence suggesting that tube worms have lived for 100 years at some cold sites. Ecologically, she says, killing them during mining might be equivalent to cutting down an old-growth forest. Indeed, she argues, animals at these sites “may turn out to be more sensitive than those at [still-hot] vents. We don’t yet know.”
Cradles of life?
Vent sites are unstable. Every 10 to 100 years, a new flow of lava kills off life and repaves the surrounding ocean bottom. Yet only days later, animals begin recolonizing a vent. That finding, which is less than 2 decades old, has established the “simple but profound truth,” Baker says, that “volcanoes and the presence of water are sufficient [to support] life.”
Direct ancestors of vent bacteria “may extend back 4 billion years,” he notes. Studying current generations of these microbes may therefore offer clues to life’s origins (SN: 1/9/99, p. 24: https://www.sciencenews.org/pages/sn_arc99/1_9_99/bob1.htm) and the possibility that geologic activity has similarly fostered life elsewhere “in and beyond our own solar system,” Baker says.
Although large fauna such as tube worms (SN: 4/15/06, p. 228: Into Hot Water: Lab test shows that worms seek heat) and hairy blond crabs (SN: 4/1/06, p. 205: Available to subscribers at Hairy crab lounges deep in the Pacific) garner most of the public attention given to deep-sea vents, Boetius points out that the biggest share of undiscovered species there are microbial. As part of the International Census of Marine Microbes, under way through 2010, she hopes to resolve whether bacteria at vent sites are novel or just the same marine species—perhaps in different proportions—that occur elsewhere in the ocean.
However, some scientists challenge the ethics of conducting these and other studies, charging that they all pose risks to life at vent sites.
For instance, in a Jan. 13, 2005, letter to Nature, former vent scientist Magnus Johnson of the University of Hull in Scarborough, England, cited a 1977 report that vent shrimp were probably blinded by floodlights on exploratory submarines (SN: 4/3/99, p. 219). “The Worldwide Fund for Nature has recognized that one of the greatest threats to hydrothermal vents comes from ‘uncoordinated and unregulated’ research,” Johnson pointed out.
Charles Fisher of Pennsylvania State University in University Park, who has just returned from a South Pacific research cruise, concedes that scientists and their equipment are probably the greatest threats to deep sites—but only because few other people get down there. However, he argues that scientists aren’t inflicting much injury on vent ecosystems. Moreover, he and others note that the new code of conduct is intended to limit even that small potential for researchers’ doing harm.
