Deep Ocean Mini-Reviews

Our planet looks blue (as seen from space) because water absorbs red light while scattering bluer wavelengths. The deeper one goes into the ocean, the more light that is absorbed or scattered. Venturing about 200 meters down to the lower edge of the photic zone, less than one percent of the sunlight hitting the surface remains. Enough sunlight penetrates into the photic zone to sustain photosynthesis, but below this surface layer, it is simply too dark and primary energy production shuts down.

This is roughly where the "deep ocean"—containing about 95 percent of the total water by volume—begins. The word "deep" is appropriate, since average ocean depths are about two-and-a-half miles. The deepest spots yet measured extend downward almost seven miles. This realm constitutes the Earth's largest continuous habitat, as well as one of the planet's last great frontiers. "Less than five percent of the ocean has been seen even once, let alone explored or understood," writes the oceanographer Sylvia Earle.

We have mapped out the surface of the moon, Mars, and Venus in far greater detail than we have surveyed the ocean floor. We can point our telescopes at other planets and take pictures, waiting, if necessary, for a cloudless night to do so. But the deep ocean is opaque, and no matter how long we wait, we'll never get clear, unobstructed views from the surface.

Fortunately, new tools have become available in recent decades to observe phenomena previously beyond reach. These include manned submersibles, robotic vehicles, subsurface floats, undersea laboratories and seafloor drills. We now know that, far from being mostly devoid of life, the oceans are filled with an astonishing variety of creatures of all sizes and at all depths, even extending into the subsurface.

Despite its enormous size, the ocean is not immune to human insults—pollution, dumping, mining, and overfishing—all of which can have profound effects on marine ecosystems in ways we don't fully appreciate. But the lesson is clear nevertheless: We can no longer take the deep ocean, and its resources, for granted by continuing to act as if out of sight is out of mind.

It is still the case that most of what is known about the deep ocean concerns the seafloor (benthic) environment. Yet even there, "the data needed to evaluate the consequences of biodiversity loss on the seafloor [is almost] completely lacking," a recent study in Current Biology concluded. An international study conducted by marine scientists from Italy, Belgium and the United Kingdom tried to fill some of the knowledge gaps by studying 116 deepwater sites in the Atlantic, Pacific and Southern Oceans, as well as the Mediterranean and Black Seas. The most salient finding to emerge from this global investigation is that deep-sea ecosystems, which are essential for cycling carbon, nitrogen, and phosphorus, are more delicate than their terrestrial counterparts. The loss of a few key marine species can have disproportionate effects, causing profound disruptions that ripple throughout the system as a whole.

The team, headed by Roberto Danovaro of the Polytechnic University of Marche in Ancona, Italy, focused, in particular, on nematodes (roundworms). Nematodes are the most abundant multicellular animals on earth, accounting for about 80 percent of the multicellular creatures found in terrestrial ecosystems and about 90 percent of those found in the benthic zone. Well over 20,000 nematode species have been identified to date. Previous studies have established that nematode diversity correlates strongly with the diversity of other deep-sea fauna. Danovaro and fellow researchers discovered that a higher diversity of nematodes supports exponentially higher rates of ecosystem processes, and a higher efficiency at which these processes are carried out. Conversely, a 25 percent decrease in nematode biodiversity led to a 50 percent decline in such functioning, whereas a 50 percent species loss could result in total ecosystem collapse.

Although biological diversity has long been known to promote ecological stability, on the basis of this study it appears to be especially important in deep-sea environments. Preserving biodiversity in the deep ocean may not only be essential for the health of ecosystems there, but may also be essential for the health of our planet as a whole.

"We humans devalue what we do not know," the Scripps oceanographer Tony Koslow wrote in his book, The Silent Deep. That tendency ties in with the premise of the Census of Marine Life—a massive international effort launched a decade ago, involving more than 2,000 scientists from over 80 countries.

The census, which ended in 2010, set out to answer a simple question that is, nevertheless, of overwhelming importance: "What lives in the oceans?" Part of the idea behind this collaboration—called one of the most important experiments undertaken in the modern era—is that we'll be in a better position to protect marine ecosystems, as well as more motivated to do so, once we have a better understanding of the animals that make up these communities. We also need to establish some baseline data, which is what a census is all about, in order to see how marine populations are changing, for better or worse, due to either natural or human-induced causes.

The scope and breadth of this endeavor has spurred many advances in ocean exploration, particularly in terms of identifying and probing the genetic structure of microbial organisms that constitute about 90 percent of all marine life. One newly developed approach that was put to great use in the census, DNA barcoding, can identify marine species through DNA sequencing in a matter of hours. "It is not an exaggeration to state that over the past decade, the census has played a fundamental role in transforming the study of life in the sea," claims Cornell University, USA, oceanographer Charles Greene.

To date, more than 5,000 new species have been discovered, including tubeworms that burrow into the seafloor in search of oil, crabs covered with ersatz hair, and sponges that secrete chemicals that could potentially be utilized in the fight against cancer. Perhaps the oddest of all are the mats of microbial filaments that sit on the seafloor off the western coast of South America, covering an area the size of Greece. Although these animals may seem strange to us, they are far more representative of life on this planet than Homo sapiens.

That, of course, is only the beginning. Census scientists estimate that for every marine species identified to date, there are at least four more yet to be found. And once a new organism is found, that leaves us with an even more challenging task— figuring out its ecological significance. "The first 10 years has been very much focused on discovery," says Paul Snelgrove, a biological oceanographer at Memorial University in Newfoundland, Canada. "The next step is to ask, what do these species do and how important are they to the way the earth works?" That knowledge, in turn, should guide us in our attempts at safeguarding the ocean environment from an array of human threats.

Since then, hydrothermal vents have been discovered throughout the world's oceans—from Antarctica to the Arctic waters near Greenland. Some of the hottest vents, which emit fluids exceeding 700°F, are called "black smokers", because the fluids contain a dark cloud of particles that are shot out of tall rock formations that resemble chimneys or smokestacks. Scientists now know that vent environments are oases of life, literal hotspots of biodiversity near the ocean bottom. Unlike ecosystems on or near the Earth's surface, which are sustained by sunlight and photosynthesis, these deep ocean ecosystems are sustained by chemical energy sources and chemosynthesis.

Sitting at the base of this unusual food chain are bacteria that oxidize—and thereby draw energy from—the hydrogen sulfide gas and other minerals and organic compounds spewed out of the springs. Other microbes that flourish in this high-temperature realm are part of a distinct and evolutionarily independent branch of the kingdom of Archaea. Scientists now speculate that life on Earth, and perhaps on other worlds, may have originated in warm and stable vent habitats.

However, these vent habitats are not entirely stable. Violent volcanic eruptions can periodically wipe out multicellular organisms within the immediate vicinity. While people had assumed that creatures from nearby vents would repopulate the area, new research, led by Lauren Mullineaux of the Woods Hole Oceanographic Institution, suggests otherwise. Mullineaux found larvae recolonizing one site in the east Pacific that appears to have travelled at least 200 miles to get there. As for how weak-swimming larvae could have traveled such a great distance, Mullineaux and colleagues have identified ocean-bottom jets that might have carried the larvae to their current home. But if the journey lasts longer than the animal's expected lifespan, one has to wonder whether these animals live longer than scientists had previously thought or whether the theory itself is wrong. This, of course, is but one of many mysteries surrounding hydrothermal vents and the strange communities that have sprouted in their midst.

Although Alvin has revolutionized our picture of the undersea realm, it is only designed to withstand pressures to depths of 4,500 meters or 2.8 miles. "Right now, Alvin allows us to see 63 percent of the ocean," says Dan Fornari of the Woods Hole Oceanographic Institution (WHOI), USA, the organization that operates the submersible. "We want to see 99 percent." To reach that goal, the vessel would need to be capable of going deeper than 4 miles. A plan is already in place to overhaul Alvin and replace key components so that it can safely dive to such depths.

Meanwhile, scientists are developing more permanent installations for studying the oceanic realm. In the past, researchers monitoring various marine sites and phenomena had to make do with a passing glance or the occasional peek. They'd get a snapshot view of the ocean once, or if they were lucky twice, but they'd still miss a lot, explains WHOI oceanographer Alan Chave. "We're approaching the limit of what we can do with that. We need to understand processes that occur episodically over long time periods and large areas. We need instruments measuring continually."

More than just a pipedream, this capability is coming to fruition, as various undersea observatories, analogous to astronomical observatories, are being deployed at places of keen scientific interest. In 1998, WHOI built H₂0—the first permanent deep-ocean observatory (more than three miles deep)—midway between California and Hawaii, where it monitors seismic activity, among other tasks. The MARS Observatory began operating in Monterey Bay, CA, a decade later, where one experiment is charting the effects of ocean acidification on deep-sea animals. NEPTUNE, the world's largest ocean observatory, started up in 2009. Located off the coast of Washington State and Canada, NEPTUNE is ideally located to study hydrothermal vent systems, seafloor volcanism, earthquakes, and other phenomenon. By establishing a long-term presence on the seafloor, these and other observatories are providing an unprecedented window into the fathoms, transforming oceanography in the process.

Based on current knowledge, about 30 percent of the creatures inhabiting the water column are "planktonic" drifters possessing translucent, gelatinous bodies that will, most likely, never touch a hard surface. They move silently, incapable of making a sound. Most have no brain, heart, lungs, eyes or protective shell, yet they can sense light, navigate, find prey and defend themselves.

Among the oddest of these so-called "jellies" are siphonophores—wispy, chain-like carnivores that can outcompete bulkier predators such as sharks, squids and whales. Composed of modular pieces with multiple stomachs, tentacles and other body parts, some siphonophores have been measured at 40 meters, making them longer than the blue whale and among the longest animals found anywhere.

With their unusual shapes and iridescent, pulsing bodies, jellies may seem bizarre to humans. Yet these animals hold great ecological significance, even though their exact roles are far from understood. Researchers are finding, for example, that the sinking of various types of jellies transports carbon and other nutrients to the ocean floor in quantities sufficient to affect global climate regulation. Woods Hole biologist Laurence Madin and colleagues discovered that in one patch of the North Atlantic, jelly-like salps consumed almost 75 percent of the carbon-containing plants on the ocean surface, transporting up to 4,000 tons of carbon each day to deeper waters, so that it would not reenter the atmosphere as carbon dioxide, a greenhouse gas. Robison and collaborators have made similar findings regarding deep carbon transport by gelatinous larvaceans in Monterey Bay, while a German-led team noted the efficiency of jellyfish-like Pyrosoma atlanticum in delivering tons of carbon to the seafloor near Africa's Ivory Coast. Collectively, these results draw attention to the underappreciated importance of jellies, while illustrating how much remains to be learned about the midwaters and its strange inhabitants.

While the discovery of bacterial mats was interesting in its own right, the finding had even greater implications—namely that "previously unimagined and potentially huge communities of microbial life" were thriving below the ocean bottom, Wirsen says, "in conditions we had considered too extreme. It shattered our narrow preconceptions and stretched our view of the places and circumstances that can harbor life."

As a result of that horizon-expanding experience, new initiatives have been launched to search for life in the deep biosphere—far below the earth's surface and below the seafloor. The effort to explore the vast subsurface, says Wirsen, "is a quest to find the limits of life." The premise is that the deeper we go, the hotter it gets. Eventually, it is assumed, we will reach a point where life cannot survive because it's too hot. We have found microbes a mile below the seafloor in rocks more than 110 million years old, but we have not yet established any limits to life, nor come across conditions so harsh as to render life impossible.

Interestingly, the microbes found beneath the seafloor tend to be different from those found on the surface. A large fraction of them are from the domain Archaea—a group of single-celled microorganisms that resemble bacteria, but are genetically and metabolically distinct. A 2008 report published in Nature by German and Japanese researchers found, for example, that bacteria predominate in the upper 10 centimeters of seafloor sediments, whereas Archaea take over at levels below that, ultimately constituting about 87 percent of the deep biosphere.

Because of their extremely low metabolism as compared to bacteria—with colonies doubling in numbers over the course of hundreds of years rather than tens of minutes—"Archaea appear to be better adapted to the extreme, chronic deficiency of energy that characterizes this habitat," says Julius Lipp, one of the Nature authors. The abundance of unicellular life in seemingly inhospitable subseafloor sediments is yet another example of life's resilience and its amazing knack for filling the most unlikely niches.

Far from being anomalous outcroppings, seamounts are "among the most ubiquitous landforms on Earth," according to University of Hawaii geologist Paul Wessel and his colleagues, David Sandwell and Seung-Sep Kim. Approximately 13,000 seamounts greater than 1.5 kilometers in height have been identified so far. Wessel, Sandwell, and Kim estimate that more than 100,000 seamounts greater than a kilometer high have yet to be discovered. If seamounts 100 meters or bigger are included in the tally, there may be 25 million of them scattered, somewhat unevenly, among the ocean basins. The surface area of these seamounts, collectively, is larger than South America. That, combined with the abundance and diversity of life forms found at seamounts, makes them, says Peter Etnoyer of Texas A&M, USA, "one of the predominant ecosystems on the planet" —home to lush assemblages of corals and sponges, and fish such as orange roughy, sablefish, and the pelagic armorhead.

Deep-sea corals found at these sites include the oldest animal yet known—a 4,200-year-old black coral discovered near the Cross Seamount in the Pacific Ocean. Two-thousand-year-old gold corals have also been observed in other seamount locations. Because many of these animals grow in layers—similar to the growth rings seen on trees—they can help reveal the climate history in the ocean's past. Etnoyer calls deep-sea corals "flagship species for an ecosystem in need of conservation."

While more than 99 percent of all seamounts remain unexplored, they have not been untouched by overfishing, bottom trawling, mining and other intrusive and destructive activities. Seamounts were once viewed simply as hazards to human-occupied submarines, but there has been a belated realization that they are valuable resources—biological, geological and ecological—that are worthy of protection.

Such protective methods include the banning of bottom fishing at a "bio-observatory" established at the Condor Seamount near the Azores Islands. Scientists involved in that effort call for fishing closures at other seamount sites that would be dedicated, instead, to research, so that information can be gathered about managing these areas in more sustainable ways.

It is difficult to predict how ocean life will fare in the wake of these changes, but the data collected so far is not encouraging. Experiments have shown that elevated acidity levels reduced fertilization success among Australian sea urchins, led to abnormal development in temperate brittlestar larvae and caused the death of copepods commonly found off the California coast.

The consequences of increased ocean acidification are of special relevance for coral reefs and shell-bearing creatures. Coral and protective shells are both made out of calcium carbonate—a compound that degrades under acidic conditions. Although higher acidity is expected to have a negative impact on marine ecosystems, a 2009 study, headed by Justin Ries of the University of North Carolina, USA, found that some organisms would be harder hit than others.

In experiments, Ries and collaborators at the Woods Hole Oceanographic Institution placed 18 species of marine creatures, which had either calcium carbonate skeletons or shells, into seawater tanks that were exposed to air containing different concentrations of carbon dioxide: the current level, that predicted 100 years from now, that predicted 200 years from now, and finally at levels that can dissolve, outright, the form of calcium carbonate found in shells. The researchers then measured shell growth rates at the various concentration levels, observing clear-cut divergences among the species. Conch shells, as well as the spines of pencil urchins, deteriorated dramatically. Shell thickness increased, however, among the crustacean species tested—the blue crab, American lobster, and large prawn.

The broader implications of ocean acidification have yet to be assessed. Ries admits that it's "difficult to predict how even subtle changes in organisms' abilities to calcify will ultimately work their way through these ecosystems." Since ocean acidification is already occurring and will be hard to turn off, a much bigger experiment is underway in this planet's oceans, and we can only hope for a favorable outcome.

Although icebergs are clearly a significant feature of the waters around Antarctica, many people assume that they are barren masses of ice that have little bearing on life, apart from the hazards they pose to unwary seafarers, especially under conditions of darkness, fog, and storm. While the perils of icebergs cannot be denied, as the 2007 sinking of the Antarctic Explorer attests (as well as the Titanic's fate in 1912), these frozen, free-drifting mountains are, nevertheless, hardly devoid of life.

A 2007 article in Science magazine found that icebergs in the Weddell Sea, off the Antarctic Peninsula, were "hot spots of chemical and biological enrichment." The scientific team, led by Kenneth Smith of the Monterey Bay Aquarium Institute, USA, selected two icebergs that were scrutinized in detail from their research vessel, the Laurence M. Gould, and from NASA satellite images. One iceberg, called A-52, was more than thirteen miles long. The second, W-86, had a smaller surface area, but was deeper, extending nearly 1,000 feet below the surface. Smith and his collaborators described the icebergs as moving estuaries that deliver nutrients and minerals from land that might elsewhere be supplied by rivers. Elevated concentrations of these nutrients and minerals boosted, in turn, the abundance of phytoplankton—photosynthetic and generally microscopic organisms—especially diatoms. The availability of diatoms and other forms of phytoplankton, meanwhile, attracted Antarctic krill, jellyfish, and worms, which themselves attracted predators higher up the food chain—various species of fish and seabirds.

The zone of heightened biological activity extended about 2.3 miles from the icebergs, before dropping off significantly. However, given the large number of icebergs in the Weddell Sea, the researchers estimated their "combined area of influence" to be about 40 percent of the surface waters. Raised biological production in the vicinity leads to the removal of more carbon dioxide from the atmosphere, the authors note, and the "sequestration of organic carbon to the deep sea, a process unaccounted for in current global carbon budgets."

"No longer can we look at icebergs as mere passive beauties," writes Jeff Rubin of the American Polar Society. "They are active agents of change, each one an icy oasis, trailing a wake of life as it drifts on its inexorable oceanic journey to melting."

Kevin Raskoff, a biologist with Monterey Peninsula College, led a team that discovered a previously unknown jellyfish in Monterey Bay and the Gulf of California, which used bump-like stinging cells in place of tentacles to capture prey. "The coast of California is one of the more well studied parts of the world's oceans," Raskoff says. "Yet we are still discovering new species there." The moral of the story, for him, is that you don't have to go far to find something new. "You just have to dive deep."

Deep was the operative term for voyages near Tasmania in 2008 and 2009, where the remotely-operated submarine Jason ventured 4 kilometers below the surface. From samples collected by Jason, researchers discovered new species of barnacles and sea anemones, as well as a carnivorous, "Seuss-like" sea squirt that "looks and behaves like a Venus fly trap," according to Jess Adkins of the California Institute of Technology, USA.

A 2007 study in the journal Science reported on bacteria—found growing on the seafloor inside 6-foot tall tubeworms—that can convert carbon dioxide into organic carbon in two separate ways. These bacteria are the first known organisms capable of this. Perhaps stranger still are the multicellular organisms (from the phylum Loricifera) found deep in the Mediterranean Sea that live and reproduce in the complete absence of oxygen. In an apparent triumph of adaptation, the new species lack mitochondria—cell structures that can generate energy from oxygen and sugar—instead relying on organelles that can produce energy without oxygen.

Future exploration of the deep sea will undoubtedly yield not only species that are new to humanity, but possibilities for life that have never been contemplated before. "Who knows what else we might find?" Raskoff asks. "It's heartwarming to know that there's still a lot of mystery in the deep ocean. There are still a lot of big things moving around out there that we don't know about."

Ocean Alliance's ongoing initiatives include a 40-year study of a population of right whales that use the bays of Península Valdés, Argentina, as a nursery ground—the longest continuous study of any great whale based on known individuals.

In recent years, OA has broadened its interests to include whale habitats—specifically, the study of marine pollution using whales as a bioindicator species. OA recently completed a 5½-year Voyage of the Odyssey expedition, collecting benign biopsy samples from sperm whales in all equatorial oceans and analyzing the whales' exposure to heavy metals and anthropogenic contaminants.

This expedition established the first-ever, baseline data set of toxic contaminants in the world's oceans. The 'Voyage of the Odyssey Final Report' (whose findings included shockingly high levels of chromium—a known human carcinogen—in whales) was made public at the 2010 meeting of the International Whaling Commission in Morocco.

On July 17, 2010, in the wake of the BP Deepwater Horizon disaster, OA's research vessel Odyssey set sail for the Gulf of Mexico to conduct toxicology studies to assess the short- and long-term impacts of oil and chemical dispersants on whales and ocean life. Working in partnership with the Wise Laboratory of Environmental and Genetic Toxicology at the University of Southern Maine, USA, OA collected more than 50 Gulf whale biopsies and created the first-ever cell lines from Sperm, Humpback and Bryde's whales. The next field season in the Gulf is scheduled for late Spring 2011.

Ocean Alliance believes that rigorous science and widespread public education are basic requirements for long-term conservation of the world's oceans and their inhabitants. With a generous grant from the Annenberg Foundation, the organization recently purchased the historic Tarr and Wonson Paint Factory on the Gloucester, MA waterfront. Ocean Alliance is working to restore the site as an oceanographic research and public education center, and it will serve as its new headquarters.

For more information, please contact:

Ocean Alliance
191 Weston Road
Lincoln, MA 01773
www.oceanalliance.org