Only 150 years ago, marine scientists painted the deep sea as a mysterious, barren, and lifeless abyss. Dark and cold, with excruciatingly high pressure, the ocean deep surely couldn’t sustain any sort of life forms.
Ocean exploration and new technologies have upended those ideas. Particularly in the last 30 years, scientific investigations have transformed our understanding of the deep sea from an environment of quiet desolation to one of vibrant biodiversity featuring a dynamic mix of exotic marine life. In fact, scientists now believe a large fraction of the planet’s organisms reside in the deep sea.
A handful of researchers at Scripps Institution of Oceanography at UC San Diego have helped shape recent advances in the continually evolving depiction of the deep and its environment. Among them, Lisa Levin has investigated a variety of bizarre creatures that have adapted to extreme deep sea settings, from toxic methane seeps to massive zones where oxygen levels are next to nothing.
Tony Koslow spent a decade exploring marine life on undersea mountains called seamounts. Such species, he says, were once believed to reside too deep to be caught by fishermen, but today are regularly ending up on our dinner plates.
Indeed, the deep sea, previously considered untouchably remote, is a lot closer than many perceive, researchers are learning. Human-generated pollution, overfishing, and climate change are having a direct impact on deep-sea environments and their inhabitants.
Much remains to be studied and explored in the deep sea, Earth’s final frontier, say Levin and Koslow, especially before the long arm of humanity reaches down into the dark ocean depths and irreversibly changes what’s there.
Now a professor of biological oceanography, Levin’s interest in the deep sea was sparked when she was a graduate student at Scripps. She points to a deep-sea biology class taught by pioneering Scripps researcher Robert Hessler as the source of her enthusiasm.
Levin has conducted research with one foot in shallow water and the other in the deep sea, concentrating in each environment on how animals adapt to highly stressful settings. Such extreme organisms have become Levin’s research specialty.
Her research has taken her to methane seeps, patchy areas found at depths of more than 7,800 meters (4.8 miles) below the ocean surface. Methane, a clear, highly combustible gas, resides in the earth’s crust under the seafloor. When the planet’s tectonic plates at certain locations shift, methane squeezes and “seeps” upward.
Researchers first discovered animal communities living at such seeps in 1984 off Florida. New methane seeps are being discovered every few months, Levin says, and scientific understanding of their ecology is still in its infancy.
Microbes at these sites consume methane and interact with bacteria to create a setting rich in sulfide, the “rotten egg”-smelling, typically toxic, chemical compound.
Levin focuses on a family of sediment-dwelling polychaete worms (sometimes known as bristle worms) called Dorvilleidae and their survival mechanisms.
“Because it’s such a nasty setting, they don’t have very many competitors,” said Levin. “We’ve been able to track their different strategies, so they’ve been a really great model for studying adaptation to high stress in the deep sea.”
Levin’s interest also covers enormous water masses that experience minimal circulation and, thus, extremely low oxygen levels. These areas are called hypoxic zones and can be up to 1,000 meters (3,200 feet) thick.
Levin recently coauthored a study showing that oxygen-minimum zones cover 1.1 million square kilometers (425,000 square miles) of the deep ocean. One such zone exists off California at roughly 600 to 1,000 meters (2,000 to 3,200 feet) deep, part of a continuous zone that spans from Alaska to central Chile.
Because such zones occur naturally, Levin says they have become an important comparative tool for studying human-produced “dead zones” created by pollution and ocean warming. The animals living in hypoxic zones employ a variety of strategies to make a living there, including those with specialized gills or tentacles to extract what little oxygen is available.
The lack of oxygen prevents food decay, so those that are able to brave such conditions — certain polychaetes, for example — can find themselves stepping into an unchallenged feast.
Mobile animals such as certain fish species can venture inside these zones for periods of time and repay their oxygen “debt” later in the day by swimming back to more oxygen-rich waters outside the low-oxygen zone.
In addition, Levin studies chemosynthetic mud dwellers that convert chemicals into energy. By Levin’s own description, this community features a host of “really strange organisms,” including creatures that live in symbiotic, or mutually beneficial, arrangements. One extreme example is an oligochaete worm (related to earthworms) that lives with six types of symbiotic microbial organisms under its skin. Each microbe has a role to play, some synthesizing sulfide, others using it as an energy source, others converting nitrogen, and so on.
“It’s like having a worm with an entire ecosystem inside,” said Levin. “We found it in .02 milliliters (.0007 ounces) per liter of oxygen, which is about as low as you can measure.”
Like Levin, Tony Koslow received graduate training at Scripps before moving on to a career that includes deep sea research.
He was influenced by John Dove Isaacs, the maverick Scripps scientist who was a pioneer in studying seamounts and creating new ways to study and photograph deep-sea animals.
After working as a fisheries oceanographer, Koslow spent ten years studying the previously unknown and extraordinarily rich marine life on seamounts at the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia. There he developed acoustic and egg survey methods to monitor deepwater fish species, including the orange roughy and blue grenadier. While the orange roughy has become a popular dish at seafood restaurants, its situation is delicate because it can’t easily rebound from severe depletions due to a 150-year lifespan and late maturation.
“Since the 1980s, we have seen virtually every orange roughy stock in the world fished down to less than 10 percent of its original size,” said Koslow.
While at CSIRO, Koslow’s work led to one of the world’s first deep-water marine reserves. He was recently recruited to Scripps to lead the California Cooperative Oceanic Fisheries Investigations (CalCOFI) program, and he continues his efforts to protect and conserve deep sea life through marine protected areas. In his 2007 book: “The Silent Deep: The Discovery, Ecology, and Conservation of the Deep Sea,” Koslow provides a historical analysis of discoveries, along with a detailed description of deep-sea ecology. The most stirring accounts in the book, however, are those that describe the imprint of the human footprint on the deep, even in the most remote reaches of the abyss.
Technological advances in commercial fishing, Koslow uses as one example, have devastated deep-sea habitats.
“When trawlers go through sensitive habitats in the deep sea, such as deepwater coral reefs, it’s like clear-cutting a forest,” said Koslow.
A variety of human contaminants—from garbage to pesticides—often sink to the ocean depths, making the deep “the ultimate sink” for many pollutants, according to Koslow.
But Koslow believes that the most imposing threat may be human-induced climate change. As global warming accelerates, the deep will be threatened on several fronts.
Increased infusions of carbon dioxide levels will make the deep sea—as the rest of the ocean—more acidic, causing problems for many deep-sea habitats. Methane-hydrate deposits locked away in the deep could destabilize in a warming ocean and release massive quantities of carbon and accelerate further warming.
Yet another threat comes from ocean circulation. Climate models predict that global warming will cause a broad weakening of circulation in the deepest areas of the ocean, choking off links to oxygen-rich near-surface waters and increasing deep oxygen minimum zones.
“There will be less and less oxygen going down there. If that happens, the deep sea will just grow more stagnant,” said Koslow. “The climate changes we’re causing now will continue for a long time to come and those changes could have great impact on the deep sea.”
Every time Lisa Levin has explored the deep—whether in a remotely operated vehicle or a submarine, sometimes hundreds of miles from shore and thousands of feet deep—she has seen an assortment of urban items, from shoes to beer cans.
She and Tony Koslow say the deep sea, in many ways, is much closer than we think and that human impact there is great and growing.
“People often think of the deep sea as out of sight, out of mind, so remote from human activities that it must be the most pristine environment on Earth,” said Koslow. “That’s simply not the case. And it’s much less resilient than other environments.”
But Levin lights up at the prospect of the novel discoveries yet to be made. She believes new kinds of ecosystems will be found. Novel organisms await discovery in environments that fuse geology and chemistry in unheard-of ways.
“The deep sea is not that far away in terms of our activities and impact,” said Levin. “By not knowing what’s down there, we run the risk of destroying things before we’ve discovered them. Every time we make a new discovery we learn more about how life works and how it evolved.”