The Kuril-Kamchatka and Aleutian trenches lie between Russia and Alaska in the United States. Photo / Janna Graber
When her three-person submersible descended more than 9000m into one of the Pacific Ocean’s deepest trenches, Mengran Du wasn’t sure what they would find.
What she saw, she recalled, was “unbelievable”.
Dense clusters of tubeworms with tentacles tinged bloodred, jutting up like skyscrapers.
Iridescent snails scaling the worms, like window washers.
Bristly, white creatures wriggling between them like rush-hour commuters trying to get home for dinner.
An international team of researchers has discovered the world’s deepest known ecosystem sustained by chemicals seeping from the seafloor, submerged in water and darkness. The discovery expands the limits of where we know life can live on Earth.
“It’s a unique ecosystem,” said Dominic Papineau, an exobiologist who co-wrote the study on the deep-sea discovery published today in the journal Nature.
“It’s a totally new thing that has not been seen before.”
The bottom dwellers, found in the Kuril-Kamchatka and Aleutian trenches, between Russia and Alaska, “alter our understanding of trench ecosystems”, said Lisa Levin, a professor emeritus of biological oceanography at Scripps Institution of Oceanography, who was not involved in the study.
For decades, scientists have studied organisms that thrive around hydrothermal vents, fissures spewing superheated fluids.
The creatures that live around cold seeps - places where gases such as methane and hydrogen sulphide ooze from the seafloor at near-freezing temperatures, often where tectonic plates meet - have been understudied.
So, to investigate, an expedition to the northwest Pacific last northern summer used the crewed submersible Fendouzhe to dive into the hadal zone, the ocean’s deepest region, named for Hades, the Greek god of the underworld.
China, the United States and others have been seeking to capitalise on the mineral wealth of the seafloor, mining metals for use in electric cars and other technology - and prompting concern about upending deep sea life.
The trenches examined in the new study are probably too deep for mining. Their exploration demonstrates newly acquired abilities by countries and companies to investigate the open ocean.
“People know very little about the bottom of the trench,” said Du, a geochemist with China’s Institute of Deep-sea Science and Engineering, which led the study. For decades, she added, researchers lacked the “high technology to enable us to go there”.
The researchers, once down there, found they weren’t alone. They discovered communities of animals dominated by marine tubeworms and molluscs spanning over 2410km of total darkness.
Elsewhere on Earth, sunlight sustains life.
Photosynthesis performed by plants or algae is the base of almost all food webs.
Ocean scientists previously assumed trench creatures eked out an existence by feasting directly on dead animals and other organic matter that fell from sunlit parts of the ocean into the crevasses.
But in the hadal zone, life appears to sustain itself through a more meandering method.
Analysis of gases seeping from the seafloor suggests microbes are consuming organic matter that accumulates in the trenches and belching methane after their meals.
Symbiotic bacteria inside the tubeworms and molluscs, in turn, absorb the methane and hydrogen sulphide from those cold seeps to produce organic matter to nourish their hosts.
The process, called chemosynthesis, may seem like an alien way for an animal to score dinner, but Papineau noted that humans have their own colonies of microbes that aid in digestion. “We ourselves have bacteria in our gut,” he said.
The researchers expect several of the specimens they plucked from the trenches will yield species new to science, though they don’t know how many. “This is the next paper,” Papineau said.
They also don’t know exactly how these animals survive the extraordinarily high pressures found in the trenches.
“They must have some trick, or they must have some unique metabolic pathway, to adapt to the high pressure,” Du said.
But the most surprising finding, according to biologist Lesley Blankenship-Williams, is about where all that nourishing methane is coming from: not from geological processes deep in the Earth, but from microbes in the sediment.
“Flourishing chemosynthetic communities had long been postulated to exist in the trenches, but this is the first paper that documents their existence below 9km and at multiple locations,” said Blankenship-Williams, a professor at Palomar College in California who was not involved in the study.
The research team found life on 19 of 23 dives over a 40-day period, suggesting that hadal ecosystems may be common in Earth’s ocean trenches.
The extreme adaptability of organisms in those trenches gives hope to those searching for evidence of life in oceans on other worlds, such as Jupiter’s icy moon, Europa.
“There is about 3700 million years of Earth evolution between the oldest animal fossils to the oldest microbial fossils,” Papineau said.
“So, if deep extraterrestrial oceans existed for billions of years, then perhaps similar chemosynthetic-based ecosystems with animal-like creatures could also exist there.”