Scientists say they cruised the ocean in a deep-sea submersible and came across an undiscovered ecosystem
Geochemist Mengran Du had 30 minutes left in her submersible mission when she decided to explore one last stretch of the trenches that lie between Russia and Alaska, about 5,800 to 9,500 meters (19,000 to 30,000 feet) below the ocean's surface in what's called the hadal zone. She said she began to notice 'amazing creatures,' including various species of clam and tube worm that had never been recorded so deep below the surface.
What Du stumbled upon was a roughly 2,500-kilometer (1,550-mile) stretch of what her team says is the deepest known ecosystem of organisms that use the chemical compound methane instead of sunlight to survive. Du is a co-lead author of a study describing the findings that was published July 30 in the journal Nature.
The hadal zone is primarily comprised of oceanic trenches and troughs — some of the deepest and least explored environments on Earth. At these depths, 'life needs tricks to survive and thrive there,' explained Du, a professor and researcher at the Institute of Deep-sea Science and Engineering at the Chinese Academy of Sciences.
One of those tricks lies in bacteria that have evolved to live inside the clams and tube worms, according to the National Oceanic Atmospheric Administration. The bacteria convert methane and hydrogen sulfide from cold seeps — cracks in the seafloor that leak these compounds as fluids — into energy and food that the host animal can use, allowing organisms to live in zero-sunlight conditions.
The discovery suggests that these communities might also exist in other hadal trenches, Du said, opening opportunities for further research into just how deep these animals can survive.
After analyzing sediment samples collected from the expedition, Du and her team said they detected high concentrations of methane. The find was surprising, since deep-sea sediments normally contain very low concentrations of the compound.
The scientists hypothesized that microbes living in the ecosystem convert organic matter in the sediments into carbon dioxide, and carbon dioxide into methane — something the researchers didn't know microbes could do. The bacteria living inside clam and tube worm species then use this methane for chemosynthesis to survive, Du said.
There was another revelation, too. Scientists previously thought chemosynthetic communities relied on organic matter — such as from dead organisms and drifting particles from living species — that fell from the ocean's surface to the floor. But this discovery, Du said, reveals that these methane-producing microbes are also creating a local source of organic molecules that larger organisms such as clams can use for food and energy.
Methane, as a carbon-containing compound, is part of the carbon cycle. So, this discovery also indicates that the hadal trenches play a more important role in that cycle than previously thought, Du explained.
Scientists have long understood that methane is stored as compressed fluid deep in the subduction zone, where tectonic plates meet below the ocean floor, which ultimately releases through 'cold seeps' at the bottom of hadal trenches. Now that Du's team has discovered chemosynthesis at such depths, they hypothesize that the hadal trenches act not only as reservoirs, but also as recycling centers for methane.
This suggests, Du said, that 'a large amount of the carbon stays in the sediments and (is) recycled by the microorganisms.' Indeed, scientists have recently estimated that hadal zone sediments could sequester as much as 70 times more organic carbon than the surrounding seafloor. These so-called carbon sinks are crucial for our planet given that methane and carbon dioxide are two major greenhouse gases driving global warming in the atmosphere.
Chemosynthetic communities themselves are not new to science. Previous research has hinted that it was possible for them to thrive at such great depths, said Johanna Weston, a deep ocean ecologist at Woods Hole Oceanographic Institute in Massachusetts who was not involved with the new study. She was impressed, however, with the extent of the recent discovery, she told CNN.
In an age of widespread biodiversity loss, the finding highlights the importance of new technology that can withstand high pressure in deep-sea environments to document undiscovered organisms, said Weston, who is part of a team actively exploring the deep-sea offshore from Argentina.
Even though the hadal trenches are remote, they aren't completely isolated, she added. Weston and her colleagues discovered a newfound species in 2020 in the Mariana Trench named Eurythenes plasticus for the microplastic fibers detected in its gut. And near Puerto Rico, Weston newly identified an isopod that exclusively eats sargassum, a type of abundant seaweed in the Atlantic Ocean that can sink to the ocean floor in just 40 hours. 'The deep ocean is very connected to what's happening on the surface,' she said.
Research on deep-sea ecosystems is only a few decades old, and the technology for new discoveries is improving. But Du added that it's important for different countries and scientific disciplines to collaborate on future efforts. The Global Hadal Exploration Program, which is co-led by UNESCO and the Chinese Academy of Sciences, aims to do just that by creating a network of deep-sea scientists from multiple countries.
Du hopes she and her team can learn more about hadal trench ecosystems by studying how these species have adapted to such extreme depths.
'Even though we see the hadal trench as a very extreme environment, the most inhospitable environment … (chemosynthetic organisms) can live happily there,' Du said.
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