For decades, explorations into the planet’s deepest oceanic trenches primarily focused on the sediment, with researchers anticipating the discovery of viable life within the soft seafloor deposits. The bare rock faces lining these trenches were largely disregarded, deemed devoid of life.
This established assumption persisted until the advent of manned submersibles capable of directing their cameras onto the rock itself. The findings from this novel investigation have been published in the journal Science.
The rock that was examined turned out not to be inert, but rather a living crust, so densely populated that thousands of organisms occupied a mere hand-sized area.
These trenches had remained inaccessible for decades, leaving a region of mystery – an area of the ocean approximately 3.7 miles deep, named after the Greek underworld – largely unexplored.
It stands as one of the least understood environments on Earth. Simply reaching the seafloor and retrieving samples presents a significant engineering challenge in itself.
China’s manned submersible, “Fendouzhe,” has altered this landscape. Between 2020 and 2024, it undertook 98 dives into the world’s deepest trenches. This effort was spearheaded by Sikun Song of the Institute of Deep Sea Science and Engineering (IDSSE).
His team specifically directed their attention toward the rocky trench walls and exposed stone surfaces that previous explorations had largely overlooked.
The cameras captured not barren stone on these rock faces, but a dense, living crust. This surface was covered in filamentous and tubular growths.
On an area no larger than approximately 10 cm, as many as 4,300 such organisms were found. Throughout the study, the team documented 32 species belonging to six distinct branches of life.
Some were single-celled, while others were tiny animals. The majority of these had never been previously described, a gap that earlier surveys of the deepest ocean zones had long indicated.
The most abundant life forms were foraminifera, single-celled organisms that construct their own shells using fine filaments attached to the rock. Researchers affectionately nicknamed them “rock feathers.” Four of these minute species, entirely new to science, constituted the majority of the observed organisms.
Some mechanism must have been supporting life in conditions of perpetual darkness, devoid of plants and sunlight. Scientists had a strong hypothesis, informed by peculiar oases found around deep-sea hydrothermal vents.
Near these vents, tube worms and mussels subsist on bacteria that convert chemicals seeping from the seafloor into sustenance.
This conjecture had been supported by years of research. Many scientists proposed that the rock-dwelling organisms survived through a similar process. However, the evidence pointed elsewhere. No chemosynthetic organisms, creatures that feed on chemicals, were detected on any of the rock surfaces.
Genetic analysis of the foraminifera revealed none of the necessary mechanisms for chemosynthesis. Prior to this research, no one could definitively explain how these deep-sea communities on rock faces sustained themselves.
Foraminifera, it appears, are feeding on slowly sinking dead plankton and organic detritus that drifts down from the sunlit ocean far above. They are scavengers, not chemical factories.
Individual organisms appear as minuscule, millimeter-sized specks on the rock. But collectively, spread across the trenches, they represent a significant biomass. The team estimates they account for between two and eleven percent of all living carbon within the world’s deepest trenches.
This establishes a surprisingly active node in the ocean’s carbon cycle. Foraminifera consume material descending from above. Therefore, they intercept carbon transferring from the surface to the ocean depths, a process previously described in research on the chemistry of oceanic trenches.
In models of the deep ocean, carbon calculations had largely omitted these rock communities, as their existence in such numbers was unknown.
The inclusion of these populations could revise estimates of how much carbon the deepest trenches absorb and store. The most significant discoveries were made in two trenches: the Kermadec Trench off the coast of New Zealand and the Mariana Trench in the western Pacific Ocean.
The most mineral-rich communities were found at depths ranging from approximately 5.5 to 6.8 miles below the surface. The pattern didn’t end there. Subsequent expeditions identified similar rock communities in five additional trenches scattered across the Pacific Ocean and beyond.
If these communities are so widespread, they represent not a localized anomaly but a standard characteristic of the planet’s deepest environments. This alone recalibrates our understanding of where life congregates in the sea.
Before this work, scientists perceived the deepest trenches as little more than barren rock and a thin layer of mud. They believed that the few organisms present on exposed rock survived on chemicals seeping from the seafloor.
Researchers now understand that the deepest reaches of the ocean floor host dense, largely undocumented communities that subsist on material falling from above. This particulate matter contains a proportion of the deep ocean’s carbon. This offers oceanographers new and concrete evidence for tracking and studying.
The next series of expeditions will investigate other trenches for these same rock aggregations and quantify the amount of carbon they are actively transporting. The deepest parts of the ocean, long overlooked, appear to be teeming with life.
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