Ocean depths triple the microbial diversity of the upper levels of terrestrial seas, but most of that life is unknown to science: the analysis of almost 1,700 samples and two billion DNA sequences collected around the world reveals .
The abyssal zone of the terrestrial oceans harbors at least three times more diversity of microbial life than the waters located in the upper layers, according to new research.
This study has also found that most of the eukaryotic organisms that live in the deepest oceans on Earth are unknown to modern science.
To reach these conclusions, scientists scanned the seabed that surrounds the entire planet. Sediment collected at each location was analyzed for environmental DNA (eDNA), which marine animals shed as they go about their lives.
Although marine creatures discard some of the eDNA, that material contains evidence of microbes and other tiny animals that make up the dark ecosystem at the bottom of the world.
The researchers compared the results with other existing plankton DNA data sets, collected from the upper layers of the ocean, to make sure they only identified creatures from the deep.
This is the first time that scientists have assembled a consistent set of molecular data on the ocean realm on such a global scale, and while the meta-analysis is not exhaustive, it is an impressive start, the researchers say.
“We compared our deep-sea benthic DNA sequences with all available reference sequences for known eukaryotes,” says geneticist Jan Pawlowski of the University of Geneva in Switzerland, in a statement.
“Our data indicates that nearly two-thirds of this benthic diversity cannot be attributed to any known group, revealing a major gap in our knowledge of marine biodiversity.”
Earth’s oceans contain different levels, depending on their depth. The first level, called mesopelagic, covers marine waters located between 200 and 1,000 meters deep,
Next is the bathyal or bathypelagic zone, which covers the waters and seabeds located between 1,000 and 4,000 meters deep.
Below it is the abyssal zone, which corresponds to the oceanic space located between 4,000 and 6,000 meters deep, which is what this research has focused on.
That deep ocean floor is the least explored ecosystem on the planet, despite covering more than 60% of the Earth’s surface.
Life present in abyssal sediments, from benthic animals to microbes, is poorly understood, despite the fact that these life forms help to recycle and/or sequester sinking (in)organic matter originating from pelagic communities. , the researchers point out.
Therefore, benthic ecosystems support two important ecosystem services of planetary importance: the healthy functioning of oceanic food webs and the burial of carbon on geological time scales, both of which are critical regulators of Earth’s climate.
The new study provides the first unified view of entire ocean eukaryotic biodiversity, from the surface to deep ocean sediments, addressing marine ecological questions for the first time on a global scale and across the three-dimensional space of the ocean. it represents a big step towards “single-ocean ecology,” the researchers add.
“With nearly 1,700 samples and two billion DNA sequences from the surface to the deep ocean floor worldwide, high-throughput environmental genomics greatly expands our ability to study and understand deep-sea biodiversity, its connectedness with upper-level water masses and with the carbon cycle,” explains Tristan Cordier, lead author of the study.
Analysis of the abundance and composition of plankton DNA in deep-sea sediments confirmed that the polar regions are hotspots for carbon sequestration.
In addition, the DNA composition of plankton in the sediments predicts the variation in the strength of the biological pump, an ecosystem process that transfers atmospheric carbon dioxide to the deep ocean, thus regulating global climate.
“For the first time, we can understand which members of the plankton communities are contributing the most to the biological pump, possibly the most fundamental ecosystem processes in the oceans,” explains Colomban de Vargas, a CNRS researcher in Roscoff, France.
“Our data will not only help resolve global questions about the biodiversity, biogeography and connectivity of marine eukaryotes. They can also serve as a basis for reconstructing the past functioning of the biological pump from ancient sedimentary DNA archives. They can also inform the future importance of this biological community in a warmer ocean, which is key to modeling the future carbon cycle under climate change”, explains Tristan Cordier.
“Our study further demonstrates that deep-sea biodiversity research is of paramount importance. A large number of unknown organisms inhabit the sediments of the ocean floor and must play a fundamental role in ecological and biogeochemical processes. A better understanding of this rich diversity is crucial if we want to protect these vast and relatively pristine ecosystems from the impacts of possible future human incursions and understand the effects of climate change on them”, concludes Andrew J. Gooday, who was also involved in the research.
Patterns of eukaryotic diversity from the surface to the deep-ocean sediment. Tristan Cordier et al. Science Advances, 4 Feb 2022, Vol 8, Issue 5. DOI: 10.1126/sciadv.abj930