The ocean floor is the least explored ecosystem on the planet, even though it covers more than 60% of the Earth’s surface. Life in the deep sea, including microbes and bottom dwellers, is still a mystery, but it provides an essential role in recycling the sinking organic matter coming from surface waters, according to a study published in the journal Science Advances.
To shed some light into this vastly unknown ecosystem, researchers from the Norwegian Research Centre (NORCE), Bjerknes Centre for Climate Research, the University of Geneva, as well as from the CNRS/Genoscope and IFREMER in France, sequenced DNA from deep-sea sediments originating from all oceans and compared this data to existing plankton data obtained by the Tara Oceans and Malaspina circumglobal expeditions.
This is the first unified database of all eukaryotes in the ocean, from the surface to the deep-sea sediment. Importantly, it allows researchers to address ecological questions globally and across all aquatic habitats.
“With nearly 1700 samples and two billion DNA sequences from the surface to the deep-ocean floor worldwide, high-throughput environmental genomics vastly expands our capacity to study and understand deep-sea biodiversity, its connection to the water masses above and to the global carbon cycle”, said Tristan Cordier, Researcher at NORCE and Bjerknes Centre for Climate Research, Norway, and lead author of the study.
Who lives in the dark?
By comparing DNA sequences from sediment areas and open water, the team managed to separate bottom dwellers from sinking plankton that had reached the seafloor from the water surface. Results suggest that this biodiversity at the bottom of the sea could be three times larger than the open water above, with many taxonomic groups yet unknown.
“We compared our deep-sea benthic DNA sequences to all references sequences available for known eukaryotes. Our data indicates that nearly two-thirds of this benthic diversity cannot be assigned to any known group, revealing a major gap in our knowledge of marine biodiversity”, said Jan Pawlowski, Professor at the Department of Genetics and Evolution of the University of Geneva and the Institute of Oceanology of the Polish Academy of Sciences in Sopot.
What can we learn from this analysis?
This analysis also confirms that deep-sea areas are hotspots for carbon sequestration, with plankton DNA found at the bottom showing the strength of the biological pump. In simple terms, the biological pump — also known as marine carbon pump— is the way that carbon is transported from the atmosphere and land to the ocean and finally to the sea bed. This is a vital mechanism to regulate global climate.
“For the first time, we can understand which members of plankton communities are contributing most to the biological pump, arguably the most fundamental ecosystem processes in the oceans,” said Colomban de Vargas, Researcher at CNRS in Roscoff, France.
“Our study further demonstrates that deep-sea biodiversity research is of paramount importance. Huge numbers of unknown organisms inhabit ocean-floor sediments and must play a fundamental role in ecological and biogeochemical processes. A better knowledge of this rich diversity is crucial if we are to protect these vast, relatively pristine ecosystems from the impacts of possible future human incursions and understand the effects on it of climate change”, concluded Andrew J. Gooday, Emeritus Fellow at the National Oceanography Centre, Southampton.
(1) Cordier T., Barrenechea Angeles I., Henry N., Lejzerowicz F., Berney C., Morard R., Brandt A., Cambon-Bonavita M.A., Guidi L., Lombard F., Martinez Arbizu P., Massana R., Orejas C., Poulain J., Smith C.R., Wincker P., Arnaud-Haond S., Gooday A.J., de Vargas C., Pawlowski J. 2022. Patterns of eukaryotic diversity from the surface to the deep-ocean sediment. Science Advances. http://www.science.org/doi/10.1126/sciadv.abj9309.