A first unified vision of the full ocean eukaryotic biodiversity

The abyssal world: the last terra in­cog­nita of the Earth sur­face.


Deep-ocean sediment (DOS) remains one of the least explored ecosystems on the planet. This vast and heterogeneous environment provides habitats for diverse biological communities.

Largely unknown life forms in the abyssal zone help recycle and/or sequester the sinking (in)organic matter originating from pelagic communities that are numerically dominated by microscopic plankton. Genomic assessments of their biodiversity have failed to separate indigenous benthic organisms from sinking plankton.

Benthic ecosystems have two major ecosystem services of planetary importance: the healthy functioning of ocean food webs and carbon burial on geological timescales. Both are critical drivers of the Earth’s climate.

Scientists from the Norwegian Research Centre (NORCE), Bjerknes Centre for Climate Research, the University of Geneva, and the CNRS/Genoscope and IFREMER in France have massively sequenced eukaryotic DNA contained in deep-sea sediments from all major oceanic basins. They then compared these new data to existing global-scale plankton datasets from the sunlit and dark water column, obtained by the Tara Oceans and Malaspina circumglobal expeditions.

The study offers the first unified vision of the full ocean eukaryotic biodiversity, from the surface to the deep-ocean sediment. It also addresses marine ecological questions on a global scale and across the three-dimensional space of the ocean. In simple words, this represents a significant step towards ‘Once ocean ecology.’

Tristan Cordier, Researcher at NORCE and Bjerknes Centre for Climate Research, Norway, said, “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.”

Comparing sediment DNA sequences with the ones from pelagic realms allowed scientists to differentiate indigenous benthic organisms from sinking plankton that had reached the seafloor from the overlying water column. It also reveals that the benthic biodiversity could be three times larger than in the water masses above.

Jan Pawlowski, Professor at the Department of Genetics and Evolution of the University of Geneva, said, “We compared our deep-sea benthic DNA sequences to all references sequences available for known eukaryotes. Our data indicate 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.”

Studying the abundance and composition of plankton DNA in deep-sea sediments affirmed that polar regions are hotspots of carbon sequestration. The composition tells about the strength of the biological pump, an ecosystem process that transfers atmospheric carbon dioxide into the deep ocean, hence regulating the global climate.

Colombian de Vargas, Researcher at CNRS in Roscoff, France, said, “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.”

Tristan Cordier said, “Our data will not only address global-scale questions on the biodiversity, biogeography, and connectivity of marine eukaryotes. It can also serve as a basis to reconstruct the past functioning of the biological pump from ancient sedimentary DNA archives. It would then inform its future strength in a warmer ocean, which is key for modeling the future carbon cycle under climate change.”

Andrew J. Gooday, Emeritus Fellow at the National Oceanography Centre, Southampton, said“Our study further demonstrates that marine biodiversity research is of paramount importance. Many 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.”

Journal Reference:

  1. Cordier T., Barrenechea Angeles I et al. Patterns of eukaryotic diversity from the surface to the deep-ocean sediment. Science Advances. DOI: 10.1126/sciadv.abj9309
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