Written by Tristan Cordier, researcher at the Bjerknes Centre and NORCE.
The deep-ocean floor is the least explored ecosystem on the planet, despite covering more than 60 percent of the Earth's surface. The sediments of the abyssal seafloor host a vast, largely unknown biodiversity, from benthic animals to microbes, that helps to recycle the sinking organic matter originating from pelagic communities that are numerically dominated by microscopic plankton organisms. Benthic ecosystems thus underpin two major ecosystem services of planetary importance: the healthy functioning of ocean food-webs and the burial of carbon on geological timescales, both of which help to regulate the Earth's climate.
Together with researchers from the French institutes CNRS, IFREMER and Genoscope, I have massively sequenced eukaryotic DNA contained in deep-sea sediments from all major oceans and compared these new data to existing global-scale plankton datasets from the sunlit and dark water column, obtained by the Tara Oceans and Malaspina oceanographic expeditions. This resulted in a massive dataset composed of nearly 1700 samples and more than two billion DNA sequences.
These are exciting times, because high-throughput environmental genomics has vastly expanded our capacity to study and understand deep-sea biodiversity, its connection to the water masses above and to the global carbon cycle.
This study provides the first unified vision of ocean eukaryotic biodiversity, from surface to seafloor, allowing marine ecological questions to be addressed for the first time at a global scale and across the three-dimensional space of the ocean. This represents a major step towards a “one ocean” ecology.
What lives in this dark and hostile environment?
By comparing sediment DNA sequences with the ones from pelagic realms, it was possible to distinguish indigenous benthic organisms from sinking plankton that had reached the seafloor from the overlying water column. Our results indicate that deep-sea benthic biodiversity may be three times larger than that of the water column, and that nearly two third of this strictly benthic diversity cannot be assigned to any known eukaryotic group. This underlines our lack of knowledge of the vast biodiversity thriving in one of the largest biomes on Earth.
What can plankton DNA in deep-ocean sediments tell us?
Phytoplankton communities produce half the oxygen we breathe. They also form the basis of marine food-webs that collectively contribute to store carbon in the deep ocean via the biological carbon pump. For the first time, we can analyse the abundance and composition of plankton DNA that has settled on deep-sea sediments.
Our data confirmed that polar regions are hotspots of carbon sequestration and indicate that eukaryotic parasites likely play an underestimated role in the carbon export of oligotrophic oceans. Furthermore, the taxonomic signature of plankton DNA in sediments predicts the variation of the strength of the biological pump, opening the possibility to develop plankton DNA-based proxies to reconstruct the past functioning to the biological pump.
How will the deep-sea be impacted by global changes?
This genomic dataset represents the first consistent snapshot of whole eukaryotic diversity in the modern ocean. It thus provides a unique opportunity to reconstruct ancient oceans from the DNA contained in the cumulative sediment record, to assess how climate has impacted plankton and benthic communities in the past. Such knowledge can then serve as a basis to reconstruct the past functioning of the biological pump from ancient plankton DNA archives, a key information to better inform on its future intensity under climate change. By extension, this work would lead to better models of the future carbon cycle under climate change, which is presently crucial for policy makers.
Altogether, our study underlines that deep-sea biodiversity research is of paramount importance, not only for climate research, but also to provide the basis of a more informed deep-sea stewardship, especially if we are to protect these vast, relatively pristine ecosystems from the impacts of anticipated deep-seafloor mining activities.
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 AJ.J., de Vargas C., Pawlowski J. 2022. Patterns of eukaryotic diversity from the surface to the deep-ocean sediment. Science Advances. 8:5, eabj9309.