Hidden Diversity: Three times as many eukaryotic organisms live on the sea floor of the deep sea than in the entire ocean above them – and two-thirds of this diversity is determined by species that are still completely unknown. Researchers discovered this during a first gene-based “census” in deep-sea sediments. To do this, they sampled and examined this “terra incognita” of the earth’s biosphere in all of the world’s oceans.
The deep sea and the seabed in the dark depths of the oceans form the last ecosystem of the earth’s living environment that has hardly been explored. Although the deep seabed makes up around 60 percent of the earth’s surface, it has so far only been examined selectively. The studies were also mostly limited to larger animals or to microbial organisms in the sediment – for example in the Mariana Trench or in the license area for future deep-sea mining.
Genetic census in the world oceans
Now a research team led by Tristan Cordier from the Norwegian research center NORCE has closed an important gap in knowledge about this “terra incognita”. Because they have for the first time compiled an inventory of all eukaryotes – the organisms that carry cell nuclei – from the sea surface down to the sediments of the deep sea. “This represents the first molecular metadata set covering the three major ocean kingdoms – pelagic euphotic, pelagic aphotic and deep-sea sediments on a global scale,” the scientists explain.
To do this, they collected 418 samples from deep-sea sediments in all the world’s oceans during 15 research expeditions and analyzed them using high-throughput gene sequencing. For comparison, they also analyzed data from 1,160 samples from the light-flooded, euphotic, open-water areas of the ocean and 138 samples from the dark, deeper water areas.
“With almost 1,700 samples in total and two billion DNA sequences from the surface to the sea floor, this advances our knowledge of deep-sea biodiversity, its connection to the water masses above and the global carbon cycle,” says Cordier.
Biodiversity in the seabed three times higher than in the water
The result: the researchers identified a total of 242,465 gene signatures from eukaryotic organisms. Of these, only 1.6 percent occurred in all three ocean kingdoms. While plankton algae dominated in the near-surface zones of the oceans, it was mainly heterotrophic nanoflagellates that were not dependent on light in the darker water layers. As expected, the gene signatures of some of these organisms were also found in the deep-sea sediments – because these creatures sink there after death.
What was surprising, however, was how much life there is on the sea floor of the deep sea: “The diversity in the deep sea sediments could be more than three times higher than in the pelagic habitats,” report Cordier and his team. In addition to some well-known species such as amoebas, nematodes, foraminifera and ciliates, they found countless completely unknown gene signatures.
Two thirds still completely unknown
“Almost two-thirds of the benthic eukaryotes could not be classified taxonomically into any known species,” the researchers report. More than 10,000 of these gene signatures also differed by more than 90 percent from any known, suggesting that these creatures do not just represent new genera or species. Instead, there are apparently still many completely unknown families or even orders of the organism kingdom in these deepest zones of the sea.
“Our results suggest that most of the biodiversity in deep-sea sediments consists of unknown high-level eukaryotic groups and their numerous sublineages,” the team said. The samples also revealed that the lifeworld on the sea floor shows much larger small-scale and local differences than those in the overlying ocean. In some cases, even cores that were only 30 centimeters apart from one borehole revealed different compositions of organisms.
Big local differences
The researchers suspect that these local differences are primarily related to the availability of nutrients in these deep sea regions. Because most inhabitants of the sediment feed on the organic material that sinks from higher water layers, even a lump of droppings lying at the bottom can spawn a small community of its own.
It is fitting that the researchers found the highest biodiversity in the sediment where there was also a high density of plankton in the layers near the surface. Above all, nematodes, foraminifera and molluscs were particularly numerous and species-rich where many organic floating particles sank to the sea floor, as the scientists determined. However, this only explains part of the distribution.
Basis for more research and protection
“Our global molecular inventory from sea surface to deep-sea sediments provides the first unified view of eukaryotic biodiversity patterns in all three dimensions of the ocean,” say Cordier and his colleagues. “It shows that the deep-sea sediments in particular are an extremely diverse and unique realm with a strong connection to the water masses above.”
According to the research team, their data can now form the basis for further research on the unrecognized environment in the deep ocean. “We also hope that they will enable a more informed and effective conservation strategy for these unique and still relatively untouched deep-sea ecosystems – especially as the exploitation of seabed resources is gaining momentum,” say the scientists. (Science Advances, 2022; doi: 10.1126/sciadv.abj9309)