From mollusks and worms to microscopic creatures: Which creatures inhabit the largest terra incogniata on our planet – the deep sea floor? A genetic study now provides at least a basic impression of the unknown diversity of species. The biodiversity of the eukaryotes on the sea floor is therefore at least three times greater than in the open water above. About two thirds of the creatures represent previously unknown organisms, according to the genetic comparisons. As the researchers emphasize, these unexplored inhabitants of the sediments are of enormous importance for the global material cycle.
It covers more than 60 percent of the earth’s surface – and yet the deep sea floor is the least explored area of life on our planet. Technically, these zones are called benthic ecosystems. It is generally known that, in addition to bacteria, there are also many different animals and microscopic organisms. They belong to the eukaryotes – creatures that, in contrast to prokaryotic bacteria, have cells with a more complex structure. The benthic eukaryotes include animals, but above all microorganisms, which sometimes consist of only one cell. It seems clear: These creatures provide ecosystem services of planetary importance. Because they help to recycle or bind organic matter in the oceans. They ensure the healthy functioning of the food webs of the oceans and the “burying” of climate-relevant carbon.
Samples from the Terra Incogniata
So far, however, it is largely unclear what diversity of organisms exists in the deep sea floors. At least basic information could be provided by studying the genetic material in the sediments, which comes from the inhabitants. However, there has been a problem up until now: in genomic assessments of biological diversity, it was not possible to distinguish benthic organisms from the likewise eukaryotic organisms of the open water zone above them. Because above all the representatives of the plankton constantly trickle down to the sea floor, form layers there and provide the benthic organisms with food.
In order to obtain a differentiated picture of eukaryotic biodiversity, an international team of researchers has now carried out extensive comparisons: They sequenced the DNA of living beings in deep-sea sediments from all of the world’s major ocean basins and then compared this data with corresponding information from the water column. “With nearly 1700 samples and two billion DNA sequences from the surface to the deep-sea floor worldwide, high-throughput environmental genomics expands our ability to study and understand deep-sea biodiversity, its connection to overlying water bodies, and the global carbon cycle,” explains first author Tristan Cordier from NORCE and Bjerknes Center for Climate Research in Norway.
Genetic traces of unknown diversity
As the researchers report, their results show that the diversity of species on the deep sea floor is on average three times greater than in the water masses above. This diversity is in turn composed of very different taxonomic groups, most of which are unknown: “We compared our benthic deep-sea DNA sequences with all reference sequences available for known eukaryotes. Our data show that almost two-thirds of this benthic diversity cannot be assigned to any known group, revealing a major gap in our knowledge of marine biodiversity,” says co-author Jan Pawlowski from the University of Geneva.
The study thus points to the need for further research: “Investigating the biological diversity of the deep sea is of the utmost importance,” emphasizes co-author Andrew Gooday from the National Oceanography Center in Southampton. “A huge number of unknown organisms appear to inhabit the seafloor sediments and have to play fundamental roles in ecological and biogeochemical processes. A better knowledge of this rich diversity is important if we are to protect these vast, relatively pristine ecosystems from the impacts of possible future human encroachment and to understand the impacts of climate change on them,” says Gooday.
The study also provides, for the first time, a more comprehensive picture of the overall eukaryotic biodiversity in the oceans. In this way, marine ecological issues could now be addressed on a global scale and across the ocean’s three-dimensional space, say the researchers. “Our data not only help to answer questions about biodiversity, biogeography and connectivity of marine eukaryotes on a global scale. They can also serve as a basis for reconstructing how the biological pump worked in the past using ancient DNA archives in the sediments. This would then shed light on their future strength in a warmer ocean, which is crucial for modeling the carbon cycle in the wake of climate change,” says Cordier.
Source: MARUM – Center for Marine Environmental Sciences, University of Bremen, specialist article: Science Advances, doi: 10.1126/sciadv.abj9309