Light filters through the ocean surface into dark water

Life in the ocean ranges from the surface all the way to the abyss, with many deep-living organisms yet to be discovered. Image © Franziska Reinhardt/Shutterstock

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Two thirds of life in the seabed is unknown to science

Up to two thirds of the life living in the deep oceans could be unknown to science.

A new study trying to understand this diversity found that 60% of DNA sequences from marine sediments could not be identified at a higher taxonomic level, demonstrating the huge gap in scientific knowledge as a new era of deep sea mining is set to begin. 

The deep seafloor could be up to three times as diverse as the overlying waters, with much of this diversity yet to be discovered by science.

This is the finding of a study analysing hundreds of sediment samples from expeditions around the world's oceans, which revealed that almost two thirds of the deep sea's diversity is unknown. Some of these completely new forms of life may be associated with the biological carbon pump, which helps to regulate the planet's climate.

Dr Andrew Gooday, co-author and scientific associate at the Museum, says, 'It's been known since at least the 1960s that life is very diverse on the ocean floor, and that most species that are found in the deep sea are undescribed.

'What is new in this study is that a considerable proportion of this novel diversity is at a higher taxonomic level, suggesting that many unknown groups of organisms live in the deep sea. Most of these new lineages are likely to be small protozoans rather than larger organisms that most studies focus on.'

While the majority of the deep seafloor remains relatively untouched by humans, the potential for seabed mining in certain areas has raised controversy over its impact on marine ecosystems. The scientists hope their research, published in the journal Science Advances, will help to promote better protection for this final frontier of Earth exploration. 

Polymetallic nodules on the seabed

Mining for metals found on the seabed is controversial, with much of the deep sea yet to be explored. Image © Philweb, licensed under CC BY-SA 3.0 via Wikimedia Commons

The unknown ocean

Despite covering 71% of the Earth's surface the oceans are relatively understudied when compared with the land, with just 21% of the seabed mapped at high resolution. This leaves much of the life which lives there unknown, with one study estimating that as much as 91% of the ocean's species are yet to be described.

One of the main difficulties associated with discovering these species is the sheer size of the ocean. While terrestrial life is generally constrained to the planet's surface, marine species are spread vertically from the surface to a depth of 11,000 metres at the ocean's deepest point in Challenger Deep.

The challenges of studying the environment of the deep ocean, such as the high pressure, lack of light and the great depth, makes the discovery of species that live there even more difficult, particularly when they live beyond the sunlit and twilight zones of the water column.

This has also meant that the deeper abyssal parts of the ocean have remained relatively undisturbed. However, the presence of valuable metal deposits on the seabed means this may not be the case for much longer.

Proponents of deep sea mining say that these metals are needed for the construction of green technologies such as electric cars and wind turbines, which require the elements to generate and store electricity. 

Demand for these metals is rising, so extracting them from the seabed may help to alleviate shortages, cut greenhouse gas emissions and reduce the impacts of terrestrial mining.

However, the lack of knowledge of seabed ecosystems means that there could be devastating impacts on marine life. As well as the impact of the mining vehicles themselves, the sediment plumes created by the mining could damage habitats, disrupt nutrient cycling and deplete oxygen over wider areas. 

Opponents of deep sea mining have called for it to be banned, or at least postponed until its impacts are better understood.

'At the moment, it's very difficult to say how the diversity of the deep sea will be impacted by deep seabed mining, although there certainly will be some impact, at least locally,' Andrew says. 'It is likely that many species await discovery, while others have been found only in a single sample.

'We don't know if that's because they are only found in restricted areas, and are therefore at risk of extinction from mining, or if they have wider distributions but are rare. In the latter case, the extinction risk would be much lower.'

While mining is currently still at the prospecting stage, commercial operations may not be too far away. In June 2021, the nation of Nauru triggered a two-year countdown on the implementation of deep sea mining rules for international waters. 

As such, the race is on for scientists to understand marine diversity before the impact of mining begins. 

A bioluminescent siphonophore against a black background

Previous studies have generally focused on discovering larger organisms, like siphonophores, that live in the deep, rather than smaller forms of life. Image © NOAA Ocean Exploration & Research, licensed under CC BY-SA 2.0 via Flickr

Diversity at depth

To assess the diversity of oceanic species, the team of international researchers analysed 418 samples of sediment from different depths and areas of oceans across the world. These were then compared with thousands of samples from the overlying water, which had been taken on previous expeditions.

They ended up with almost two billion DNA sequences, which were grouped into 240,000 sequence variants. This allowed them to distinguish between the DNA of organisms which live in the seabed from those that had fallen from the higher ocean, and compare them for the first time.

Analysis of the results suggests that the diversity of life in marine sediment is significantly higher than organisms living in the waters above. Of the major groups that could be identified, nematode worms together with amoebae, ciliates, foraminifera, and a variety of other single-celled organisms dominated at the bottom of the sea.

However, 60% of the deep sea DNA sequence variants could not be placed in any known higher-level grouping, given our current level of knowledge of marine life. The researchers suggested that the unknown DNA could represent entire lineages of marine life that have yet to be described.

Some of these lineages were found to be associated with the biological carbon pump, a natural cycle which transports vast amounts of atmospheric carbon in the deep sea. As these organisms die and sink to the bottom of the ocean, they take the carbon with them. This can be preserved permanently in the deep sea sediment, or eventually returned to the upper ocean.

'Some of the unknown taxa seem to be involved in this process,' Andrew says, 'so the study throws new light on the delivery of carbon to the seafloor, which is very important in the context of climate change.'

The authors of this paper call for greater effort to study and protect the deep ocean, concluding, 'Our results highlight deep ocean sediment as one of Earth's richest modern ecosystems and fossil archives. 

'They underline the need for concerted international efforts to further understand its biodiversity and ecological role in planetary biogeochemical cycles.'