About two-thirds of the organisms on the ocean floor don’t really fit taxonomically, according to new research.

It is no secret that many life forms can be found at the bottom of the oceans. And we even know quite accurately what those life forms are up to there: recycling and storing (in)organic matter – such as carbon. This matter comes from ecosystems that live in higher water layers (and are dominated in absolute numbers by plankton). As a result, these life forms are of crucial importance for the functioning of the marine food chains and the long-term storage of CO2 – and thus also the regulation of the Earth’s climate.

Identifying Life Forms

It is less clear, however, what kind of life forms are involved; the ocean floor—despite occupying some 60 percent of the Earth’s surface—is the least studied habitat on Earth. A new study – published in the magazine Science Advances – is now trying to change that. For the study, researchers searched for DNA in sediments from the bottoms of all major oceans on Earth. And that DNA was subsequently analyzed in more detail and compared with DNA from organisms found higher up in the water during previous studies. The latter enabled the researchers to distinguish between DNA from organisms that lived higher in the water, but later – after their death – descended to the bottom and organisms that actually live or had lived on the bottom of the sea.

Surprising insights

The research yields several surprising insights. For example, the results indicate that the diversity in the so-called benthic zone – the lower part of a water body, such as an ocean – can be up to three times greater than in the water mass above it. In addition, the benthic diversity appears to cover very diverse taxonomic groups, most of which have yet to be created because of these benthic organisms. “Our data indicates that nearly two-thirds of the benthic diversity cannot be attributed to the groups known to us,” said researcher Jan Pawlowski. “And that reveals a huge gap in our understanding of marine biodiversity.”

Marine Carbon Pump

The research also reveals more about the marine carbon pump: a collection of processes that ultimately store atmospheric CO2 deep in the ocean (see box).

The ocean takes in carbon in two ways. First of all, there is the physical pump that transports surface water in which a lot of CO2 is dissolved downwards to deeper water layers. And then there’s the marine biological pump. This article is about the latter pump. This pump ensures that CO2 is stored deep in the ocean via marine organisms. How does that work exactly? On the water surface we find photosynthesizing plankton (also called phytoplankton) that extract CO2 from the atmosphere through photosynthesis and bind it to itself. Some of this carbon trapped in marine particles sinks – for example after the death of the phytoplankton – where it is stored for a very long time in sediments at the bottom of the ocean.

Previous research suggested that plankton concentration is closely related to the intensity of the marine carbon pump. And this study confirms that. For example, the abundance and composition of plankton DNA in sediments can tell more about the power of the marine carbon pump. “For the first time, we now understand which members of the plankton community contribute the most to the marine carbon pump,” said researcher Colomban de Vargas.

From past to future

With the new research, we have for the first time a fairly complete picture of what lives in the entire present-day oceans – from the epipelagic zone (near the surface) to the benthic zone. And that can also help us better understand the oceans of the past. In theory, for example, researchers can analyze older sediments to clarify which organisms lived on the seabed at that time and, on the basis of this, also gain a picture of how the marine carbon pump worked then – in the climate of that moment. And if we zoom in on periods when the oceans were warmer than now, this can in turn provide more insight into how the marine carbon pump will function in the future – if the earth and therefore the oceans warm up further.

In addition, however, it is also important that we take a closer look at the organisms that live on the ocean floor and of which we have caught a first glimpse in this study, says researcher Andrew J. Gooday. “Large numbers of unknown organisms live in the sediments on the ocean floor and must play a fundamental role in ecological and biogeochemical processes. A better understanding of this rich diversity is crucial if we are to protect these vast, relatively pristine ecosystems from possible future human interference and better understand the effects that climate change is having on them.”