Abyssal fauna keeps polymetallic nodules of mining interest on the seabed

Species inhabiting the deep ocean floor play a key role the preservation of abyssal habitats and associated biodiversity. This is the main conclusion of research co-led by the National Oceanography Centre (NOC) in the UK and the Institute of Marine Sciences (ICM-CSIC) in Barcelona. Specifically, the study demonstrates that benthic fauna, living in contact with the seafloor, is primarily responsible for keeping polymetallic nodules—mineral deposits targeted by deep-sea mining industry—on the surface of the seabed, preventing them from being buried or dissolving into abyssal mud over time.

This discovery not only reveals an unforeseen ecosystem function, but also highlights a new and unexpected ecological impact that could worsen the already known consequences of deep-sea mining on a global scale.

Polymetallic nodules (also known as manganese nodules) are rock accretions of variable size—generally between 1 and 20 centimeters—found scattered yet highly abundant across some abyssal plains, at depths between 4,000 and 6,000 meters. They form extremely slowly over millions of years and provide an unusual yet essential habitat for a wide variety of deep-sea species. For decades, the scientific community has wondered how these nodules managed to remain exposed on the surface rather than being growingly covered by sediments settling on the seafloor.

Seafloor engineers

Now, using time-lapse photography surveys of the seabed, the research team has documented how various mobile species actively clear sediment from the surface of the nodules. These include sea cucumbers, sea urchins, and sea snails, among other fauna. Furthermore, the activity of organisms such as polychaete worms, which live within the sediments and lift the nodules from below, has also been recorded.

These daily activities of benthic fauna—the species living in contact with or just above the seabed—act as a continuous maintenance mechanism for the nodules, supporting a hypothesis first proposed over four decades ago. By keeping the nodules clean and exposed on the surface, these "ecosystem engineers" allow other sessile species—those growing attached—such as corals and sponges, to colonize the hard substrate, thereby fostering the high biodiversity characteristic of these abyssal habitats.

For this reason, the authors emphasize the need to understand not only which species inhabit deep-sea environments, but also how they interact with and modify their environment, particularly within the current context of evaluating the true impacts of deep-sea mining.

The impact of mining

According to the scientific team, if mining extraction activities reduce or eliminate mobile abyssal fauna, this natural process of cleaning and lifting nodules will be directly disrupted. As a result, and enhanced by mining sediment plumes generated during nodule extraction, the remaining hard substrates could end up buried or dissolved in abyssal mud, eliminating the ecosystem's capacity to regenerate and support biodiversity in the long term.

In this regard, Erik Simon-Lledó, researcher at ICM-CSIC and senior author of the study, warns of the risks associated with deep-sea mining: 

"Abyssal nodule mining could trigger cascading ecological impacts that had not been considered until now. By reducing the mobile fauna, a functional mechanism keeping the hard substrate exposed would be affected making it harder for species like corals and sponges to recolonize the environment, aggravating the already known effects of mining on the rich biodiversity of these ecosystems."

For his part, Loïc Van Audenhaege, lead author of the study, highlights: 

"We are only just beginning to uncover the hidden processes that sustain deep-sea ecosystems. Understanding these interactions is essential to predicting the extent to which these ecosystems are capable of being resilient."

Overall, the findings of this research open up new opportunities to monitor the ecological state, resilience, and long-term dynamics of abyssal zones. As the authors conclude, accounting for the role of ecosystem engineering species will be essential for improving predictions of how nodule fields will respond to human disturbances and for advancing toward science-based environmental management of the deep sea.