Experiments suggest that deep-sea mining and warming trigger stress in a midwater jellyfish

The study is led by the GEOMAR Helmholtz Centre for Ocean Research and co-authored by NORCE researcher Helena Hauss. Their scientific work is published in the journal Nature Communications.

The deep sea is home to one of the world's largest communities of animals - about which we still know very little. Yet it is already subject to a growing number of human-induced environmental pressures. Also, despite the importance of midwater ecosystems on a global scale, little research has so far focused on species-specific responses of midwater animals to environmental stressors.

This is the gap that the researchers set out to fill in the recently published study.

Hauss and her colleagues carried out experiments on a research vessel, in special 30 litre tanks, to simulate conditions created by mining activity. One of their findings is that the helmet jellyfish (owing to its hat-like shape) is "highly sensitive" to plumes of sediment. Since the experiments took place in a controlled environment - tanks on a vessel - the researchers cannot at this point conclude on the jellyfish's stress response out in the open ocean. Therefore, more research is needed on the topic.

Although mining operations will target seafloor minerals, they will also disturb and pump up fine sediment off the seafloor, generating suspended sediment ‘clouds’ (known as plumes) along the seafloor. Since there is usually little sediment in the midwater, it is expected that midwater animals will be highly sensitive to mining-induced sediment plumes.

This is concerning, as Dr Helena Hauss, co-first author of the study and Research Director of Marine Ecology at NORCE, explains:

“The midwater is crucial for the global ocean’s capacity to store carbon, but its inhabitants are also the main food source for many fish, squid, and marine mammal species and therefore resemble a critical link in the marine food web. They have evolved under much more stable conditions compared to surface-dwelling animals, under a constant scarcity of food, and are therefore potentially more susceptible to changing conditions in their environment,” Hauss says.

This is the first time researchers have investigated the stress response of a midwater organism to simulated sediment plumes.

“Since determining ‘stress’ in a jellyfish is not a straightforward process, we investigated their response from multiple angles and combined insights gained from their physiology, gene expression, and the microbial symbionts on the jellyfish’s exterior,” explains Vanessa Stenvers, co-first author of the study and a doctoral candidate at GEOMAR and the Smithsonian Institution.

The strongest visual effect of suspended sediment was the aggregation of sediment particles on the jellyfish after just ~1.5 hours of incubation, to which the jellyfish started to produce excess mucus that slowly sloughed off.

“While mucus helped jellyfish maintain a stable microbiome, continuous mucus production is an energetically costly response and can demand a substantial portion of the total energy budget of an animal,” adds Stenvers.

The authors are concerned that stressors leading to increased energy expenditure, as they observed for the helmet jellyfish, will have to be met with increased food intake. Since food in the deep sea is generally scarce, this could ultimately lead to starvation.

Although more data from different midwater species are needed to better understand the environmental impacts of deep-sea mining, the stress response in the helmet jellyfish may be representative of other gelatinous animals. Based on their overall findings, the researchers urge caution with regard to deep-sea mining, as many of the deep ocean’s important ecosystem services could be compromised.

Reference:

Stenvers, V.I. et al (2023): Experimental mining plumes and warming trigger stress in a deep pelagic jellyfish. Nature Communications. doi: 10.1038/s41467-023-43023-6