Stressor impacts on planktonic organisms

To investigate the impacts of global warming and sediment plumes released by deep-sea mining on deep planktonic organisms, experiments were conducted with the helmet jellyfish Periphylla periphylla (led by GEOMAR; conducted by V.I. Stenvers & H. Hauss and coordinated by H.J.T. Hoving).

Travel restrictions due to Covid-19 led to the cancellation of the proposed land-based expedition to Norway and collection using GEOMAR’s submersible JAGO. Instead, experiments were performed on shipboard expeditions to Norway. Experiments with P. periphylla were carried out during two cruises in 2021 (HE570 and AL568). During incubation experiments, animals were exposed to varying levels of warming (in situ, +2°C, +4°C for 7–9 hours) and five abyssal sediment suspension concentrations (0, 16.7, 33.3, 166.7, 333.3 mg·L^-1 for 24 hours). Both behavioural (posture, mucus sloughing) and physiological responses were measured, including respiration and ammonium excretion rates as well as gene expression. Samples were also obtained for microbiome analyses.

P. periphylla increased its metabolic activity in response to warming and sediment plume exposure. An increase of 4°C, which is a large increase given projections of future midwater conditions, resulted in an oxygen consumption that is twice as high compared to the in situ temperature. Exposure to sediment had a strong physical and metabolic effect on the jellyfish showing production of excess mucus, increasing ammonium production and respiration, and expression of genes related to wound repair and anaerobic respiration. Strikingly, the sediment treatment with the highest load, resulted in a similar respiration increase as the most extreme temperature treatment of 4°C. Sequencing results of the microbiome showed no significant treatment effect on microbial community composition on the outer bell, and mucus production appeared to be an efficient strategy to maintain the microbial community despite sediment smothering.

The increase in metabolic rates observed in the sediment experiments will impact the energetics and demand for food in this deep-sea jellyfish that is adapted to a slow metabolic pace in an environment with scarce food sources. Future plans for mining should consider these results since seafloor disturbance could have detrimental effects on deep pelagic communities, if our results are representative for other midwater fauna.

Periphylla was not available on a third cruise to the Arctic (HE605) in August 2022 (during which we had intended to increase our sample size). Instead, pilot experiments were conducted with the amphipod Themisto abyssorum, measuring responses of respiration rate to a range of sediment plume concentrations. Data collected for T. abyssorum will be processed over the coming months.

This work has since been published in Nature Communications (November 2023).