Contrasting metabolic strategies of two co-occurring deep-sea octocorals

Cold-water coral energy reserves and calcification in contrasting fjord environments

The relationship between energy reserves of cold-water corals (CWCs) and their physiological performance remains largely unknown. In addition, it is poorly understood how the energy allocation to different metabolic processes might change with projected decreasing food supply to the deep sea in the future. This study explores the temporal and spatial variations of total energy reserves (proteins, carbohydrates and lipids) of the CWC Desmophyllum dianthus and their correlation with its calcification rate. We took advantage of distinct horizontal and vertical physico-chemical gradients in Comau Fjord (Chile) and examined the changes in energy reserves over one year in an in situ reciprocal transplantation experiment (20 m vs. 300 m and fjord head vs. mouth). Total energy reserves correlated positively with calcification rates. The fast-growing deep corals had higher and less variable energy reserves, while the slower-growing shallow corals showed pronounced seasonal changes in energy reserves. Novel deep corals (transplanted from shallow) were able to quickly increase both their calcification rates and energy reserves to similar levels as native deep corals. Our study shows the importance of energy reserves in sustaining CWC growth in spite of aragonite undersaturated conditions (deep corals) in the present, and potentially also future ocean.

Vulnerability of six cold-water corals to sediment resuspension from bottom trawling fishing

Bottom trawling can significantly affect benthic communities, directly through immediate removal of sessile organisms and indirectly through sediment resuspension. Submarine canyons, often surrounded by fishing grounds, are important habitats for cold-water corals (CWC). Vulnerability of CWCs to increased suspended sediment concentration (SSC) is key to understanding the severity of bottom trawling effects on those communities. Here we show survival, growth, and physiological response of six CWCs from a Mediterranean submarine canyon (Dendrophyllia cornigera, Desmophyllum dianthus, Desmophyllum pertusum, Madrepora oculata, Leiopathes glaberrima and Muriceides lepida), exposed to a long-term, aquarium-based sedimentary disturbance experiment. Compared to cup coral and octocoral, which did not exhibit symptoms of distress, our data indicate that colonial scleractinian corals and black coral, which experienced substantial polyp mortality in enhanced SSC treatments, are more vulnerable. Indirect impact of bottom trawling could thus contribute to structural simplification of CWC communities posing an additional stressor alongside with global climate change.

Trophic ecology of Angolan cold-water coral reefs (SE Atlantic) based on stable isotope analyses

Cold-water coral (CWC) reefs of the Angolan margin (SE Atlantic) are dominated by Desmophyllum pertusum and support a diverse community of associated fauna, despite hypoxic conditions. In this study, we use carbon and nitrogen stable isotope analyses (δ¹³C and δ¹⁵N) to decipher the trophic network of this relatively unknown CWC province. Although fresh phytodetritus is available to the reef, δ¹⁵N signatures indicate that CWCs (12.90 ± 1.00 ‰) sit two trophic levels above Suspended Particulate Organic Matter (SPOM) (4.23 ± 1.64 ‰) suggesting that CWCs are highly reliant on an intermediate food source, which may be zooplankton. Echinoderms and the polychaete Eunice norvegica occupy the same trophic guild, with high δ¹³C signatures (-14.00 ± 1.08 ‰) pointing to a predatory feeding behavior on CWCs and sponges, although detrital feeding on ¹³C enriched particles might also be important for this group. Sponges presented the highest δ¹⁵N values (20.20 ± 1.87 ‰), which could be due to the role of the sponge holobiont and bacterial food in driving intense nitrogen cycling processes in sponges' tissue, helping to cope with the hypoxic conditions of the reef. Our study provides first insights to understand trophic interactions of CWC reefs under low-oxygen conditions.

Phenology in the deep sea: seasonal and tidal feeding rhythms in a keystone octocoral

Biological rhythms are widely known in terrestrial and marine systems, where the behaviour or function of organisms may be tuned to environmental variation over periods from minutes to seasons or longer. Although well characterized in coastal environments, phenology remains poorly understood in the deep sea. Here we characterized intra-annual dynamics of feeding activity for the deep-sea octocoral Paragorgia arborea. Hourly changes in polyp activity were quantified using a time-lapse camera deployed for a year on Sur Ridge (1230 m depth; Northeast Pacific). The relationship between feeding and environmental variables, including surface primary production, temperature, acoustic backscatter, current speed and direction, was evaluated. Feeding activity was highly seasonal, with a dormancy period identified between January and early April, reflecting seasonal changes in food availability as suggested by primary production and acoustic backscatter data. Moreover, feeding varied with tides, which likely affected food delivery through cyclic oscillation in current speed and direction. This study provides the first evidence of behavioural rhythms in a coral species at depth greater than 1 km. Information on the feeding biology of this cosmopolitan deep-sea octocoral will contribute to a better understanding of how future environmental change may affect deep-sea coral communities and the ecosystem services they provide.

Beyond deep-sea mining sublethal effects: Delayed mortality from acute Cu exposure of the cold-water octocoral Viminella flagellum

The potential release of metals, especially copper (Cu) during mining of seafloor massive sulphides (SMS), represents a potential toxicological threat to cold-water coral (CWC) habitats. Herein, we evaluated for the first time the response of the whip coral Viminella flagellum to short-term acute Cu exposure. Nubbins of V. flagellum were exposed to Cu concentrations of 0 (control); 60; 150; 250; 450 and 600 μg/L for 96 h. After exposure, V. flagellum nubbins were transferred to a continuous flow-through aquarium and feed once a day for 3 weeks. No immediate mortality was detected during the short-term Cu exposure. However, a delayed mortality, which was concentration dependent was observed. The first signs of tissue loss occurred after 1 week of recovery in non-contaminated conditions in V. flagellum nubbins previously exposed to Cu concentrations of 60 and 150 μg/L followed by nubbins exposed to Cu concentrations of 250, 450 μg/L after 2 weeks and 600 μg/L after 3 weeks. A delayed mortality impact should be considered in future Cu tolerance experiments and scenarios of deep-sea mining exploitation.