Combined in situ niche analysis and pulse-chase labelling experiments unravel energy acquisition strategies in cold-water corals

Cold-water corals (CWCs) are reef-building foundation species occurring worldwide in the aphotic zone of the ocean. Those reefs are considered biodiversity hotspots in the deep-sea. In the absence of endogenous production, CWCs mostly depend on exported photosynthetic production for their nutrition. Energy acquisition is therefore a major challenge for CWCs. In this context, we investigated feeding habits of three syntopic CWC species forming reefs in the Lampaul Canyon (Bay of Biscay) at depths ranging from 800 to 1200 m: Desmophyllum pertusum, Madrepora oculata and Solenosmilia variabilis. Specifically, we tackled the following questions: 1) What is the realized trophic niche of CWCs in the Lampaul Canyon?; 2) Do the three species rely on the same resources?; and 3) Are the trophic niches of CWCs overlapping with those of associated fauna, particularly suspension feeders?
Joint carbon, nitrogen and sulfur stable isotope analysis showed that CWC exhibit marked resource segregation with associated fauna. However, coral niches largely overlapped. Species-specific trends in niche size were present, suggesting variable trophic diversity. Fatty acid analysis highlighted the importance of zooplankton for CWC feeding, which could favor this resource for its high nutritional value. Furthermore, pulse-chase experiments in pressurized tanks suggested that all species are, to some extent, able to function as holobionts. They notably obtain part of their nitrogen from mutualistic relationships with the microbiome living in the mucus secreted by the polyps. Ultimately, reliance upon these multiple, non-mutually exclusive ecological mechanisms could be a key factor to facilitate CWCs future survival under changing environmental conditions.