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Scleractinian cold-water corals (CWC) represent key taxa controlling deep-sea reef ecosystem functioning by providing structurally complex habitats to a high associated biodiversity, and by fuelling biogeochemical cycles via the release of organic matter. Nevertheless, our current knowledge on basic CWC properties, such as feeding ecology and key physiological processes (i.e. respiration, calcification and organic matter release), is still very limited. Here, we show evidence for the trophic significance of zooplankton, essentially sustaining levels of the investigated key physiological processes in the cosmopolitan CWC Desmophyllum dianthus (Esper 1794). Our results from laboratory studies reveal that withdrawal (for up to 3 weeks) of zooplankton food (i.e. Artemia salina) caused a significant decline in respiration (51%) and calcification (69%) rates compared with zooplankton-fed specimens. Likewise, organic matter release, in terms of total organic carbon (TOC), decreased significantly and eventually indicated TOC net uptake after prolonged zooplankton exclusion. In fed corals, zooplankton provided 1.6 times the daily metabolic C demand, while TOC release represented 7% of zooplankton-derived organic C. These findings highlight zooplankton as a nutritional source for D. dianthus, importantly sustaining respiratory metabolism, growth and organic matter release, with further implications for the role of CWC as deep-sea reef ecosystem engineers.