Sponge grounds are hotspots of biomass and biodiversity in the otherwise barren deep sea. It remains unknown how these ecosystems can thrive in such food limited environments, since organic matter settling from the surface ocean covers only small parts of their carbon demand. In this study, the food-web interactions and potential food sources of a North Atlantic deep-sea sponge reef were identified by bulk and compound-specific stable isotope analysis of amino and fatty acids. The elevated bulk δ15N values of sponges with relatively low abundance of associated microbes (LMA) is in line with a position at the top of the benthic food web, while the relatively high δ13C and intermediate δ15N values of high microbial abundance (HMA) sponges suggest considerable reliance on an alternate resource. Trophic positions based on amino acid δ15N values placed HMA sponges at the base of the food web. Fatty acid analysis of δ13C indicated transfer of sponge derived organic matter to the wider food web. Our results show that sponges drive both bottom-up and top-down processes, shunting organic carbon to higher trophic levels that would otherwise be inaccessible to other fauna. In this way, sponges are key to the sustenance of thriving deep-sea ecosystems.