Sponges as primary producers of a deep sea sponge ground?

Sponge grounds are hotspots of biomass and biodiversity in the otherwise barren deep sea. It is unknown how these benthic ecosystems can thrive in such a food limited environment, since sinking organic matter from the surface ocean is expected to cover less than 20% of their carbon demand. In this study food sources, food web interactions, nitrogen flow in the food web as well as microbial mediated transformations of nutrients of an Arctic deep-sea sponge ground were identified by bulk and compound specific stable isotope analysis. Sponges did not show the expected isotopic composition of suspension feeding fauna but a deviation depending on their classification as high (HMA) or low microbial abundance (LMA) sponges. LMA sponges had an unexpectedly enriched δ¹⁵N value which would locate them at the top of the food chain (>19 ‰), whereas HMA sponges had a more depleted isotopic value expected of filter feeding fauna (10 ‰) and a relatively enriched d¹³C value (Δ3.8 ‰). This enriched d¹³C value suggests that HMA sponges receive their energy not solely from suspended matter, but may also utilize dissolved (in)organic matter as a resource. This incorporation may be facilitated through their microbial symbionts (bacteria & archaea) to supplement their nutrition and provide access to products from other metabolic pathways (de-/nitrification and anammox). On the other hand sponges might be able to take up dissolved nutrients themselves, since the isotopic composition of amino acids showed a signatures which cannot be explained by microbial heterotrophy. HMA sponges seem to inhabit the first trophic position and likely transfer organic matter to the benthic fauna, which was confirmed by the isotope mixing model. Our results show that sponges play an important role in the benthic pelagic coupling by utilizing food sources which are inaccessible for other organisms and presumably release detritus that supports the associated fauna, thus enabling a thriving deep sea ecosystem.