Unveiling in situ oxygen, carbon and nutrient cycling of a sponge-driven biological hotspot in the arctic.

Deep-sea sponge grounds are habitat-forming benthic communities characterized by high biomass and structural complexity. Despite their ecological significance, their role for the deep-sea environment remains poorly understood and their functioning is often inferred from ex situ studies. We hypothesized that deep-sea sponge grounds exhibit substantially higher respiration and nutrient turnover than surrounding soft sediments, making them hotspots of carbon and nutrient cycling in the deep sea. Integrated respiration and nutrient cycling were quantified in a sponge ground on the summit of an Arctic seamount (Schulz Bank, ~ 580 m depth). We used in-situ incubation chambers measuring oxygen consumption, prokaryotic cell removal, and inorganic nutrient fluxes. Respiration rates ranged from 0.13 to 0.93 mmol O₂ m⁻² h⁻¹, which is comparable to cold-water coral reefs and up to 7–21 times higher than reported for soft sediments of the Arctic deep sea. This indicates a high organic carbon demand exceeding surface-derived supply, suggesting the uptake of additional food resources. All incubations showed net release of ammonium, phosphate, nitrite and nitrate, with fluxes correlating with sponge biomass. Our results demonstrate that deep-sea sponge grounds function as hotspots of carbon and nutrient cycling and suggest distinct functional contributions of sponge groups and their microbiome.