Mass occurrences of large sponges, or 'sponge grounds', are found globally in a range of oceanographic settings. Interest in these grounds is growing because of their ecological importance as hotspots of biodiversity, their role in biogeochemical cycling and bentho-pelagic coupling, the biotechnological potential of their constituent sponges, and their perceived vulnerability to physical disturbance and environmental change. Little is known about the environmental conditions required for sponges to persist and for grounds to form, and very few studies have explicitly characterised and interpreted the importance of oceanographic conditions. Here, results are presented of the first observational oceanographic campaign at a known sponge ground on the Schultz Massif Seamount (SMS; Arctic Mid-Ocean Ridge, Greenland / Norwegian Seas). The campaign consisted of water column profiling and short-term deployment of a benthic lander. It was supported by multibeam echosounder bathymetry and remotely operated vehicle video surveys. The seamount summit hosted several environmental factors potentially beneficial to sponges. It occurred within relatively nutrient-rich waters and was regularly flushed from above with slightly warmer, oxygen-enriched Norwegian Arctic Intermediate Water. It was exposed to elevated suspended particulate matter levels and oscillating currents (with diurnal tidal frequency) likely to enhance food supply and prevent smothering of the sponges by sedimentation. Elevated chlorophyll a concentration was observed in lenses above the summit, which may indicate particle retention by seamount-scale circulation patterns. High sponge density and diversity observed on the summit is likely explained by the combination of several beneficial factors, the coincidence of which at the summit arises from interaction between seamount geomorphology, hydrodynamic regime, and water column structure.Neighbouring seamounts along the mid-ocean ridge are likely to present similarly complex 2 oceanographic settings and, as with the SMS, associated sponge ground ecosystems may therefore be sensitive to changes over a particularly broad range of abiotic factors.

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In the deep ocean symbioses between microbes and invertebrates are emerging as key drivers of ecosystem health and services. We present a large-scale analysis of microbial diversity in deep-sea sponges (Porifera) from scales of sponge individuals to ocean basins, covering 52 locations, 1077 host individuals translating into 169 sponge species (including understudied glass sponges), and 469 reference samples, collected anew during 21 ship-based expeditions. We demonstrate the impacts of the sponge microbial abundance status, geographic distance, sponge phylogeny, and the physical-biogeochemical environment as drivers of microbiome composition, in descending order of relevance. Our study further discloses that fundamental concepts of sponge microbiology apply robustly to sponges from the deep-sea across distances of >10,000 km. Deep-sea sponge microbiomes are less complex, yet more heterogeneous, than their shallow-water counterparts. Our analysis underscores the uniqueness of each deep-sea sponge ground based on which we provide critical knowledge for conservation of these vulnerable ecosystems.

Shallow-water sponges are often cited as being 'climate change winners' due to their resiliency against climate change effects compared to other benthic taxa. However, little is known of the impacts of climate change on deep-water sponges. The deep-water glass sponge Vazella pourtalesii is distributed off eastern North America, forming dense sponge grounds with enhanced biodiversity on the Scotian Shelf off Nova Scotia, Canada. While the strong natural environmental variability that characterizes these sponge grounds suggests this species is resilient to a changing environment, its physiological limitations remain unknown, and the impact of more persistent anthropogenic climate change on its distribution has never been assessed. We used Random Forest and generalized additive models to project the distribution of V. pourtalesii in the northwest Atlantic using environmental conditions simulated under moderate and worst-case CO 2 emission scenarios. Under future (2046-2065) climate change, the suitable habitat of V. pourtalesii will increase up to 4 times its present-day size and shift into deeper waters and higher latitudes, particularly in its northern range where ocean warming will serve to improve the habitat surrounding this originally subtropical species. However, not all areas projected as suitable habitat in the future will realistically be populated, and the re duced likelihood of occurrence in its core habitat on the Scotian Shelf suggests that the existing Vazella sponge grounds may be negatively impacted. An ef fective monitoring programme will require tracking changes in the density and distribution of V. pourtalesii at the margins between core habitat and where losses and gains were projected. Species distribution models showed that the suitable habitat of the deep-water glass sponge Vazella pourtalesii will expand under climate change scenarios.