Nitrification in a Seagrass-Sponge Association

In the Mediterranean Sea, the demosponge _Chondrilla nucula_ can occur in close association with the native seagrass _Posidonia oceanica_. _C. nucula_ harbors a diverse and abundant microbial community, including potential nitrifiers. Thus, the sponge may contribute to the nitrogen (N) demand of the seagrass holobiont. In this study, we investigated potential nitrification rates (PNR) and inorganic N fluxes within this association at a site where _C. nucula_ covered 18 ± 3 % of the seagrass meadow area, during plant growth (spring) and senescence (autumn). Using incubation experiments with 15N-labeled ammonium, we measured PNR and inorganic N of the seagrass-sponge association, and of sponge and seagrass independently, under light and dark conditions. We supplemented these experiments with 16s rRNA gene amplicon sequencing to characterize the microbial community of the sponge. PNR was exclusively measured when the sponge was present (alone or in association with the seagrass). PNR was highest in the dark and when _C. nucula_ was associated with the seagrass, ranging from 21 ± 7 to 267 ± 33 nmol N g DW-1 h-1 in spring and autumn, respectively. Sponge-mediated PNR can support 8% of the N demand of the _P. oceanica_ holobiont during growth and 47 % during senescence. We identified key nitrifying bacterial and archaeal groups as members of the sponge’s microbial community. While _C. nucula_ released inorganic N, potentially sustaining the seagrass, it benefitted from dissolved organic carbon released by _P. oceanica_. These results suggest that the interaction between _C. nucula_ and _P. oceanica_ is mutually beneficial, ultimately supporting and stabilizing the seagrass ecosystem.