The progressive increase of CO2 emissions into the atmosphere due to anthropogenic activities and the consequent absorption by the oceans are causing important changes in the carbonate chemistry of the seawater and a lowering of its pH. This phenomenon, known as Ocean Acidification, is expected to lead to profound changes in the biodiversity and functioning of marine ecosystems in the next decades. In this context, calcifying organisms are predicted to be severely threatened, while non-calcifying or photosynthetic organisms are forecasted as winners in future acidified scenarios. Among non-calcifying organisms, marine sponges can be one of the winners. They belong to the phylum Porifera, which is widespread in the marine environments, and are characterized by simple anatomical structures constituting one of the oldest sessile taxa. These organisms are of paramount relevance in benthic communities for their capability to recycle nutrients, for their rich associated microbiomes and for their metabolic versatility and secondary metabolite production, representing model holobiont systems. In this dissertation four Mediterranean sponge species (i.e. Petrosia ficiformis, Chondrosia reniformis, Crambe crambe and Chondrilla nucula) have been selected to conduct an in situ investigation to evaluate adaptation traits to ocean acidification. The experimental work took place along the volcanic CO2 vents off Ischia island, which mimic the lowered pH conditions predicted for the end of this century. The aims of this study included: the investigation of sponges distribution across acidified and control sites, and the assessment of acclimatization responses in the morphology, associated microbiomes and metabolomic profiles.