Microbiomes are generally referred as plastic entities, able to adapt their composition and functionality in response to environmental change, with a possible impact on corals acclimatization to phenomena related to climate change, such as ocean acidification. Ocean sites characterized by natural gradients of CO2 provide models to investigate the ability of marine organisms to acclimatize to the decrease of seawater pH. Here we compare the microbiome of the temperate, shallow water, non-symbiotic solitary coral Astroides calycularis that naturally lives at a volcanic CO2 vent in Ischia Island, Naples, Italy, to corals living in non-acidified sites on the same island. Bacterial DNA associated to the different coral anatomic compartments, i.e. mucus, tissue and skeleton, of A. calycularis was differentially extracted and a total of 68 samples were analyzed by 16S rRNA gene sequencing. In terms of phylogenetic composition, microbiomes associated to the different coral anatomic compartments were different among themselves and from the surrounding seawater microbial population. The mucus microbiome was the most affected by the ocean acidification. The effect of the acidification on the different microbial communities was characterized by a general increase in subdominant bacterial groups, some of which are involved in different points of the nitrogen cycle, including potential nitrogen fixing bacteria and bacteria able to degrade organic nitrogen. As expected in acidified conditions, our data hint at a general increase of nitrogen fixation and recycling in A. calycularis living close to the CO2 vent system.