Impact of marine aquaculture on the microbiome associated with nearby holobionts: the case of Patella caerulea living in proximity of sea bream aquaculture cages.

Aquaculture plays a major role in the coastal economy of the Mediterranean Sea. This raises the issue of the impact of fish cages on the surrounding environment. Here we explore the impact of aquaculture on the composition of the digestive gland microbiome of a representative locally dwelling wild holobionts, the grazer gastropod Patella caerulea, at an aquaculture facility located in Southern Sicily, Italy. The microbiome was assessed in individuals collected on sea bream aq-uaculture cages and on a rocky coastal tract located far from the cages, as control site. P. caerulea microbiome variations were explained in the broad marine metacommunity context, assessing the water and sediment microbiome composition at both sites, and characterizing the microbi-ome associated with the farmed sea bream. The P. caerulea digestive gland microbiome at the aquaculture site was characterized by a lower diversity, the loss of microorganisms sensitive to heavy metal contamination, and by the acquisition of fish pathogens and parasites. However, we also observed possible adaptive response of the P. caerulea digestive gland microbiome at the aquaculture site, including the acquisition of putative bacteria able to deal with metal and sul-phide accumulation, highlighting the inherent microbiome potential to drive the host acclimata-tion to stressful conditions. This work was carried out in the context of the Controlling Microbiomes Circulations for Better Food Systems (CIRCLES) project, which was funded by the European Union's Horizon 2020 research and innovation program under grant agreement no. 818290.