Shifts in the taxonomic composition of microbial communities associated with Anemonia viridis (Cnidaria, Anthozoa) species complex tentacles along a natural pH gradient

Environments characterized by acidified pH conditions and increasing CO2 concentration select for the establishment of more resistant organisms as anemones, which may thrive under ocean acidification conditions. Interestingly, sea anemones are known for their capacity to buffer intracellular pH conditions, even when exposed to low seawater pHs, suggesting that overall physiology may not be strongly affected by ocean acidification. Here, the pH gradient deriving from volcanic activity at a shallow hydrothermal vent in the Levante Bay (Vulcano Island, Italy) was taken into consideration to investigate the effects of ocean acidification on the microbial communities associated with the tentacles of Anemonia viridis (Cnidaria, Anthozoa) collected at different pH conditions. Microbial enzymatic activities (leucine aminopeptidase, LAP; beta-glucosidase, GLU; alkaline phosphatase, AP) within anemone tissues were investigated by using fluorogenic substrates. High values of LAP enzymatic activities (from 74 and 67 mmol g-1 h-1) were detected in anemone tentacles in more acidified sites. Results from advanced generation sequencing techniques showed an overall microbial community dominated by Proteobacteria, followed by Bacteroiodota. Some differences in the taxonomic composition of symbionts were detected with a general decrease in biodiversity from less to more acidified sites. Some microbial taxa, i.e. Planctomycetota, Firmicutes and Desulfobacterota were less represented in anemones from most acidified sites. The characterization of the A. viridis symbionts was useful to provide deeper understanding of the holobiont response and its acclimation mechanism to naturally acidified environmental conditions.