Close but different: Metabarcoding analyses reveal different microbial communities in ancient Roman nymphaea

In this study, we used a metabarcoding approach to characterize the prokaryotic and eukaryotic communities colonizing two adjacent but structurally different Roman nymphaea in the archaeological park of Baia (Phlegraean Fields, Campania region, Italy). These two environments likely belonged to a thermal complex and are still mostly unearthed because of the effect of bradyseism. This condition, together with infiltrations by underground brackish water, exposed the sites to different degrees of biodeterioration. We sampled 17 biofilms across the two sites and found differences in microbial communities both between sites and within one site. No particular patterns of diversity were detected when considering the composition of the substratum. At finer taxonomic level, the influence of marine aerosols and submerged water was highlighted by the presence of several marine taxa. Moreover, we found some bacteria previously reported elsewhere from thermal vents or deep waters, confirming that cave environments often harbour extremophiles. Overall, this study provided a detailed overview of whole community colonizing the nymphaea and confirmed the metabarcoding approach as a powerful tool to gather taxonomic information useful for restoration purposes.