More than meets the eye: unravelling anthropic land use impacts on skin microbiota of an opportunistic amphibian species

As amphibians register population declines worldwide as a result of human-driven habitat modifications and emerging dis- eases, their skin microbiota has attracted major interest as a means of adaptation to the changing environment and a barrier against pathogens. A substantial body of research has been published on the characterization of the bacterial taxa found on amphibian skin, or their variation between species and/or life stages, as well on the interaction between amphibian skin micro- biota and potential pathogens, especially the fungus Batrachochytrium dendrobatidis, showing how bacterial skin communities can influence the resistance of amphibian wild populations to this pathogen. However, the diversity of amphibian skin myco- biota and its potential interaction with the bacterial component of microbiota is still largely unexplored. This study aimed to characterize the diversity patterns of both bacterial and fungal communities living on the skin of Bombina variegata (Linneaus 1758), and to highlight the associations between these patterns with biotic and abiotic parameters of their habitats, including the microbiota of the wetland itself. Skin swabs of about ten individuals were collected from populations of four habitat cat- egories, with varying degrees of human impact. Alpha and beta diversity indices were calculated for both communities from the 16S (bacteria) and ITS1 (fungi) amplicon variants (ASVs) retrieved. Differences across habitats were statistically tested, as well as the influence of abiotic factors and that of one community on another. We found a clear association between skin and water microbial communities in terms of relative abundance of bacterial and fungal ASVs shared between the two microbial communities. Both alpha and beta diversity indices calculated from B. variegata skin swabs were found to be statistically different across habitats, although different patterns were found for bacterial and fungal diversity. Beta diversity indices also showed close clustering of water and skin communities for each habitat and sampling site. Water pH, temperature and dis- solved oxygen proved to be statistically significant factors affecting both bacterial and fungal diversity, but, again, with different patterns between alpha and beta diversity.