Microbiota of the therapeutic Euganean thermal muds with a focus on the main cyanobacteria species

The Euganean Thermal District has been known since Roman times for the therapeutic properties of peloids, obtained from natural clays that have undergone a traditional maturation process. This leads to the growth of a green microbial biofilm with Cyanobacteria and the target species Phormidium sp. ETS-05 as fundamental components for their ability to synthetize anti-inflammatory molecules. Currently, in-depth studies on the microbiota colonizing Euganean peloids, as in general on peloids utilized worldwide, are missing. This is the first characterization of the microbial community of Euganean thermal muds, also investigating the effects of environmental factors on its composition. We analysed 53 muds from 29 sites (Spas) using a polyphasic approach, finding a stable microbiota peculiar to the area. Differences among mud samples mainly depended on two parameters: water temperature and shading of mud maturation plants. In the range 37–47◦ C and in the case of irradiance attenuation due to the presence of protective roofs, a statistically significant higher mud Chl a content was detected. Moreover, in these conditions, a characteristic microbial and Cyanobacteria population composition dominated by Phormidium sp. ETS-05 was observed. We also obtained the complete genome sequence of this target species using a mixed sequencing approach based on Illumina and Nanopore sequencing.