Comparison of bacterial community between rock varnish and rock without varnish

Rock varnish is a thin layer of iron and manganese oxihydroxides that glue different clay minerals together. It has been recently raised the interest from the geomicrobiological perspective due to the uncertainness of its formation, its abnormal Mn:Fe ratio (˜ 1 : 1 to 50 : 1) compared to the Earth’s crust (1 : 60), its oligotrophic and extreme nature. We aimed to investigate the variation of the microbial communities colonizing the varnish and non-varnish rock surfaces with regards to their mineralogical composition using a combination of molecular and geochemical methods. Three samples of rock surfaces with varnish and three without evident varnish were sampled from five sites in Matsch valley (South Tyrol, Italy), a south-east oriented valley at about 2500 m a.s.l. with a metamorphic lithology. The ICP-AES analysis showed some variations in the elemental composition among sites, while the EDS mapping showed marked differences in the surface elemental composition. The SEM micrographs showed a wide array of microbial forms colonizing the micro-landscape of the rock surfaces. The 16S-illumina run showed that the bacterial communities on the studied rock surfaces are dominated by Proteobacteria and photosynthetic taxa. There were significantly less autotrophic taxa on varnish environment. The taxonomic-indipendent analysis showed that there were variations both among sites and between varnish and non-vanish. Our findings suggest that the varnishing process results in a compositional shift of the microbial community towards an heterotrophic metabolism and by SEM pictures are visible structures that occur only on varnish sites.