![]() Direct and indirect evidence for the biogenic Mn coatings, such as crusts, and nodules, can occur in various environments on the Earth's surface and subsurface 3. Even if Mn occurs in seven different oxidation states (from 0 to + 7), manganese interacts as reduced soluble or adsorbed Mn 2+, insoluble Mn 3+ and Mn 4+ oxides in the natural environment 3. ![]() The geochemical behaviour of Mn differs based on the oxygen profile 2. The manganese oxides control the availability of several elements because of their strong oxidizing capacity. It represents a crucial trace nutrient for the growth and survival of numerous living organisms, contributing, for instance, to oxygen production, redox reactions, or protecting cells from toxic metals, UV radiation and oxidative stress 1. Manganese is the second-most abundant transition metal available on the terrestrial surface and covers 0.1% of the Earth's total mass. Overall, the findings on structure, microbiota, and predicted metagenome functions showed a similar microbial contribution to epigean and hypogean black deposits Mn metabolism. Genes coding for essential Mn metabolism were present in all samples. The predicted metagenome functions analysis revealed diverse metabolic profiles of the Cave and Gorge groups. However, the communities of all samples shared the presence of uncultured ASVs belonging to the Gemmatales family (Phylogenesis: Gemmataceae Planctomycetes Planctomycetota Bacteria). The amplicon sequence variants (ASVs) analysis at the different taxonomic levels showed a diverse genera composition. The 16S rRNA metabarcoding alpha-diversity metrics showed similar Simpson's and Shannon indices and different values of the Chao-1 index. Micrographs showed that samples presented microbial biosignatures and microanalyses highlighted Mn concretions and layers on Al-Si surfaces. Based on 16S rRNA sequencing results, PICRUSt software was used to predict metagenome functions. Metabarcoding of the V3–V4 regions of 16S rRNA was used to investigate Bacteria and Archaea communities. Scanning electron microscopy coupled with energy dispersive spectroscopy was used to analyse the presence of microbial biosignatures and the elemental composition of samples. This study revealed how Bacteria and Archaea communities and their metabolic functions differed between two groups of black deposits identified in gorge and cave environments.
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