## 3 Results
### Scanning Electron Microscopy Coupled with EDX Microanalysis (SEM-EDX)  
SEM-EDX was initially used for observation of the stone slabs without any preparation to detect the presence of microorganisms both in the crust and in the bedrock. On the black coating, no biological colonization was detected, but at the transversal section of the samples, putative bacteria (cocoid shape) were revealed (Figure 6a), occupying the microcavities close to the surface beneath the black layer. In other areas of the samples, cyanobacterial biofilms were detected (Figure 6b). In addition, on the surface of sample EZ4, fungal hyphae were observed (Figure 6c). Observation of surface morphology was also performed by SEM-EDX, and punctual analyses were carried out to locally characterize the elemental composition of the black crust, which exhibits strongest maxima of Si and Al, followed by Fe, Mg and Mn and in lower proportions Ca, K and Ti, thus confirming the results of XRF analysis. The black coating observed from the external surface shows a structure with mounds and depressions, where the upper parts appear smoother and more compact while the lower parts appear more crumbled. EDX performed on different points revealed that Mn is present in higher amounts on the more compact parts, while in areas that appear more crumbled, the peak of Mn is lower or not detected at all. \begin{table}
\begin{tabular}{c c c c} \hline
**Elements \({}^{\star}\)** & **Black Varnish (\%)** & **Reddish Layer (\%)** & **Bedrock (\%)** \\ \hline
**Mg** & 1.1 & 1.1 & 2.0 \\ \hline
**Al** & 8.4 & 7.0 & 3.2 \\ \hline
**Si** & 66.1 & 73.4 & 48.9 \\ \hline
**P** & 1.1 & 0.5 & 0 \\ \hline
**S** & 1.0 & 0.3 & 0.1 \\ \hline
**K** & 1.1 & 1.1 & 0.4 \\ \hline
**Ca** & 8.3 & 9.9 & 40.9 \\ \hline
**Ti** & 0.6 & 0.5 & 0.1 \\ \hline
**Mn** & 3.1 & 0.8 & 0.1 \\ \hline
**Fe** & 8.0 & 4.7 & 1.2 \\ \hline
**Ba** & 0.3 & 0 & 0 \\ \hline \end{tabular} \({}^{\star}\)Only the main elements are shown.  
\end{table} 
Table 2: XRF data (%) of the three layers of sample AV1. Examination of the black varnish in the petrographic thin sections revealed a clear micron-size laminated pattern (Figure 7a) with a curved shape and oriented parallel to the surface. This micro texture is defined as a stromatolite structure and is typical of desert varnish [17,19,27]. Mappings of the elemental composition of the black crust revealed that Mn and Fe are not homogeneously distributed, but their content varies in the different layers of the coating. Specifically, we observed that the upper part of the varnish is Mn-depleted, along with the part in contact with the bedrock, while an accumulation of Mn is present in the central area. Other elements such as Al, Mg and K are equally distributed throughout the crust. In addition, accumulation of detrital particles was observed in the depressions of the outer layer. These particles were mostly composed of clay minerals (Si, Al, K, Mg and Fe), while the Mn-rich material that characterizes the varnish is only present in the coating itself (Supplementary Figure S5). Surprisingly, no calcium was detected in the rock beneath the crust, but only silicon, which was in contrast with the results of XRF analyses. Therefore, a more thorough analysis of the bedrock was carried out through EDX punctual measurements, which revealed that the bedrock is composed of a calcite matrix with lower content of Mg in which silicate grains and silicified microfossils are embedded. Interestingly, backscattered images of the thin sections (sample AV1 is reported as an example in Figure 7b) showed that the bedrock is not homogeneous and, in the areas corresponding to the porous weathered zones previously observed with the optical microscope, the calcitic matrix (light grey) is missing except for sporadic grains, and only the silicate component (dark grey) is left. This phenomenon is clearly illustrated in Figure 7c, where elemental mapping of the weathered area of sample EZ3 is reported.