# Effects of Microbiota on the Soiling Process of PV Modules in Arid Zones of the Atacama Desert
## 3 Results and Discussion
### Soiling Mechanism
In contrast, summer was less dry and showed two peaks in frequency: one around 25% and another at 70%. Nevertheless, during this season, the RH could also reach 100%. Despite being a desert area, the RH reached 100% throughout the year. This phenomenon occurred due to local fog events called \"Camanchaca\". This fog forms from a bank of stratocumulus clouds that develop along the Chilean coast. As these clouds move into the Atacama Desert, they transform into a thick fog that does not produce rain but results in high RH values that  
Figure 7: Frequency of atmospheric parameters measured for each season in 2022 at PSDA and UA. The measurements corresponded to wind speed (m/s), relative humidity (%), and temperature (°C). can be condensed. Lastly, the T\({}_{\text{amb}}\) at the PSDA showed significant oscillations. It can be observed that in autumn, winter, and spring, the T\({}_{\text{amb}}\) ranged from \(-4\)\({}^{\circ}\)C to 35 \({}^{\circ}\)C, while in summer, the temperature ranged from 4 \({}^{\circ}\)C to 35 \({}^{\circ}\)C. This condition of the T\({}_{\text{amb}}\) was due to the area's arid and desert nature. During the day, the intense solar radiation heats the Earth's surface, which raises the temperature [52]. At night, the absence of clouds [53] causes a rapid loss of heat, which results in the temperature dropping below 0 \({}^{\circ}\)C.  
Given the atmospheric conditions and the chemical composition of soiling at both locations, it was possible to define the fouling mechanisms. At both the UA and PSDA, the first condition for cementation was met: the presence of soluble and hygroscopic material [1]. According to the XRD analyses, gypsum was present at the PSDA, and halite at the UA. The second condition was the presence of a high RH in the environment. In both locations, the RH exceeded 70%. It was demonstrated that when the RH surpassed 70%, particles, regardless of their nature, adhered to the surface of the photovoltaic glass. However, if a hygroscopic material is present, only a 20% RH is needed to condense and adhere the particles [54]. Based on this, it can be defined that the hygroscopic material found at the UA and PSDA interacted with the RH, where it dissolved partially or completely. When the temperature increased during the day, this material re-precipitated, and thus, formed crystals. This recrystallization of the soluble material created solid bridges between the soluble particles, less soluble particles, and the glass surface, which resulted in cementation. The bacteria contained on the PV panels at the UA and PSDA were capable of forming a biofilm in a 10% LB medium on standard PV glass slides as substrates. The bacteria formed biofilms, and they were easily visible after 24 h. For further studies, the biofilms on PV glass slides were harvested from 10% LB medium at 120 h (Supplementary Figure S2). The results of the biofilm metagenomic analysis revealed a prevalence of _Bacillus_, _Sporosarcina_, _Bhargawaea_, _Mesaobacillus_, _Cytokacillus_, _Calladakilab_, and _Planococcus_ genera at the UA, whereas _Peribacillus_ and _Kocuria_ were predominant at the PSDA (Figure 8). These bacteria have been typically described as biofilm formers [55; 56; 57; 58; 59; 60; 61; 62; 63]. The formation of a biofilm at the laboratory scale demonstrates the potential formation on PV panels at the UA and PSDA under favorable conditions, which contributes to microbiota as a component of soiling. This contribution may play an important role in the traditional soiling mechanisms previously studied. Figure 8: **(A)** Metacoder heatmap showing the log2 relationship of the median proportions of bacterial taxa that formed biofilms on standard PV glass. Nodes are weighted by the OTU count and colored on a gradient from the greatest decrease in median proportion (teal green) to the greatest increase in median proportion (red). **(B)** Taxonomic diversity of the biofilms. Samples obtained from the FV glass analyzed by 165 rRNA gene sequencing (Nanopore platform).