# Microbial Depolymerization of Epoxy Resins: A Novel Approach to a Complex Challenge
## 3 Results
### Microbiological Water  
3.2.1. Observation on Older Samples from Microbiological Water  
ESEM imaging on the epoxy samples incubated with microbiological water for more than 3 years revealed a various and abundant microbial community growing on their surface. A few examples of the diverse microbes are shown in Figure 5. The most abundant type showed a quasi-spherical shape with a diameter of 8-10 \(\upmu\)m (Figure 5a-c). These cells were mostly in dense colonies connected to one another by some filamentous matrix, but in some cases, they were also present as sessile cells (Figure 5a-e). The size of these cells is on the border between typical prokaryotic cells (1-10 \(\upmu\)m) and eukaryote cells (>10 \(\upmu\)m). Therefore, it is not possible to assign it to one of the two reigns just based on size. The second most abundant type of microbes was rod-like microorganisms (Figure 5c-e). The rod structures were characterized by a length varying between 10 to 15 \(\upmu\)m and a width around 1 \(\upmu\)m.  
These dimensions are typical of prokaryotic cells (1 \(\upmu\)m), but it was not clear whether they were single rod-shaped cells or whether they consisted of more than one cell connected to each other. ESEM investigations also revealed some quasi-spherical cells growing in colonies with a far smaller size compared with the one previously described (1.5-3 \(\upmu\)m; Figure 5f,g). This size suggests that they should be prokaryotic cells. Some areas with a mixed biofilm composed of microbes with different sizes and shapes, including filamentous  
Figure 5: ESEM images of the microbiome colonizing the surface of the epoxy samples removed after more than 3 years of incubation in microbiological water. (**a–c**) The most abundant type of microbes observed in dense colonies or as sessile cells of quasi-spherical shape with crystals aggregates visible between cells, (**d,e**) rod-like microbial cells, (**f,g**) quasi-spherical cells with echinulate shape, (**h**) mixed biofilm composed of microbes of different sizes and shapes and (**i**) colony of a filamentous structure with crystals aggregates. structures, were also observed (Figure 5h). Finally, isolated filamentous structures with larger diameters were also found in well-defined colonies (Figure 5i). Crystal aggregates were also detected close to the microbial colonies. These structures were not isolated, but microbial cells and filamentous matrix was observed between the mineral aggregates (Figure 5a,d,i). To determine the elemental composition of these mineral structures, EDX analysis was performed. The detected elements in the analyzed spot were carbon, oxygen, nitrogen and calcium (Figure 6). Since the analysis revealed the presence of Ca as only detectable mineral, it is plausible that this element was part of the mineral salt composition. Mineral precipitation can occur under different conditions, but in natural ecosystems, where microorganisms are present, minerals are often involved in microbial metabolisms. Precipitation and/or dissolution of Ca-containing minerals is a widespread ability in the microbial world [34]. As previously described, filamentous matrices are present on and between the crystal structures. This could confirm our hypothesis that microbes are producing and/or exploiting these mineral aggregates. While the imaging was performed, several surface modifications, such as cracks and holes, were also noticed. Interestingly, the majority of these structures were compatible in size and shape with the microbial cells previously described (Figure 7a,b). A closer look to the epoxy surface below the microbial cells revealed some depressions with a shape similar to the microbial structures (Figure 7c,d). Additional tests should be performed to draw definitive conclusion, but these results may suggest a first step of microbiologically driven depolymerization of parts of the samples surface.