## 4 Discussion
### Selection of Methods Used for Assessing Photoautotrophic Growth on Plaster
For the M1 inoculation and incubation procedure, 3N-BBM medium was used instead of standard Bold's Basal Medium, containing 3-fold nitrogen concentration. Higher nitrogen levels might contribute to increased microalgal growth. [PERSON] [32] draws attention to the dependence of myxrosporine-like amino acid (MAA) synthesis on increased nitrogen bioavailability. For photoautotrophic organisms such as terrestrial algae, MAAs are a crucial component for counteracting UV radiation. Therefore, a sufficient nitrogen concentration might be necessary for the physiological performance of terrestrial algae irradiated at regular intervals. For studies conducted mostly in aquatic environments, nitrogen concentration is proven to affect algal cell density and biomass composition [33]. For the M1 method, an additional layer of solidified 3N-BBM medium on top of which technical material samples were placed could also supply higher levels of humidity inside the Petri dish, circumvent possible sample desiccation, and allow for higher nutrient availability. As a result, a much more intense growth occurred on samples inoculated and incubated with the M1 method. For the M2 method, the application of a microbial medium layer on top of the material surface can be implemented to simulate especially exposed facade areas, where dirt and dust particles start to accumulate. Furthermore, this technique visibly facilitated the process of inoculation and prevented the inoculum from spilling from the targeted surface which can be a noteworthy advantage when dealing with large sample sets. The fact that, for long-term warranty studies, repeated cycles are usually necessary, the abovementioned method can further improve the cleaning of tested surfaces between experimental cycles and lower the risk of damaging the substrates. To minimize the impact of medium layer application on the artificially changing porosity of tested substrates, alternative, superior medium layer application techniques should be explored. For all methods used, the silicone coatings exhibited on average higher resistance to microalgal growth than mineral plasters. This was visible during both research stages and is probably associated with higher hydrophobicity [3; 17; 19]. Furthermore, the color change components determined with the spectrophotometric method showed that for silicone plasters, discoloration shifted strongly toward yellow, which could be related to biofilm desiccation (increased production of auxiliary pigments) [3; 34]. Both inoculation and incubation methods could be potentially used to determine the longevity of protective coatings; however, the period of effectiveness should be further correlated with environmental results. Variables controlled in the laboratory studies will determine the warranty period of tested coatings and should faithfully represent environmental conditions typical for the geographical region where the coating will be used [7]. In order to allow for the creation of the most reliable research method enabling faithful representation of the plaster coating resistance, further research accurately correlating laboratory scale methodology with environmental tests should be performed. The performed research strongly suggests that the spectrophotometric color determination and visual assessment methods can be easily used to reliably determine the resistance of plaster coatings against photoautotrophic growth, with environmental conditions being simulated in the laboratory. With simple solutions such as water leeching, UV irradiation, and medium manipulation, the environmental conditions characteristic for different geographical regions can be imitated. However, the amount of water used, the composition of the microbial medium, as well as the time of soaking and irradiation used should be further determined in accordance with environmental studies as even small changes introduced can strongly influence the rate and severity of photoautotrophic colonization.