## 4 Discussion
### Selection of Methods Used for Assessing Photoautotrophic Growth on Plaster
Such a solution can also reduce the risk of not observing changes on the surface of the material despite the actual presence and cellular activity of microorganisms. For the performed analysis, a 5-point scale for the fouling degree assessment was incorporated from European standards PN-EN 15457 [28], originally dedicated to the effectiveness of protective coatings against the growth of fungi. In the European standard PN-EN 15458 [20] normative method applicable to the assessment of the effectiveness of protective coatings against algae growth, only a 3-point scale is used. Given the low complexity and the speed of the analysis, the visual assessment method remains especially suitable for large sets of samples tested and at the same time can be easily reinforced with other analytical methods. Additionally, macroscopic visual changes affecting colonized building substrates are most directly related to functional biodeterioration [3]. To improve on the visual growth assessment method, a more precise scale determining both the microbial growth area and the intensity of biofilm formation should be further examined. The results acquired with spectrophotometric color change measurements strongly correlated with the observations made during the visual assessment. Additionally, the mean scope of the changes corresponded to the mean \(\Delta\)E values acquired during other laboratory and field studies [8; 27; 31]. The effectiveness of the method used depends to a large extent on the number of measurements for each of the tested samples, which should be as large as possible, especially in the case of porous materials that show differences in color between different areas of the surface of the technical material. Differences in the color parameters will also depend on the species of microorganisms used in the inoculation mixtures and the conditions of incubation. In order to maintain the high accuracy of the method, it is recommended to use and compare changes occurring under the influence of an inoculum mixture developed always with a consistent taxonomic composition and in constant incubation conditions. Regardless of the differences in specific color components, the numerical representation of the overall color change (\(\Delta\)E) allows the assessment of the intensity of algae growth and the degree of fouling of the material. The threshold levels determining the interpretation of numerical values should be determined experimentally for different coating types. With a proper assessment model and the technological advancements of spectrophotometric tools used by manufacturers, further automatization of the analysis is possible and should facilitate the testing of large numbers of samples and their variants. Based on the analysis above, the visual assessment technique in conjunction with spectrophotometric CIE Lab color change were selected as the methods most suitable for further experiments. The conducted studies confirmed that both the visual evaluation and spectrophotometric determination of color change (performed with the CIE L*a*b color system) can be successfully used for the assessment of microbial growth on plaster samples tested for resistance against photoautotrophic fouling. At the same time, the acquired results prove that even relatively small changes introduced in the sample inoculation and incubation procedure might strongly affect the results of the performed tests. Differences visible in the acquired values suggest that the M1 method allowed for easier surface colonization and biofilm formation than the M2 method. Given the same species used for the inoculation procedure, the equal cell density between inoculation mixtures, and identical incubation conditions (i.e., temperature, relative air humidity, irradiation time, and intensity as well as incubation time), the factors differentiating the two methods tested were the composition of the medium used during inoculation and the presence (M1) or lack of (M2) a solid microbial medium poured into Petri dishes.