###### Abstract  
The aim of this study was to develop a method for assessing the growth of photoautotrophs on plaster coatings, which will be used to reliably assess the resistance of these materials to photoautotrophic growth in the simulation of long-term exposure. In the course of the study, mineral and silicone plaster substrates were inoculated with a mixture of _Stichococcus bacillaris_, _Nostoc commune_, _Pseudochlorella signiensis_, and _Coenochloris signiensis_, and incubated for 28 days in model conditions. At 14 and 28 days after inoculation, the degree of photoautotrophic growth was determined using hemocytometer cell counting, a HY-LiTE 2 ATP measuring system, chlorophyll a concentration quantification, CIE La*b spectrophotometric color change evaluation, and visual assessment. The acquired results allowed us to select visual assessment and spectrophotometric color change evaluation as quick-to-perform and reliable techniques for further laboratory studies. The impact of minor changes introduced in the inoculation and incubation procedures on the rate of biofilm formation and severity of microbial fouling was studied. Differences in inoculation and incubation procedures strongly affected the results of the performed tests. Both methods have shown high potential and should be further expanded upon in environmental studies. photoautotrophic biofouling assessment; plaster coatings; methods for assessing photoautotrophic growth; biodeterioration of building materials +
Footnote †: journal: Applied Sciences  
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Footnote †: journal: Applied Sciences  
1  
conditions and intrinsic properties of the substrate, are called bioreceptivity [3]. Academic studies on the biodeterioration of building materials have allowed us to determine that for terrestrial algae the most important factors are temperature, water availability and relative air humidity, insolation, as well as the porosity and roughness of the substrate. However, it should be noted that the adaptation of algae to different environmental conditions is not yet fully understood [4]. Several studies have already expanded on the bioreceptivity of inorganic materials in response to, inter alia, photoautotrophic growth [5; 6; 7; 8; 9; 10]. Some of them point to the urgent need for further laboratory and field studies [5; 6; 7] performed in various climatic regions. This is especially true considering recent climate changes influencing the weathering of building facades [7]. Unfortunately, the laboratory procedures used to study biofilm communities on building substrates have not yet been standardized, especially for resistance tests [5; 7; 11]. Furthermore, the majority of research relates to the bioreceptivity of cultural heritage buildings and not newly manufactured facade coatings usually more resistant to biofouling. One of the most known methods against green biofilm colonization is the use of hydrophobic protective layers (e.g., silicone-based coatings) aimed at the dependence of algal growth on water availability. The effectiveness of the abovementioned solution is now also supported by the common use of antimicrobial agents, i.e., biocides [12]. These can be applied to either remove biofilms already existing on building facades or be incorporated into the coating during the production process. As typical biocides, those based on quaternary ammonium salts [13], photoactive compounds including TiO\({}_{2}\) and ZnO\({}_{2}\)[3; 14], metal salts (Ag, Cu, and Zn), and nanoparticles (AgNPs, AuNPs, and ZnONPs) [3; 15] are predominantly used. In recent years, polymer coatings with TiO\({}_{2}\) have been rapidly gaining popularity and are now commonly implemented. Such solutions can be applied to ceramic materials [3; 16; 17], mortars [18], and other materials, combining the benefits of hydrophobic coatings and photoactive biocides. For the abovementioned systems, the main antagonistic factor in relation to photoautotrophic biofilms is the photo-dependent oxidation that causes ruptures in the bonds forming between microbial cells and the colonized substrate.