Growth strategy of aerial green algae on building materials in the temperate climate zone and its relevance to substrate biodeterioration.

Aerial green algae colonising building materials in temperate climates form complex biofilms that contribute to the deterioration of man-made structures, reducing their aesthetic value. The present study seeks to address whether algae, already at an early stage of colonisation, can adversely influence building substrates, contributing to biodeterioration beyond mere aesthetic impact. Early-stage colonisers - Chloroidium saccharophilum, Klebsormidium nitens, Bracteacoccus minor, Diplosphaera chodatii, and Stichococcus bacillaris, were isolated from brick and plaster walls, identified using traditional and molecular methods, and grown on experimental brick and plaster substrates. Over six months, algal growth in laboratory and environmental conditions was visualised and monitored by measuring chlorophyll a concentration, fluorescence intensity, and substrate profile height. All taxa were able to grow on both types of substrates at their surface and in the interior part. Most of them formed compact biofilms that filled substrate pores and overgrew the cavities, with some producing membrane-like layers (K. nitens and S. bacillaris). Algae with thick cell walls (C. saccharophilum, D. chodatii, S. bacillaris) and filamentous K. nitens detached the mineral grains from the brick surface during biofilm separation, as a consequence of prolonged colonisation over time. Except for B. minor, all taxa contributed to the geophysical biodeterioration of brick.