New insights into the growth and diversity of subaerial biofilms colonizing granite-built heritage exposed to UV-A or UV-B radiation plus red LED light

The combined effects of UV-A or UV-B radiation and red LED light on subaerial biofilm formation were studied. Biofilms were generated using cultures isolated from natural phototrophic biofilms growing on the granite facades of the Palace of Xelmirez and the Monastery of San Martin similar to o Pinario, both historic buildings in Santiago de Compostela (NW Spain). Red LED and UV-A or UV-B radiation were tested separately and together in the same experiment, along with white LED (positive control) and no LED or UV lights (i.e. darkness, negative control). The initial target biofilms had different ratios of green algae: cyanobacteria [cells mL-1] of 100:1 (Xelmirez) and 5:1 (Pinario). Relative to the other treatments, the combination of UV-A and red LED significantly enhanced growth of the biofilm mainly comprising algae. Relative to the negative control, proliferation of Isocystis sp. and Stichococcus bacillaris also increased (by respectively 10% and 36% in Xelmirez and by 2% and 44% in Pinario), while the occurrence of Bracteacoccus minor decreased (by 47% in Xelmirez and 46% in Pinario). UV-B plus red LED also impeded the proliferation of Bracteacoccus minor (by 7% in Xelmirez and 14% in Pinario), but conversely resulted in poor biofilm growth.

Automated summary

The combination of UV-A and red LED light significantly enhanced the growth of algae-dominated biofilms, while UV-B plus red LED hindered the proliferation of Bracteacoccus minor, resulting in poor biofilm growth.