Environmental Proteomics Elucidates Phototrophic Biofilm Responses to Ornamental Lighting on Stone-built Heritage

Recent studies are showing that some lights suitable for illuminating the urban fabric (i.e. that do not include the red, green and blue sets of primary colours) may halt biological colonisation on monuments, mainly that caused by phototrophic subaerial biofilms (SABs), which may exacerbate the biodeterioration of substrates. However, the light-triggered mechanisms that cause changes in the growth of the phototrophs remain unknown. Environmental proteomics could be used to provide information about the changes in the SAB metabolism under stress inflicted by nocturnal lighting. Here, laboratory-produced SABs, composed of Chlorophyta, Streptophyta and Cyanobacteriota, were subjected to three types of lighting used for monuments: cool white, warm white and amber + green (potentially with a biostatic effect). A control without light (i.e. darkness) was also included for comparison. The nocturnal lighting impaired the capacity of the SABs to decompose superoxide radicals and thus protect themselves from oxidative stress. Cool white and warm white light both strongly affected the proteomes of the SABs and reduced the total peptide content, with the extent of the reduction depending on the genera of the organisms involved. Analysis of the photo-damaging effect of amber + green light on the biofilm metabolism revealed a negative impact on photosystems I and II and production of photosystem antenna protein-like, as well as a triggering effect on protein metabolism (synthesis, folding and degradation). This research provides, for the first-time, a description of the proteomic changes induced by lighting on SABs colonising illuminated monuments in urban areas.