Chromatic bio-indicators of biofilm community structure on stone heritage

Colorful microorganisms in form of subaerial biofilms (SABs) colonize the surfaces of outdoor stone monuments worldwide. These colored patinas, beyond being an esthetic issue, could indeed have a deteriorative, neutral, or even bio-protective role for the stone. Since color is the hallmark of all SABs, we argue that the SAB spectral reflectance can be a way to obtain information on the effect of a SAB on the underlying stone. To this end, the funded project “CHROMA(Bio)” wants to develop a simple decision-making tool based on SABs’ spectral profiles (the color) to guide heritage professionals in making the appropriate choice to remove or keep the SABs on lithic surfaces. Thus, the color fingerprints of SABs can be used as a (bio)indicator of SH susceptibility, being associated with the deteriorative, neutral, and bioprotective roles of the biofilms. Knowing the composition of a SAB is the first step in understanding the effects it can have on the lithic substrate. The correlation between color and SABs has been widely studied to analyze early biological colonization, substrate bio-receptivity, and to evaluate the effectiveness of stone- cleaning treatments. However, the main studies from the literature are all performed through the CIELAB color space coordinate system, and classical colorimetry may be insufficient to obtain precise compositional information. As far as we know, nobody has used spectral reflectance data to obtain information on the composition of the SAB community. The present work describes this approach, presenting studies about the correlation between the structure of the active bacterial community and the reflectance spectra of SABs. Preliminary results from a measurement campaign on tombstones are here presented. For each SAB sampled on different graves, we acquired the reflectance spectra and collected biomass for the analysis of the metabolically active bacterial community. Even though more in-depth studies need to be performed, the preliminary statistical analysis suggests a correlation. The proof of concept about the correlation between spectral reflectance and SAB community composition can be the starting point for a more in-depth evaluation of the role of the SAB on the lithic substrate, which will serve as a bioindicator for heritage professionals to determine the SABs’ impacts on the stone.