System-Level understanding of microbial interactions in subaerial biofilms inhabiting outdoor stone monuments

Background: Many of the world’s most precious artworks are made of stone. Their irreversible deterioration due to biological attack is a worldwide concern. Microorganisms colonize outdoor lithic surfaces and develop into biofilms at the interface solid/air (subaerial biofilms, SABs), which, in turn might cause aesthetic, chemical and physical decay. Although it has been estimated that at least 99% of the world's microbial biomass exists in biofilms, the role and behavior of microorganisms within the biofilm matrix and their complex interactions with the external environment is still unknown. This work provides a pioneering and multidisciplinary research to investigate the behavior of microorganisms within the biofilm matrix for sorting out time-spatial relationships and to elucidate microorganism-EPS, interorganism, biofilm-atmosphere and biofilm-stone interactions. Methods: This work spans sophisticated molecular, chemical, physical and data modeling techniques and it is approached from two complementary angles: 1- Lab-scale study to delineate specific transcriptional responses of mono- and multi-species biofilms as well as the biofilm-stone interactions under controlled environmental conditions. 2- Real heritage case studies to investigate the shifts in the microbial community structure and function under different environmental conditions. Through comparing phylogenetic and functional diversity under different environmental scenarios, we provide evidence that any intuition gained from the lab-scale experiments is relevant to true environmental biofilms. Results: Experimental evidence suggests the existence of strong interactions between photoautotrophic and heterotrophic microorganisms in SABs, which involve complementation of physiological and biochemical functions, emphasizing optimal metabolic performance and enhanced productivity. Conclusions: The findings obtained so far will contribute to better understand the complexity of all the interactions encountered within SAB communities, and how these interactions may influence the biofilm outcome and the biodeterioration of the stone materials in a changing environment.