Cyanobacterial biofilms in monuments and caves

Abstract

Summary: Biofilm-forming cyanobacteria are widespread inhabitants of exposed stones in archaeological and historical sites and caves. Outdoors, these phototrophic biofilms are adapted to all types of stress imposed by growth at the air-rock interface and have developed the capacity to tolerate excess solar radiation, extreme temperatures and desiccation at different latitudes. Indoors, the typology of the cave or the characteristics of confined environments strongly selects the microbial community according to light availability and air humidity. Interactions of cyanobacteria with rocky substrata serving as the source of mineral nutrients are based on the adhesion mechanisms and metabolic processes that allow the development of these biofilms. Both types of subaerial phototrophic community include cyanobacteria that support associated populations of heterotrophic populations of mostly very specialized species. The distribution of particular cyanobacterial taxa on monuments in urban or agricultural areas is related mostly to climatic conditions and the position and orientation of the hard surface with respect to water availability and air circulation. The chapter provides an overview of the more recent studies on free-living subaerophytic cyanobacteria causing discolouration and erosion of lithic faces. Emphasis is on the biodeterioration of artworks due to physical and chemical processes caused by the growth of epilithic and endolithic organisms. The methods used for studying cyanobacterial communities on rocks and buildings of historic and artistic value are summarized, with the focus on conservation issues. Study techniques which are non-invasive of the underlying substrata are essential and it is important to identify the biodeteriogens responsible for the damage.