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Cyanobacteria (blue-green algae) are a highly adaptable and widely-distributed group of prokaryotic organisms, which grow and survive in various terrestrial and aquatic habitats. As epiliths or epilithobionts, many cyanobacteria successfully colonize and grow on exposed surfaces of lithic (stone-built) temples, monuments and buildings, where they encounter frequent desiccation (water-deficit stress) and other abiotic stresses. They comprise major and dominant component of sub-aerial biofilms or crusts on exposed lithic surfaces, and are implicated in unaesthetic discoloration and biodeterioration of temples, monuments and heritage structures. The present study focuses on the occurrence and accumulation of intracellular free proline, a proteinogenic amino acid involved in desiccation/drought tolerance in a wide range of organisms, in cyanobacteriadominated crusts/biofilms (cyanobacterial crusts) sampled from 11 lithic temples located in the Garhwal region of Uttarakhand. The analysis of species composition of the crusts/biofilms showed high cyanobacterial species richness comprising, in total, 22 species of 17 different genera. Proline, in the range 11- 27 µg/g crust biomass, occurred in all the cyanobacterial crust samples when estimated following their hydration for 24 hours. When the hydrated crusts were subjected to desiccation (water-deficit stress) for 24 hours, there was 48% to 77 % increase in intracellular proline content. This study provides insights in to the physiological response of crust-forming epilithic cyanobacteria to desiccation stress, which they face during their growth and survival on exposed lithic surfaces, as well as in to their eco-physiological adaptation. [ABSTRACT FROM AUTHOR]

Cyanobacteria are a morphologically diverse, remarkably adaptable and widely distributed group of photosynthetic prokaryotes, many of which colonize, grow and survive on/in water-limited and nutrient-poor lithic substrates, such as natural rocks/stones and walls of historic monuments and buildings as lithobionts or lithobiontic organisms. They are often the primary colonizers of lithic substrates. They possess protective mechanisms against various abiotic stresses, such as desiccation, high irradiance, high levels of UV-radiation and high temperature which are often encountered on exposed rock surfaces and external walls of lithic monuments and buildings. The biocolonization and growth of cyanobacteria as lithobionts are influenced by various properties of monuments, buildings or building stones as well as by environmental factors. As photoautotrophs and primary colonizers of lithic substrates, they facilitate and promote the growth and development of heterotrophic microbes, such as bacteria and fungi. The production of extracellular polymeric substances (EPS) leads to the formation of cyanobacteria-dominated phototrophic biofilms or sub-aerial biofilms on exposed surfaces of monuments, buildings and rocks/stones. Globally, thebiocolonization, growth and activities of lithobiontic cyanobacteria and other organisms cause unpleasant discoloration, biodeterioration (bioweathering) of monuments and buildings of historical, cultural or religious importance, leading to the aesthetic damage, structural damage and economic loss. These constitute serious problems world-wide. The article provides an overview of the processes of biocolonization and biodeterioration of monuments by lithobiontic (rock-inhabiting) cyanobacteria, and their survival as lithobionts. [ABSTRACT FROM AUTHOR]