Biofilm driven bioweathering on limestone; a humidity dependent process

Abstract Phototrophic microorganisms, particularly cyanobacteria, are involved in biogeochemical cycles that alter the environment. They participate in processes such as bioweathering and biomineralization on various substrates, specifically carbonate rocks. This study investigates the interactions of phototrophic biofilms with the limestone substrate of Pasargadae, the UNESCO World Heritage Site in Iran. The initial survey revealed bedrock alterations including microfracturing, detachment, and localized mineral dissolution related to phototrophic biofilms. Further analyses showed the dominance of cyanobacteria, particularly Chroococcidiopsis spp., in endolithic biofilms. Six months of laboratory experiments with inoculation of limestone samples with Chroococcidiopsis sp., under controlled hydration-desiccation regimes and various constant humidities (29.5–100% RH) were conducted. The results demonstrated the formation of rhombohedral crystals of CaCO 3 by Chroococcidiopsis sp., on limestone samples in the hydration-dessication period and 100% humidity. The present study links cyanobacterial activity to carbonate dissolution-precipitation dynamics and rock bioweathering in the semiarid region of Pasargadae. In addition, these findings highlight the crucial role of water availability in biofilm-driven weathering of calcium carbonate. Further research is needed to clarify microbial community-level interactions.