Carbonate dissolution without acid: carbonate hydrolysis, catalyzed by photosynthetic microorganisms, in deteriorating stone monuments
Rapidly deteriorating stone (marble) monuments are generally blamed on acid rain. We suggest an alternative, not necessarily exclusive, culprit: rock-inhabiting cyanobacteria and microalgae, which may enter via fissures and dissolve carbonates as they propagate under the translucent stone surface. Cyanobacteria and algae absorb CO2 and HCO - and, in so doing, catalyze the reaction between carbonates and water itself. The resultant calcium hydroxide is expected to react with atmospheric CO2 and with adhered dust on its way out. We tested this hypothesis at the Forbidden City in Beijing, China, by inspecting stone monuments (dolomitic marble) for telltale signs of colonization and by studying an exfoliation crust with molecular and imaging techniques. The results, reported here, are consistent. Deterioration began in, and spread from, stone joints, cracks, and shattered stone edges. A cyanobacterial biofilm visible to the naked eye was present under the deteriorating stone surface. Colonized mineral grains were dissolved in a surface-controlled manner, i.e. along crystallographic and twinning planes. Secondary calcite, as well as clay minerals, were detected in the crust.