De-calcification as an important mechanism in (bio)deterioration of sandstone of Angkor monuments in Cambodia

Abstract

Stone biodeterioration has been categorically attributed to a number of key factors, including physical, chemical, and (micro)biological ones. Though colonization by microorganisms on stone has been reported widely, the specific mechanisms involved for material destruction have only been revealed recently with the biochemical reactions and mechanisms identified, involving nitrogen and sulfur cycling reactions. This article reports a finding on the dissolution and loss of CaCO3 in the destruction of stone over time of exposure, which increases the porosity of the stone to trap atmospheric depositions, and enhance the development and growth of microorganisms because of improved water holding capacity and available nutrients comparing with the fresh stone. This sharp difference in CaCO3 contents between the fresh sandstone from the quarry and those from the Bayon temple in Cambodia illustrates the direct connection between the mineral dissolution reactions and microbial catalyzed contribution to it. Such alteration to the properties of the initial stone is fundamentally important to the colonization and development of microbiome on the stone, especially ammonia-oxidizing archaea (AOA) and bacteria (AOB), and Comammox bacteria as detected. Together with other nitrogen transformation microorganisms, e.g., dissimilatory nitrate reduction to ammonium (DNRA), the microbiome can be sustained and diversified over time. Based on this and other recent results, further verification and experimental studies are needed to confirm this mechanism in a systematic way to advance a better knowledge about stone deterioration under natural conditions, which are basic information for scientific protection and management of world cultural heritage.