Microbial Formation of oxalate films on monument surfaces: bioprotection or biodeterioration?

Oxalate ® lms observed on stone monument surfaces deserve greater interest because of their possible role in protecting against deterioration. Their origin remains controversial. We present here the results of research conducted on production of oxalic acid and other organic acids by bacterial communities isolated from two monuments. Both communities were developed in vitro, and oxalate production was evaluated in a context of global metabolic activities that could eventually lead to protection or to degradation of the surface itself. HPLC analyses of organic acids production revealed that all mixed cultures produced oxalic acid but in different amounts. Besides oxalic acid, other organic acids are released that can solubilize stone calcium carbonate and have a deteriorating activity. Calcium carbonate solubilization, evaluated both by mixed cultures and isolated strains, was stronger with mixed cultures than with single strains. Our data show that oxalate production is promoted by the bacterial communities inhabiting the monument surface: Oxalate, being a minor representative among the organic acids released by the microbe cultures in a relatively short-term analysis, could form insoluble calcium salts that progressively accumulate. Keywords biodeterioration, bioprotection, microbial oxalate production, oxalate bio® lms