Microbial biodeterioration of outdoor stone monuments: Assessment methods and control strategies

Biodeterioration is the least understood decay mechanisms of outdoor stone monuments. Microbial colonisation is largely determined by the properties of the stone and environmental conditions. The literature on microorganisms on outdoor stone monuments and their decay mechanisms was reviewed. For the assessment and quantification of microbial deterioration, methods that can be carried out by cultural heritage conservators with limited microbiological skills were selected and adjusted for the application on outdoor stone monuments. To this end, the total biomass was quantified by a protein assay (Folin-Lowry method), its phototrophic contribution through chlorophyll a absorbance and the amount of extracellular substances (EPS) were assessed by carbohydrate quantification (phenol method). Microbial activity was measured through two different enzyme assays: fluorescein diacetate cleavage and dehydrogenase activity (INT reduction). In order to develop a long-term monitoring strategy, these parameters were tested in the morning (8 am) and in the afternoon (4 pm) on biofilms from a sunny and a shady sampling site on a limestone wall in the south of Mexico. The experiments were performed in the dry season and the rainy season. Changes in biofilm composition and activity during the day were very small, while seasonal changes were more pronounced. The largest differences could be seen in samples from the different sampling sites (sun and shade), where the microbial population had established over years of distinct environmental conditions. Variations in biofilm composition and activity exceeding such natural variation may indicate the necessity for an antimicrobial treatment. The choice of an antimicrobial agent is difficult and the ideal treatment does not exist. Of the various chemical antimicrobial agents tested (Mergal K14, Parmetol DF12, Troysan S97, Preventol R50 hydrogen peroxide and ethanol) on microbial biofilms on stone, ethanol (70%) was the most effective, as revealed by ATP measurements. A flexible, non-invasive in vivo system, employing the bioluminescent bacterium Vibrio fischeri, was developed to assess sub-lethal effects of antimicrobial treatments and to test combined treatments for synergy. Various biocides and ultrasound (267 kHz, 20 kHz), alone and in combination, were tested for their effect on V. fischeri (Mergal K14, Parmetol DF12, Troysan S97, Preventol R50 hydrogen peroxide and ethanol) and a microbial biofilm on stone (Troysan S97, Preventol R50 and ethanol). The tests did not reveal synergistic effects however, a systematic, comprehensive study on chemical and/or physical methods might reveal an innovative approach towards a more environmentally friendly microbial eradication method for outdoor stone monuments. Long-term monitoring of the composition and activity of a microbial biofilm may provide data to determine if an antimicrobial treatment is necessary. If an antimicrobial intervention cannot be avoided, low-toxic substances, such as ethanol, should be considered first. For the evaluation of the success of an antimicrobial treatment, ATP measurement has proven to be a reliable and simple method that does not require specialised skills