Stone Consolidation by Bacterial Carbonatogenesis: Evaluation of in situ Applications

Abstract

Precipitation of calcium carbonate minerals by bacteria, the so-called bacterial carbonatogenesis, is a promising method for the consolidation of decayed stone. Despite extensive laboratory testing that has demonstrated the efficacy of this method, little is, however, known regarding the medium-and/or long-term performance of this bacterial conservation treatment once applied in situ, on stone buildings. Here, we report on the evaluation of the performance of bacterial consolidation treatments applied in three different historical buildings placed in an urban environment and built using a highly porous, easily decayed calcarenite stone. Peeling tape tests show a significant long-lasting (up to four years) strength improvement following treatment with either a Myxococcus xanthus bacterial culture or a sterile nutritional solution that selectively activates the carbonatogenic bacteria inhabiting the stone. Total color changes, measured before and after treatment using a spectrophotometer, are systematically below the acceptable value of ΔE ≤ 5. Culture-dependent analysis of the microbiota shows that 100% of the culturable bacteria collected before and after treatment is carbonatogenic, and the total count of fungi spores remains constant or diminishes, while the population of acid-producing bacteria decreases over time after treatment application. Culture-independent microbial analyses show that no deleterious microbiota develops after treatment, being carbonate-producing Proteobacteria, Firmicutes and Actinobacteria the most abundant phyla both before and after treatment. Overall these results show that the in situ application of the bacterial consolidation method shows no detrimental side effects and is highly effective in the medium-and long-term.

Protection and consolidation of stone heritage by self-inoculation with indigenous carbonatogenic bacterial communities

“…In contrast, the efficacy of the two conventional bacterial consolidation strategies tested here (the M . xanthus and M-3P treatments) that were successfully used in situ in the past to consolidate less damaged calcarenite stones 3 , 18 , 19 , were ineffective in the case of this extremely salt weathered calcarenite stone (Fig. 1 ), and in the presence of highly saline solutions present in these porous stone blocks.…”

“…Landmarks of the world’s cultural heritage, like the pyramids and the Sphinx in Egypt, the cathedrals in Europe and America, and the Maya temples in Mesoamerica, built of or carved out of stone, are inexorably crumbling due to physical, chemical and biological weathering 1 – 3 . This gradual and irreversible deterioration has accelerated over the last century and is expected to continue at an even higher rate in the near future, due to increasing air pollution 4 and enhanced salt damage associated with climate change 5 .…”

“…This conservation strategy takes advantage of bacterially induced calcium carbonate precipitation to cement carbonate rocks, a phenomenon that is widespread in natural environments such as soils, caves, lakes, and oceans 14 – 16 . Two main strategies for decayed stone conservation that have been previously presented 3 , are also tested and presented here. The first consists of the application to weathered stone of a nutritive solution along with exogenous 8 – 13 , 17 – 19 or stone-isolated 20 single bacterial cultures with proven carbonatogenic ability.…”

“…The first consists of the application to weathered stone of a nutritive solution along with exogenous 8 – 13 , 17 – 19 or stone-isolated 20 single bacterial cultures with proven carbonatogenic ability. The second consists of the application of a sterile nutritive solution that aims to activate the carbonatogenic bacteria from amongst the microbial community of the stone 3 , 18 , 19 , 21 – 23 . Both strategies result in the formation of calcium carbonate cements that protect and consolidate decayed stones.…”

“…A limitation of the first strategy is that its effectiveness depends on characteristics of the stone substrate (e.g., porosity, mineralogy, level of degradation), the presence of soluble salts, the type of microorganism and bacterial load applied, and its interaction with indigenous microbiota 3 , 12 , 13 . Note that exogenous bacteria or stone-isolated single bacterial cultures are likely at a competitive disadvantage with respect to the microorganisms already present in equilibrium and/or better adapted to the local microenvironment in the stone.…”