Investigations on biocalcification by bacteria and its potential for limestone consolidation

Carbonate precipitation is considered to be a natural activity across a wide range of bacterial species. This process has been used to treat stone monuments for conservation purposes, using microorganisms such as the spore-forming Bacillus spp. and the slime-forming Myxococcus xanthus. This thesis investigates non-pathogenic and non-sporing bacteria for use as potential limestone consolidants and aims to develop a methodology that allows identification and quantification of the effects of bacterial biocalcification on limestone. Initially 82 bacterial strain~were isolated from calcified wood found in a freshwater stream in Somerset (England). Identification by biochemical tests and PCR allowed the selection of 3 isolates, B14, 02 and F2, by exclusion of pathogenic isolates. The experiments carried out were directed towards understanding biocalcification under different conditions of temperature, pH, as well as C02 and urea availability. F2 was shown to be the most robust isolate, precipitating the largest amount of carbonate crystals under the most extreme environmental conditions, Le. 40°C and pH 6.5. However, when the isolates were applied to limestone, 02 precipitated the greatest amount of carbonates. Light microscopy and SEM showed that crystals were formed in planar layers by bacteria, which were found in the outer layers and core of the crystals, embedded in a matrix of polysaccharides. Cell viability decreased with time. The effects of biocalcification on limestone were investigated by assessment of changes in bulk density, surface water absorption and pore space. Bulk properties were not affected, but surface properties including water absorption and colour were altered. Changes in pore space were studied by GIS, which provided not only numerical data but also visual identification confirming reduction of individual pore spaces. The results demonstrate the potential of three different nonpathogenic, freshwater isolates for use as limestone consolidants and show that GIS can be used as a valid tool to assess the effect of biocalcification on limestone.