Tertiary bioreceptivity of Hontoria limestone: assessment of secondary metabolites as natural biocides

Microbial communities colonise the surfaces of monuments and buildings, contributing to stone decay. Photosynthetic microorganisms, as primary colonisers, play an important role in creating physical and chemical environments for the development of other organisms. In order to control these organisms and to prevent their proliferation, we selected three secondary metabolites extracted from fungi, bacteria, and plants, with biocide properties. Cells free culture filtrate of fungus Trichoderma harzianum T-22, cells free culture filtrate of bacterium Burkholderia gladioli pv. agaricicola ICMP 11096 strain (Bga) and glycoalkaloids extracts from unripe berries of Solanum nigrum were tested on Hontoria limestone, a biosparitic limestone, used in many Spanish monuments, inoculated with phototrophic microorganisms. The efficacy of those natural products and the microbial inhibition were evaluated by spectrophotometric determination of chlorophyll a for the quantification of photosynthetic biomass. In addition, the quantification of surface covered areas was carried out by means of digital image analysis. The glycoalkaloids showed to be the most efficient biocide. The treatment based on the cell free filtrate of Bga showed low efficiency, while the cell free culture filtrate of T.harzianum hastened the development of phototrophic microorganisms on the stone probes.