Chlorophyll fluorescence for evaluating biocide treatments against phototrophic biodeteriogens

T o optimise cleaning procedures and conservation strategies for stone monu- ments exposed to microbial colonisation, we developed a microalgal test system based on chlorophyll a fluorescence analysis to evaluate the effectiveness of biocide treatments. In comparison with conventional methods, the proposed system is very fast, safe and easy to apply. The system has been tested using as target samples cultures of strains isolated from stone monuments, artificial biofilms developed on small Marble slabs, as well as natural biofilms occurring on marble statues and in which the predominant microorganisms were the phototrophic ones. The samples were treated with different amounts of two biocide products widely used in stone monument restoration treatments, Neo Desogen and Metatin N58-10/101. As reference we used the herbicide Diuron (DCMU). To evaluate the biocide effectiveness we measured the two fluorescence parameters Fv/Fm (maximal quantum yield) and ∆F/Fm' (effective quantum yield of photosystem II). We observed a progressive reduction of Fv/Fm and of ∆F/Fm' up to the complete inhibition of the latter with the increase of biocide concentrations. This allowed a punctual detection of the biocide con- centration effective against the phototrophic biodeteriogens. Once the complete inhibition of ∆F/Fm' was obtained, none of the tested samples showed to recover. Data obtained from artificial and natural biofilms were in agreement with those obtained on the cultures of the isolated strains. The results indicate that the microalgal test system based on chlorophyll a fluorescence analysis is a powerful tool for a rapid and not-destructive control of the bio- cide effectiveness during stone monument cleaning procedures and restoration treatments.