How traces of pollutants in the environment modify bioremediation efficiency performed with Desulfovibrio vulgaris, and the advantage of an optimization protocol using soft chemicals

In recent years, Desulfovibrio vulgaris has been used to clean marble statues affected by gypsum black crusts. Many studies showed that “biocleaning” is very selective: chemical-cleaning products cannot distinguish the decay product, which has to be eliminated, from the original stone that must be fully preserved. Besides, they may cause direct and indirect damage to limestone artefacts. Instead, Sulphate Reducing Bacteria (SRB), as D. vulgaris, are able to remove just the decay matter. D. vulgaris uses SO42− ions (gypsum constituent) in its own respiratory chain; while, it does not interact with the calcium carbonate layer underneath because it does not need carbonate ions for its catabolism. However, “biocleaning” shows a large variability in the efficiency depending on the environment. If the cleaning results are not consistent, “biocleaning” cannot become an alternative cleaning method. Therefore, it is extremely important to uncover the reasons behind these differences, and to identify possible strategies to overcome the problem. The large variability of efficiency of “biocleaning” can be explained by the close relationship between the bacterial community and pollutants previously entrapped in the substrate. The aim of this work is to ascertain this hypothesis and identify how the toxic effects of the inorganic pollutants could be mitigated to improve the efficiency of this new cleaning method. In order to identify which kinds of pollutants were present and how they interact with the bacterial community during “biocleaning”, two different steps have been carried out. The first step consists in an environmental/ecotoxicological study that characterized the substrate and proves if inorganic pollutants could affect the bacterial survival. Toxic heavy metals (i.e. Pb > Zn > Cu) were found into the chalky matrix by ICP-AES and ICP-MS analyses and the followed toxicological kinetic studies demonstrated that they reduced the bacterial growth and activity. The second step consists of an optimization of the in situ bioremediation protocol, to create a better condition for the “biocleaning” application. A soft chemical pre-treatment was performed before the bacterial application using Tween 20 surfactant. Tween 20 (non-ionic surfactant) was able to reduce the adverse effects of pollutants, decreasing their superficial amount. Moreover, the method did not influence bacterial growth and activity since it did not have superficial charges and it did not change the pH value of the surroundings. This work proves that inorganic pollutants present in the gypsum decay layer can drastically reduce the bacterial growth and, at the same time, they decrease heavily the bioremediation efficiency. This study provides a soft chemical optimization strategy, helping to overcome the problem and to improve the “biocleaning” efficiency.