Role of black fungi in color change and biodeterioration of antique marbles

Abstract

Blackening of rock and architectural surfaces by soot and dust has attracted attention of scientists and conservators for some time. Blackening of such substrates by dematiaceous yeastlike fungi, however, has practically never been reported so far. Black fungi and especially meristematically growing yeastlike forms have been known instead as human, animal, and plant pathogenic organisms. Recently we found and isolated such fungi in numerous instances from antique marble and historical limestone buildings at the Acropolis of Athens, the island of Corfu, and from sites in Germany, Italy, Portugal, Russia, Crimea, Spain, and Namibia. The so‐called black yeast are slow‐growing and difficult to isolate and identify. Infield observations, culture experiments on marble slices, and further analyses by SEM and EPR, we demonstrated that black fungi play an important role in the destruction of marble and limestone. They are ubiquitous on monuments under certain climatic and microclimatic conditions. The fungi can be classified using the EPR spectra of melanin and other black pigments. The EPR signals differ from strain to strain and when the fungi grow on marble. It is possible to identify the presence of fungi using only some milligrams of powdered infected marble. Further, in most cases it was possible to identify the deteriorative effect of the fungi as a physical process and not as chemical dissolution of marble through acid formation. In addition, it was possible to determine the origin of the marbles by EPR signals given by manganese. The origin of the fungi causing deterioration was determined by EPR signals, which are characteristic for a certain rock type and/or a certain fungal species. True melanin can also be separated from other black pigments. The melanin EPR signal usually correlated with the color intensity of the pigment. Interestingly, the color changes of antique marbles with time were related to small climatic changes that were caused by solar activity changes and/or microclimate change due to air pollution and atmospheric eutrophication in large and fast‐growing cities. Evidence from natural outcrops in nonpolluted or eutrophicated atmospheres indicates that global or local climate shifts are more important in biogenic color changes of monuments and natural outcrops than air pollution. In principle, it is concluded that many if not most of the color changes on architectural surfaces are caused by microbiota. A brief survey on black fungi and other microbial impacts on color changes in monuments is given.