Preserving the Danube Limes in Serbia: A Review on the Biodeterioration of Trajan's Bridge

Abstract

One of the most significant monuments of the Roman Danube Limes, which today stretches through several European countries, is Trajan's Bridge, built in the period from 102 to 105 AD. Its remains are located in today's Serbia and Romania. From the once magnificent bridge with masonry pillars that supported the wooden structure, only the pillars on the banks remain today, with the partially known preservation state of those under the water. The remains of the pillars on the Serbian side of the Danube were excavated and partially restored four decades ago. Today, the pillar that is closest to the river, which has not undergone conservation, is subject to different types of deterioration, primarily due to high levels of external influences. According to visual observation, the most pronounced types of deterioration at the monument are the loss of brick material and biological infestation of both bricks and stone, while it seems that the mortar is the least endangered. During the comparison between the present condition of the pillar and those recorded in the photos during the last six decades, we can estimate that its level of material loss did not overly change. However, the difference in the biological growth is visible comparing the present state with the one recorded close to the moment when the water penetrated into the defensive embankment area built because of the construction of the hydropower plant system and the rise of the Danube level. Having that in mind, the primary aim of this study was to record the actual degree of the infestation and endangerment of the monument using laboratory research and check the possible risks for future mortars’ deterioration in contact with severely infested bricks. Through this study, characterisation of the present biodeteriogens was performed. Special emphasis was given to the fungi as the known main agents of deterioration, so that a precise assessment of the amount of risk thriving communities pose to this monument can be made as a basis for the formulation and implementation of appropriate conservation treatments. Severe macrofouling of brick and stone by epilithic lichenised fungi from genera Caloplaca, Lecanora, Parmelia, Porpidia, Verrucaria, and Xanthoria, as well as mosses of Brachythecium, Grimmia, Pohlia, and Tortula genera, was documented. Areas of pillar lacking visible infestation likewise possess thriving microbial community characterised by the presence of fungal structures such as lichen soredia in mass, clusters of microcolonial fungi, and numerous sexual and asexual spores of melanised fungi, as well as structures of trichal Cyanobacteria, and Chlorophyta from genera Apatococcus, Desmococcus, Haematococcus, and Trebouxia. A total of 12 filamentous fungi from six genera were determined as part of the culturable mycobiome, with the dominance of phylum Ascomycota (Alternaria, Aspergillus, Cladosporium, Epicoccum, and Penicillium) and only one representative of phylum Zygomycota. Similarities between communities documented on brick, mortar, and stone surfaces estimated via Sørensen’s quotient of similarity were high, in the range of from 0.57 to 0.65, with the highest similarity documented between mortar and stone.