Organisms in wall ecosystems as biomonitors of metal deposition and bioavailability in urban environments

Vegetated walls are common structures in urban environments, and aiming to test the hypothesis that the biogenic crusts and plant and animal communities inhabiting these vertical surfaces can be more reliable indicators of atmospheric metal deposition than plants or animals inhabiting urban soils, we analyzed the chemical composition of the wall crusts, moss cushions (Tortula muralis) and the shells, soft tissues and feces of the stonework snail Papillifera papillaris collected in three small towns in Tuscany (Central Italy). Crusts and mosses from the same stones or bricks indicated that Cd, Pb, and Zn are the main pollutants released by vehicular traffic, while Hg and Cu probably originate from other sources. The soft tissues of P. papillaris (purged of the gut contents) showed as well higher Cd, Pb, and Zn and lower Hg concentrations at more traffic-affected sites, while data from shells and feces suggested that this species probably ingests large amounts of Al, Cr, Fe, Mn, and Pb, and avoids eating mosses. Most lithophilic elements and Pb are scarcely absorbed in the snail digestive tract and soft tissues mainly accumulate Cd and essential elements such as Cu and Mn. This study definitively confirms the extraordinary Mn bioaccumulation in P. papillaris soft tissues and reports extraordinary Mn levels also in the shell. The shells also contain unusually high Cu, Fe, and Zn concentrations and this bioaccumulation likely remains after death, potentially providing a historical record of the snail exposure to metals over lifetime.