Biomonitoring of atmospheric Hg indoor pollution: comparison of the accumulation efficiency of three different bioindicators

Biomonitoring technique, i.e. the use of biological specimens as a tool to monitor air quality, represents a simple, efficient, and economic method to assess the presence of airborne persistent pollutants, in particular heavy metals. Both living and transplanted organisms, such as mosses and lichens, or plant portions, like tree barks or leaves, may be used as bioindicators. As a matter of fact, tree tissues are not commonly used as bioindicators as compared to other more classical bio-substrates. Tree barks are potentially very efficient for the accumulation and retention of aerosol particles because of structural porosity. In this study, we tested the efficiency of Hg bioaccumulation of three different types of bioindicators: tree barks (Pinus nigra J.F.Arnold), lichens (Pseudevernia furfuracea L.), and mosses (Hypnum cupressiforme Hedw.). In particular, the efficiency of barks in retaining pollutants was compared with lichens and mosses ones, two classic and well-known organisms widely used as biomonitors. As pollutant we selected mercury (Hg) since it is possibly the ideal element to be monitored through barks because a) it is generally scarcely bioavailable i soils; b) its presence in the bark is mainly ascribable to the atmospheric transport c) it could be retained in tree barks both physically and chemically. Here, barks, lichens, and mosses have been exposed in the Central Italian Herbarium (Natural History Museum of the University of Florence, Italy) where recent studies proved high level of airborne Hg, both as gaseous elemental Hg and as particulate bound Hg. The three bioindicators were collected from an unpolluted site in the Appennino Pistoiese (Cutigliano, Italy) to avoid Hg environmental contamination: lichens and mosses were taken from the bark of fir trees at approximately 1.5 m above ground level, while pine barks were sampled at the same height directly from the trees. Once in lab, samples were cleaned and stored at -20°C until the exposition and then placed for six weeks in four Herbarium’s rooms. The total Hg of each bioindicators was measured by a direct Hg analyzer (Milestone DMA-80 evo, Department of Earth Sciences, University of Florence) before exposure (zeromeasurement), and after three and six weeks. After only three weeks of exposure, all bioindicators showed a high Hg-uptake, with concentrations (μg/kg) almost double compared to the starting conditions and accumulation percentages mostly comparable: the differences in Hg-uptake are probably ascribable to the morpho-physiological properties of the bioindicators and to their capacity to intercept airborne particles. The highest Hg accumulation (μg/kg) of all the bioindicators was reached in the room mostly used by Herbarium staff for normal work activities, i.e. the handling of packages containing plant samples for consultation and cataloguing. These actions probably produce a resuspension of atmospheric particulate matter, suggesting that Hg accumulation in the bioindicators could be related mainly to Hg-rich particles rather than to the gaseous Hg species. At the end of the six weeks of exposure, mosses and lichens showed about the same amount of Hg (μg/kg), while the concentrations accumulated by the barks, although lower, are correlated with the Hg concentrations of the other bioindicators. The results of the study clearly indicate a high Hg-uptake capacity of all bioindicators, with different accumulation rates based on the organism used and the atmospheric indoor conditions of the different Herbarium’s rooms: despite this, the performances in the Hg-uptake are comparable for barks, mosses, and lichens. Based on our knowledge, this is the first time that three different bioindicators are simultaneously used to assess their Hg-uptake efficiency in an indoor environment: from this point of view, the peculiar indoor conditions of the Herbarium give an excellent opportunity to check their performance