5:15 PM - 6:30 PM
[BCG09-P04] Lichen-substratum interactions in severe environment polluted by heavy metals
Keywords:abandoned mine site, weathering of slag, fruticose lichen, absorption of heavy metals, biomarker
Although, no studies have investigated interactions of fruticose lichens and the corresponding substrata comprehensively from the aspect of biogeochemical behavior of heavy metals. Therefore, several fruticose lichens, including Stereocaulon exutum and several Cladonia Spp., from contaminated abandoned mine sites and the corresponding substrata were investigated (1) to determine the behavior of heavy metals during the weathering of slag mediated by S. exutum, (2) to determine the distribution of the heavy metals in the thalli of S. exutum, and (3) to determine the correlations between the heavy metal concentrations of lichens and those of the corresponding substrata.
The slag that is a substratum of S. exutum consists primarily of willemite, fayalite, and/or magnetite and contains matte drops, which are mainly Cu-metals, -alloys, and -sulfides. The willemite and matte drops are ultimately converted to Fe-hydroxides during the weathering process. In addition to abiotic weathering, the heavy metals are dissolved during the biotic weathering by substances from the lichen and hyphal penetration. The dissolved heavy metals are absorbed into the lichen thalli. Absorbed Cu and Zn are distributed within the cells of hyphae, whereas Fe and As are distributed on the surface of hyphae. Fe-hydroxide-like materials are occur on the surface of hyphae (Fig).
Based on previous studies, cations in thalli are distributed into four fractions, e.g. the intercellular and surface, ion exchange site, intercellular, and residual fractions. Although the form of the ions was not identified in this study, the distribution of elements in the hyphal cells may indicated the possible absorption of ions into the cytoplasm through ion exchange sites from external solutions. For the Fe and As concentrated on the surface of hyphae as Fe-hydroxide-like materials, this distribution could be explained by elemental precipitation or the formation of compounds on the hyphae.
The concentrations of Cu, Zn, As, and Pb of Cladonia Spp. thalli were positively correlated with those of the corresponding substrata. Distribution maps of the average heavy metal concentrations of the lichens and the corresponding substrata were made to determine the practical applications of the lichens as a biomarker. The maps for the distribution of Cu, Zn, and As in the lichens had very similar distributions to those of the corresponding substrata at the scale of all study sites in southwest Japan. Therefore, a large-scale analysis of lichens with many samples successfully detected the distribution of heavy metal pollution of soil.
In conclusion, C.ladonia Spp. lichens can be used in practical applications for biomonitoring and assessment of heavy metal pollution of soil. Because lichen is a pioneer organism in polluted areas by heavy metals worldwide, the investigation of interactions between lichens and substrata could contribute to determine the elemental cycle between biosphere and lithosphere during natural recovery process of polluted areas.