Calcium oxalate and sulphate-containing structures on the thallial surface of the lichen Ramalina lacera: Response to polluted air and simulated acid rain

The formation of calcium-containing structures on the thallial surface of the lichen Ramalina lacera (With.) J.R. Laund. in response to air pollution and to simulated acid rain, was studied in in situ and transplanted thalli. In situ thalli were collected from an unpolluted site and transplanted to heavily polluted and less polluted sites for a 10 month period. Additional thalli were treated either with double distilled water or with simulated acid rain. Scanning electron microscopy and infrared spectrometry revealed that thallial surfaces of in situ R. lacera samples collected in unpolluted sites were covered with two kinds of calcium oxalate crystals: whewellite and weddellite. These aggregates of calcium oxalate crystals appear to disintegrate and provide a crystal layer on the thallial surface. Infrared spectroscopy of powder scraped from thallial surfaces of transplants, retrieved from non-polluted sites, showed the presence of whewellite and weddellite, whereas powders obtained from thalli retrieved from polluted sites contained whewellite, weddellite and gypsum. It is suggested that a certain fraction of the gypsum detected in crater-like structures in transplants from polluted sites and in thalli treated with simulated acid rain is endogenous and should be considered a biomineral.