Oxalic acid/oxalate lichens

Experimental studies of mineral weathering rates in the presence of oxalic acid demonstrate the importance of LPD. For example, in the presence of 1 mM oxalic acid, rates of silica elution from feldspar can increase up to 15-fold at circumneutral pH, while A1 elution rates can increase by two orders of magnitude (Barker et al, 1997). Similar results are reported for quartz and olivine (Grandstaff, 1986 Bennett et al, 1988), and indicate that oxalate leaching of aluminium, calcium, magnesium and other cations from primary silicate minerals can yield a silica-rich residue similar to that found in association with endolithic lichens (Johnston Vestal, 1993 Lee Parsons, 1999). 

Algae, lichen and fungi release citrate, oxalate and acetate. Humic and fulvic acids are probably the main Al ligands in most natural waters, and in their absence Ah+ predominates below pH 4.5, Al fluorides (AIF " ") from 4.5-5.5 and hydroxo-Al species at higher pH values (Bi etal. 2001b). The most soluble Al forms in acidic soil are nonsiliceous, organically bound species. 

Lichenin is obtained from various lichens by extraction with boiling water, forming a jelly on cooling it is oxidized to oxalic acid byHNOi is colored yellow by iodin and is precipitated from its solutions by alcohol. 

Garty, J., Knnin, P, Delarea, J., and Weiner, S. Calcium oxalate and sulphate-containing structures on the thallial surface of the lichen Ramalina lacera response to polluted air and simulated acid rain. Plant, Cell and Environment, 25 1591-1604 (2002). 

A very modern ecological aspect of lichens is biodeterioration, which means the deterioration and destruction of ancient frescoes, coloured windows of cathedrals and old buildings by incrustation with lichens. The lichen Dirina massiliensis Durien et Mont, has already destroyed the painted frescoes of the Palazzo Farnese in Caprarola, Italy. Similar damages have been noted on Greek sculptures and on the Arch of Trajan. Fourier transform microscopy has shown that the lichens produce oxalic acid, which converts the calcium carbonate of the substrate to calcium oxalate, changing the structure of the surface (Edwards et al. 1993). 

Jayaprakasha and Rao (2000) extracted with benzene and acetone the lichen Parmotrema stuppeum in order to investigate its antioxidant effect. Both the extracts were fractionated on 1 % oxalic acid impregnated silica gel column to obtain methyl orsenillate, orsellinic acid, atranorin and lecanoric acid, respectively. Antioxidant activities of benzene extract, acetone extract and isolated compounds were evaluated in a carotene-linoleate model system. The obtained results showed that the pure compounds and extracts have moderate antioxidant activity. 

Lichens produce a variety of chemical compounds which are of potential importance in the biogeochemical weathering of minerals and rocks. Of these compounds, carbon dioxide (CO2), oxalic acid, and a large group of substances frequently referred to as lichen acids, but called lichen compounds herein, are the most significant. 

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