Lichens as weathering agents
Biodeterioration of monuments
The value of lichens on monuments
Lichen denudation on a world scale
1. Lichens as Weathering Agents
1.1 The role of lichens as biological weathering agents in the development of soils was formerly considered in a geological context only, but recent research has shown that these organisms are capable of biodeteriorating stone substrata within a relatively short time-scale.
1.2 Although the biogeography of lichens unquestionably points to their evolutionary antiquity, their fossil record is limited and fragmentary. Nevertheless, since algae, cyanobacteria and fungi certainly do have ancient origins (Taylor 1994), there is no reason to doubt that lichens were formed relatively soon after their appearance, especially since these symbiotic organisms are apparently better equipped to cope with extreme environments (Smith and Douglas 1987). Lichens are undoubtedly one of the most successful forms of symbiosis, partnerships being based on a wide variety of symbiotic interactions which are manifested in many, often unique, morphological and physiological adaptations to widely differing environmental conditions throughout the world.
1.3 The major long-term role of lichens has been as biological weathering agents, their pedogenic action being both physical and chemical in nature (Syers and Iskander 1973, Jones 1988). Despite early controversy concerning the pedogenic significance of lichens, their effectiveness in the biodeterioration of rocks has been clearly demonstrated by recent research, which has revealed that substantial quantities of substratum can be degraded even over relatively short periods of time. Furthermore, lichens have the capacity to accumulate elements, such as nitrogen, phosphorus and sulphur, thereby increasing their potential bioavailability to successive life forms. Organic material derived from lichen decomposition, together with detached particles of the substratum and atmospherically-derived dusts trapped by thalli all contribute to the development of primitive soils.
1.4 The weathering action of saxicolous lichens can be physical, due to penetration by rhizinae and expansion and contraction of thalli, and/or chemical, due to carbon dioxide, oxalic acid and the complexing action of lichen substances. The latter have a low but significant solubility in water, forming soluble metal complexes under laboratory conditions when they react with minerals and rocks, particularly limestone (Ascaso et al. 1990). Oxalic acid, formerly considered to play a minor role in the biodeterioration process (Syers and Iskander 1973), has now been proved to be of crucial importance. The widespread occurrence of metal oxalates, particularly calcium oxalate, in lichens and in nature generally, the nature of the thallus-substratum interface, and the chemical disruption of the substratum, are significant components of the weathering process.
2. Biodeterioration of Monuments
2.1 Detailed Raman spectroscopy studies at Bradford University (Edwards et al. 1991, Edwards and Seaward 1993, Seaward and Edwards 1997) have demonstrated the highly destructive properties of calcium oxalate produced by lichen thalli. Particular attention has been directed in this work towards the dramatic effects caused by the action of certain aggressive lichen species on historic monuments, frescoes and other works of art, where biodeterioration processes have been shown to be devastatingly destructive within a surprisingly short time-scale. Such action on natural substrata is clearly of significance in a pedogenic context, since lichens are usually regarded as weathering agents on a geological time-scale.
2.2 Biodeterioration studies of the lichen Dirina massiliensis forma sorediata have revealed that calcium oxalate encrustations can be produced at the thallus-substratum interface up to depths of almost 2 mm in under 12 years. It has been calculated that 135 mg of calcium carbonate is converted into calcium oxalate monohydrate at the interface of a 1 cm diameter thallus. On certain Italian Renaissance frescoes, this lichen has in many places totally obliterated more than 60% of the surface area and in such cases, 1 m2 of fresco and underlying plaster has probably been converted into more than 1 kg of calcium oxalate. Furthermore, with the incorporation of calcite and gypsum into the thallus encrustation, it is likely that more than four times this amount of the underlying substratum has been chemically and physically disturbed (Seaward and Edwards 1995).
2.3 The short-term biodeteriorative capacity of this lichen is not specific to the above-mentioned frescoes: for example, our detailed studies of the exterior stonework of some English churches have shown that D. massiliensis forma sorediata is similarly destructive of its substratum (Seaward and Edwards 1997). In the past, encrustations generated by this lichen, and no doubt by other species, have been misinterpreted as the remaining traces of a whitish coating or rendering applied as a decorative or protective surface in a 19th or 20th century restoration programme. It is now quite apparent that this is not so, since these 'renderings' consist essentially of calcium oxalate and more often than not evidence remains of the thalli producing them. These encrustations are usually more than 0.5 mm in thickness and cover considerable areas of many church walls throughout England.
Whilst acknowledging the recent direct effects of atmospheric pollution, more particularly acid rain, on such monuments, it must also be recognised that D. massiliensis
is a relative newcomer: its dramatic spread in Europe, and more particularly England, in recent years has been facilitated by new environmental regimes, including qualitative changes in atmospheric pollution, which have allowed it to dominate substrata in the wake of the rapid disappearance of other more pollution-sensitive species. D. massiliensis
is by no means the only organism implicated in short-term biodeterioration processes: lichens and indeed other micro-organisms capable of adapting to man-made environmental disturbances can be equally destructive when ecosystem equilibrium is disrupted.
3. The Value of Lichens on Monuments.
3.1 The presence of lichens on stonework is variously interpreted by the lay public and by specialists in different disciplines, whose attitudes are inevitably coloured by differing aesthetic and practical considerations. The lichenologist, for example, regards the appearance of a lichen mosaic as a natural feature of ancient monuments, the diversity of species present being aesthetically pleasing, and taxonomically and ecologically interesting as there is direct correlation between the composition of the flora and the passage of time; lichenometry has been employed for dating rock surfaces, buildings and monuments, and the different lichen communities established on them not only reflect the various materials employed in their construction but also can often be correlated to the chronology of successive building phases, therefore assisting in archaeological interpretation. Furthermore, lichens are exceedingly sensitive to environmental change, and the diversity of the flora, for example, can be a reliable indication of the level of atmospheric pollution which in itself is one of the most serious factors in the deterioration of ancient monuments. It is ironic that in a bland, homogenous urban environment, where a lichen mosaic would be a welcome relief to the eye, the higher levels of air pollution prevent its establishment and only a monotonous flora composed of a few algae and lichen crusts can exist.
On the other hand, those in fine art concerned with the conservation of ancient monuments view the encroachment of lichens from a different standpoint: inscriptions and fine details may be obscured, and depending on the nature of the substratum, serious damage is often caused through lichen-induced biodeterioration. The chemical properties of stone surfaces vary considerably, as do their lichen floras; further differences occur as a result of micro-environmental conditions and the influence of air pollutants on substrata. Other recent environmental changes by human agencies have been conducive to increasingly detrimental invasion by certain aggressive lichens, as in the case of the establishment of nitrophilous species due to hypertrophication. Such evidence would help to explain why it is that monuments, undamaged for many centuries, now appear to be vulnerable to lichen attack, in addition to the known problems resulting from air pollution.
4. Controlling Biodeterioration.
4.1 It is essential that the real causes of deterioration in any given situation are scientifically established before well-meaning, but uninformed, action is taken; for example, cleaning techniques may well accelerate deterioration in situations where lichens have not been reliably established as the prime cause of damage. Indeed, some species may well afford a protective cover, shielding the stonework from external weathering agents. It is therefore necessary to determine which species are disfiguring but intrinsically harmless, and which cause actual physical damage.
Any treatment for the removal or discouragement of lichens from stonework should be selected with care, since although immediately effective, the long-term effects may well be deleterious. Mechanical methods involving scraping and brushing, usually followed by washing, are tedious, damaging and often ineffective. Absorbed water may adversely affect the monument, particularly under fluctuating temperature regimes; penetration can be minimised by the use of water repellents, but entrapped water and rising damp can prove highly destructive. A wide range of biocides have been employed, many of which have since been rejected due to side-effects such as crystallization of soluble salts which have penetrated the stonework, staining and discoloration of monuments where the chemicals used * have interacted with particular metals in the substratum, and the promotion of secondary biological growths, which may be even more unsightly than those removed. Furthermore, regular treatments are likely to be necessary which are expensive both in terms of the chemicals used and the labour employed for the mechanical removal of only partially detached and brittle lichen growths which remain. The biocides employed may also be harmful to the operators and, not surprisingly, dangerous to wildlife. Some success has been achieved using organo-metallic compounds, quaternary ammonium compounds and borates, but the latter have proved problematic when used in air-polluted environments where, of course, many of the monuments it is desired to conserve are located. Unfortunately, it has to be acknowledged that the problem is under-researched and much of the work published to date is of a largely empirical nature which has yet to be adequately substantiated by long-term experimentation. It remains for future generations to judge the relative effectiveness of the various conservation techniques currently employed.
5. Lichen Denudation on a World Scale.
In the meantime, as demonstrated above, many lichen species function very effectively as either short-term or long-term biodeteriorators in shaping our environment. The impact of lichen weathering of rocks on a global scale has been, and continues to be, important in terms of climatic consequences and the habitabilitv of our planet: their disappearance from particular ecosystems would be critical (Seaward 1996). In some parts of the globe the results of lichen denudation are now being detected by means of remote sensing, as in the case of the disappearance of epilithic lichens over very large areas of the Canadian Shield as a direct consequence of atmospheric pollution: the barren rock surfaces now have different reflectance characteristics, as their essentially light-absorbing lichen cover no longer exists. Such losses may well have climatic repercussions and exert a measurable influence on global warming. According to Schwatzman and Volk (1989), if today's weathering were to take place under completely abiotic conditions, dramatic increases in global temperature would result.
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