Friday, 22 May 2015

Lichens: The Natural Bio-Indicator of Air Pollution

Ajay Kumar Gautam
Department of Botany,
 Abhilashi Institute of Life Sciences,
Mandi-175008 (H.P.) India.

Lichen, a unique symbiotic association between an alga and a fungus, where algal component is called phycobiont or photobiont while fungus as mycobiont. These are the composite organisms which have an ability to colonize on a variety of substrates including rock, soil, trees and man–made structures in diverse environmental conditions. They occur in all possible environmental habitats of the world, but are diverse in tropical region and luxuriant in temperate-alpine areas. These along with mosses form dominant organism in ecosystem covering over 10 % of the earth's terrestrial habitat (Nash and Egan 1988; Hawksworth 2001).

Figure 1. A-E: Some lichens species as
 bio- indicator of different air pollutants.

Air pollution in the recent years has become a serious problem not only in developed countries, but currently it raised as a major global environmental issue mainly due to the increased fossil fuel consumption in uncontrolled and non planning manner. Moreover, the lack of proper planning to implement mitigation control measures is also a hurdle in management of air pollution. Although, the various methods used to control air pollution provide accurate and reliable data, the instruments required for such assays are expensive and cannot provide monitoring at high intensity levels across large areas at different locations. Therefore, the forecast of air pollution is one of the safe and timely measures. Numbers of physical/chemical monitoring tools, available for motoring air quality are apart not only expensive but too time consuming. Bio-indicators, in this context are one of the best, inexpensive and natural agents, forecasting the presence of pollutants in air. Lichens, the natural symbiotic organism found in almost every habitat express their growth with changed environmental conditions. Hence, can be used as pollution indicators besides of their important ethno-botanical folk uses.

Basically, lichens depend on atmospheric moisture: rain, fog and dew for growth. There are slow in growth and very sensitive towards the changing environmental conditions. Since, they absorb water and essential nutrients from atmosphere instead of from soil, hence they respond in altered manner to increased concentrations of pollutants in air. Comparison of lichens growth in polluted and healthy environment, a clear cut change in growth as well as addition or reduced growth can be observed. A number of lichens have been studied for monitoring and quantification of diverse atmospheric pollutants (Table 1 & 2). Species of Lecanora, Candelariella, Dirinaria, Buelia, Laurera, Lecanora, Trypethelium, Graphis, Cryptothecia are some of the examples of lichens indicates presence of pollutants in air. Compared with most physical/chemical monitoring tools, they are inexpensive to use in evaluating air pollution. Lichens have ability to bind toxic elemental pollutants and radioactive metals with their fungal mycelium where they concentrate them over time. Hence, they can also be used as important bio monitors to study concentrations of toxic elemental and radioactive metal pollutants in air. 

Table 1. Some lichens indicative of different levels of pollution

Highly Polluted
Hypogymnia physodes, Xanthoria parietina, Lecanora dispersa, Diploicia canescen
Moderately Polluted
Evernia prunastri, Foraminella ambigua, Lecanora chlarotera, Ramalina farinacea
Slightly Polluted
Parmelia caperata, Graphis scripta, Bryoria fuscescens, Physconia distorta
Minimal or no Pollution
Usnea subfloridana, Parmelia perlata, Degelia plumbea, Ramalina fraxinea

Table 2. Some lichens as Bio-Indicator of different air pollutants

Air pollutants
Lichens (as bio- indicators)
Pleurococcus viridis, Lecanora conizaeoides, Lecanora chlarotera, Lepraria incana, Hypogymnia physoides, Parmelia saxatilis, Parmelia sulcata, Parmelia caperata, Parmelia reticula, Parmelia reticulata, Lecidia scalaris, Lecidia expellens, Parmeliopsis ambigua, Evernia prunastri, Graphis elegans, Pseudevernia furfuracea, Usnea subfloridana, U. ceratina, U. florida, U. articulata, Lobaria pulmonaria, Sticta limbata
Anaptychia ciliaris, Lobaria pulmonaria, Ramalina farinacea, Centraria islandica
Ramalina lacera, Parmelia sulcata, Centraria islandica
Anaptychia ciliaris, Lobaria pulmonaria, Ramalina farinacea, Ramalina duriaei, Ramalina lacera, Parmelia sulcata, Centraria islandica, Umbilicaria muhlenbergii
Anaptychia ciliaris, Lobaria pulmonaria, Ramalina farinacea, Ramalina lacera, Parmelia sulcata, Cladina stellaris, Centraria islandica, Umbilicaria muhlenbergii
Anaptychia ciliaris, Lobaria pulmonaria, Ramalina farinacea, Ramalina lacera, Parmelia sulcata, Cladina stellaris, Centraria islandica
Ramalina duriaei, Cladina stellaris, Centraria islandica, Umbilicaria muhlenbergii
Anaptychia ciliaris, Lobaria pulmonaria, Ramalina farinacea, Ramalina lacera,  Cladina stellaris, Centraria islandica
Cladina stellaris, Parmelia sulcata, Ramalina duriaei, Centraria islandica
Anaptychia ciliaris, Lobaria pulmonaria, Ramalina farinacea, Parmelia sulcata, Cladina stellaris, Centraria islandica

Attanayaka ANPM and Wijeyaratne SC (2013) Corticolous lichen diversity, a potential indicator for monitoring air pollution in tropics. J Nat Sci Foundation Sri Lanka 41(2):131-140.

Conti ME and Cecchetti G (2001) Biological monitoring: lichens as bioindicators air pollution assessment- a review. Environmental Pollution 114: 471-492.

Hawksworth DL (2001) The magnitude of fungal diversity: the 1.5 million species estimate revisited. Mycological Res 105: 1422–1432.

Nash TH and Egan RS (1988) The biodiversity of lichens and bryophytes. In: Lichen, Bryophytes and air quality eds. Thomas Nash III & Vilkmar Wirth. Bibl. Carmer in der Gebr. Borntra. Verlag. Berlin, Stuttgart. Lichenol 30: 11-22.

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