Lichens as Bio - indicators

Just look around, on tree barks, rock surfaces and even in soil you can find them easily, called as lichens. They are not just single organisms but is a symbiotic association that exist between fungi and one or more species of algae or cyanobacteria. Fungus, mycobiont, mostly can be a ascomycete or rarely can be a basidiomycete protect symbiont which reside inside, from the direct light and from dessication.  
Algae or cyanobacteria, phycobiont, in turn provide fungus the photosynthates. The lichen body which is also called as thallus is composed of different layers made of parts of fungus mycelium and cells of algae, depending on the differences between these layers there are mainly three types of lichens. Foliose, fruticose and crustose lichens, out of which the most sensitive to the environmental pollutants is the foliose.   The bio-indicative nature of lichens arise due to the fact, lack of a cuticle as in plant leaves. Therefore absorb all substances. Some gases present in the air and various contaminants may harm the lichen thallus. It may result in differences in the composition of lichen communities. Most air pollutants harm lichens by inducing acid rains. Lichens in response to sulphur dioxide gas concentration in the air show a lichen zonation pattern in the species composition. Due to these effects the morphology of lichens including the diameter, nature of the middle area of the thallus, chlorophyll content may change. The air pollutants such as sulphur dioxide, oxides of nitrogen, ozone, fluorides tolerating ability of different lichen species vary. They act as good air pollutant indicators.   

• The bio-indicative nature of lichens arise due to the fact, lack of a cuticle as in plant leaves

• Some gases present in the air and various contaminants may harm the lichen thallus. It may result in differences in the composition of lichen communities

 When it comes to heavy metals and trace elements that accumulated in ecosystems due to human activities. They cause a irreversible harm to the lichens, in most cases they are lethal. The major source of the heavy metal and trace elements is the vehicular smoke. Heavy metals accumulate in the thallus, so by the analysis of thallus we can correlate those concentrations with that in the environment so they indicate such contamniants present, which is very important to humans. Lichens act as bio-indicators, are organisms that represents the quality of the environment. Understanding of the tell-tale nature of them is a clue given to found out whether the environment is polluted or not.   

 I.B. Avanthi Madushika Weerawardena   
 Microbiology (special) degree  
 University of Sri Jayewardenepura