Lichens as Monitors of Air Pollution Essay

The mutualistic association between a fungus and a cyanobacterium or algae is called lichen, usually occurring on trees, bare ground, or rocks as bushy growth or brittle patches. They are extremely sensitive organisms effectively reacting to air pollution, thus extensively used as environmental indicators or bio-indicators. Their sensitivity is a direct consequence of their efficient absorption systems that subsequently results in quick accumulation of sulfur dioxide on exposure to high levels of SO2 pollution.

Lichens are extremely sensitive to SO2 pollution in the air. When exposed to high levels of sulfur dioxide there is a rapid accumulation of the same in their systems due to their efficient absorption mechanisms. The most affected by this is the algal partner, whose chlorophyll is inhibited resulting in the inhibition of photosynthesis. Lichens can also detect and absorb dissolved SO2. They lack root, hence complete nutritional uptake is through the surface from the atmosphere with the free exchange of gases and solutions across cell surfaces. Lichens are reliable pollution indicators, if the air is polluted enough only the green algae may be found but no lichen. An abundance of leafy, shrubby, and hairy lichens in a location implies the air is clean. Some species of lichens, crusty in appearance, are tolerant to pollution and are usually found in the bark or walls of urban areas, unlike the leafy sensitive ones. Since industrialization, there is a significant reduction in the number of lichens. In addition to SO2 sensitivity, lichens are also sensitive to nitrogen pollution, toxic mercury, and lead emissions in the atmosphere. Lichens are generally classified as:

Crustose: These are crusty forms found on rocks tree barks or soil. The strong attachment of crust to the surface makes it hard to remove them.

Fruticose: These are three dimensional, bush-like growth on sides of tree barks or rocks, growing perpendicular to the surface attaches.

Foliose: These are leafy, loosely attached, and grow parallel to the surface.

Nitrogen, a major component of the atmosphere, when heated can combine with oxygen to form nitric oxides.

Nitrogen dioxide is a powerful pollutant, potentially harmful to human health when in high concentrations, which is found in fertilizers, livestock waste, and a major component in automobile emissions.

They can inflame lung linings causing respiratory symptoms like shortness of breath or cough, also decreasing immune response against lung infections.

Nitrogen deposition in lichens increases the nutrient load, which in high levels harms the algae’s chlorophyll that synthesizes the sugar which is food for both the algae and fungi.

There are some lichens tolerant of nitrogen pollution like the golden shield lichen (Xanthoria parietina). Therefore, an increase in N tolerant species accompanied by a decrease in the sensitive species indicates high levels of N in the atmosphere.

Sulfur Dioxide Sensitivity
Sulfur dioxide pollution is a direct consequence of emissions from industries and coal burning.

High SO2 concentrations can irritate the eyes, throat, lungs, and nose mucal linings and can cause tightness in the chest and coughing.

High levels of SO2 lead to acid rain which kills the lichen population. Usnea lichens (old man’s beard) are very sensitive to sulfur pollution and their presence implies the air is completely free from SO2 pollution. Some species are more tolerant to the acid conditions like Rinodinaand bryoria.

Zonation of Lichens
To determine the levels of sulfur and nitrogen and thus air pollution in an area the lichen zone pattern is observed. Typical SO2 levels in an area can be indicated by the presence of a particular species of lichen on a tree bark there.

The complete absence of lichens or presence of only crusty lichen species like Lepraria incana indicates a highly polluted area. The presence of leafy lichens like Parmelia caperata indicates moderate to good air quality. Rare species like Usnea articulata (the string of sausages) or Teloschistes flavicans (the golden wiry lichen) occur in areas with very clean air.

Lichen recolonization following a fall in atmospheric NO2 / SO2 levels or pollution in the same area occurs rarely or takes more time; as lichens are generally slow-growing and may take one or two years to recolonize the tree bark or other surfaces and are difficult to detect and identify as they are tiny specimens recolonizing the surface.

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