By KANYEMBO J. PHIRI –
IN 1866 William Nylander was the first scientist to publish observation about the lichens’ reaction to air pollution.
He noticed the poor, sparse lichen vegetation in the peripheral parts of the Jardin du Luxembourg garden in Paris.
It was noted that in general lichens were not common in cities and that urban lichens showed incomplete development.
According to him, lichens could give a good indication of the air pollutin, and so constituted a very sensitive “health meter” for the surrounding air.
Other air pollutants include, NOx, O3, heavy metals, HF, organic pollutants, which also cause disappearance of lichens from cities and industrial areas. Lichens have been used as bio-indicators of air pollution worldwide, especially in the developed countries.
What is a Llchen?
Lichens consist of two organisms (fungus and alga) which live together in partnership. This is known as a symbiotic relationship.
Essentially the alga enables the fungus to photosynthesise.
They also depend on atmospheric moisture, rain, fog and dew for growth and vary in their appearance and can look like crusty patches or leafy plants.
Lichens grow in hostile places such as rocks and bricks but can also be found on tree bark. They range in colour from greys and browns to yellows and oranges and there are about 30,000 species of lichen worldwide.
Generally Lichens have no roots and no other special organs for nutrient uptake. The nutrients are taken up directly through the thallus. Nevertheless, they are very effective in active uptake of substances from weak aqueous solutions, and can concentrate and store many different substances in their thallus.
This therefore serves as a storage place for longer or shorter periods. Metabolic turnover and growth are slow.
In many temperate regions, spring and autumn are the growing seasons for lichens. Growth is probably limited by drought in summer and by low temperatures in winter.
Lichens occur in all climatic regions of the earth, from the tropics to the arctic tundra and ice-free areas of Antarctica, from sea level to the highest mountain tops. Many of the species are widespread while others are restricted in range.
We classify them according to the substrate on which they grow, e.g. on soil, on rocks and stones, on wood, or on tree barks. Some lichens are confined to a particular substrate, whereas others can grow on almost any substrate and are therefore included in all the groups.
Importance of Lichen as biological indicator specie
Pollution levels can be measured directly. The presence or absence of certain living organisms can also act as an indicator of the amount of pollution. Lichens can be used as environmental indicators as they are very sensitive to sulphur dioxide pollution.
Sulphur dioxide (SO2) is a gas which is produced when a material containing sulphur is burned. In many countries the main source of sulphur dioxide is power stations burning fossil fuels.
Before the clean air act, domestic burning of coal contributed to high local concentrations of sulphur dioxide.
If the air is badly polluted with sulphur dioxide there may be no lichens present. Since industrialisation began between the late 18thcentury and early19th century many lichen species became extinct in Europe and other developed countries.
The most tolerant lichens are those that are crusty in appearance, whereas the leafy lichens are not very pollutant tolerant.
Those lichen species that can tolerate quite high levels of pollution can often be found on pavements and walls in urban areas. Generally the smaller the variety of lichens in an area the more polluted it is.
Why use Lichen as a pollution indicator?
It is necessarily important to use them for many reasons some of which are, they are efficient indicator of air pollution and acid depositions especially in soils, and their use for assessment is less costly compared with expensive pollution indicator equipment’s.
Furthermore appropriate techniques should be developed especially in developing countries like ours (Zambia) and many other African countries to use lichen as bio-monitoring specie for air pollution as they help to provide warning signal before severe damage occurs on the Ecosystem and health hazards are posed.
Given these facts about lichens, and considering that they can be found in almost every part of the world, it is no wonder that they are used as indicators for various types of pollution. Some of the major pollutants that harm lichens as earlier stated are sulphur dioxide, sulphuric and nitric acids, fluorides, ozone, hydrocarbons, and metals such as copper, lead, and zinc.
Pollutants come in many forms in both urban and rural areas. Sulphur dioxide has been a significant industrial pollutant for many years which is a by- product of the use of high sulphur fuels.
While efforts have been made to reduce high levels of sulphur dioxide pollution through technological innovations in many countries, it is still a significant pollutant in many areas.
Smelters produce heavy metal pollutants and car exhausts contain nitrogen oxides. With the increase in intensive animal rearing ammonia is becoming a major rural pollutant and agricultural chemicals such as fertilizers and herbicides make their way into the soil and water bodies.
Oil spills are also occasional pollutants in coastal areas. All of the above mentioned environmental hazards can have detrimental effects on lichens, though susceptibility to any particular pollutant may not be uniform across all lichen species.
Impacts of Acid Rain
When it comes to pollutants and lichens sulphur dioxide is the most intensively studied pollutant. This is a gaseous compound that dissolves readily in water to produce highly reactive acidic ions,
which are readily absorbed through lichen thalli and, once absorbed disrupt photosynthesis. Many research studies have shown that Sulphur dioxide can inhibit lichen reproduction (both sexual and asexual) as well as spore germination.
Acid rain became a recognised international problem during the 1980s resulting from the dispersion of air pollutants via tall chimney stacks.
Air pollution and acid deposition has led to problems for lichens on bark, particularly because the tree bark has often become more acidic.
In some areas, although gaseous sulphur dioxide levels have fallen, the bark of older trees is too acidic for re-colonisation, and new growth develops on twigs and younger trees.
Some species of lichens have become more widely distributed than they were a century ago as they are more tolerant of acid conditions, such as some species of Bryoria, Parmeliopsis, Pseudevernia and Rinodina.
A lichen zone pattern may be observed in large towns and cities or around industrial complexes which corresponds to the mean levels of SO2 (sulphur dioxide) recorded.
In Zambia trends have shown that the major causes of Acid rain is motor vehicle pollution due than increasing number of Automobiles which produce SO2 and due to emissions released from Industrial Areas such as Copper oxides from the Mines.
Other uses of Lichens
Lichens are used by scientists not only in the study of pollution but also as an indicator and mineral content on rocks e.g. in the Fields of Geological sciences.
Lichens store numerous minerals in their thalli, their presence and concentration is indicative of the composition of the rock that they are growing on.
Lichens are useful as food, medicine, for making dyes, in perfume manufacture, as decorations and in science.
In Japan umbilicaria esculenta is considered a delicacy where it is eaten as a soup or in salads.
Other Umbilicaria species are, or have been, eaten in other countries. In Europe records from around the 15th century suggest that by then several lichens were in regular medicinal usage.
Thus with the help of lichens, whether growing naturally on tree trunks or whether transplanted in some way an assessment can readily be made of the extent of air pollution.
In this context, because of their sensitivity, and because they normally occur almost everywhere, they are far better than any measuring instrument.
However, the difficulty lies in translating the picture of lichen distribution into exact air pollution values, a difficulty that is certainly not insuperable.
In the initial phase of investigation of damage around a source of pollution, lichens can assist in the assessment of the type of air pollution involved.
If change of substrate of the lichens, as well as their general appearance, is interpreted correctly, a good picture of the air pollution situation is already available.
Over the years vehicle pollution has become a problem has been noted to harm lichens because airborne nitrogen compounds and phosphates make lichen substrates more alkaline, while lead and other emissions are poisonous to lichens.
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