Biological effects of air pollution

Lichens as a measure of air pollution

We learned in Chapter 11 about the value of indicator species for monitoring pollution. Just as invertebrate organisms are an important group of indicator organisms for measuring water quality, a useful indicator for air quality is the variety of lichens that are present. Lichens are a group of very slow-growing organisms that you find as encrustations on stonework or as a flaky growth on tree barks. They are unusual organisms because they consist of fungal threads and microscopic green algae which live together and function as one organism. The term for this relationship is mutualism - a symbiotic relationship that is beneficial to both participating species. There are a number of different species and Cover Illustration 12 shows an example. They have different sensitivities to pollution so the cleaner the air, the more species are present.

Figure 45 shows the results of a survey of lichens at increasing distance from a city centre, whilst Figure 46 gives an indication of their distribution in England and Wales.

The reason why lichens are sensitive to air pollution is because they

Figure 45. Lichen numbers at increasing distance from a city centre

Figure 45. Lichen numbers at increasing distance from a city centre

Distance from city centre (km)

Distance from city centre (km)

Figure 46. Lichen distribution over England and Wales

Few Lichen Q^] Intermediate zone jjjjjj Rich in Lichen

Figure 46. Lichen distribution over England and Wales do not have roots and rely instead on the absorption of rainfall, and the nutrients in it, for growth. Their surfaces lack the protective layer that the leaves of plants have which allows them to block out pollutants. As a result the pollutants can accumulate within the lichens and reach levels where they break down the chlorophyll molecules responsible for photosynthesis.

Procedure for monitoring air pollution using lichens There are three main groups of lichens:

Shrubby type. These look like leafless bushes. They are grey/green in colour, about 2-5 cm high, and are very sensitive to pollution.

Leafy type. These have flat leafy lobes and grow on stones and trees. They also are grey/green and are more tolerant of pollution than the shrubby type.

Crustaceous type. These are found in a variety of colours - white, orange or green - and look like splashes of paint on surfaces on which they grow very tightly. They have varying sensitivity to pollution.

Using an Ordnance Survey map, move out from a source of pollution such as a city centre or a factory which emits fumes into the atmosphere. Look out for different types of lichen but always use the same substrate, i.e. old walls or mature tree bark. Prepare a map showing the number of species of lichen in different areas and also note whether they are rare, common or very common. Remember that you do not have to be an expert in the identification of lichens but with a little experience you should be able to distinguish the different types.

Leaf yeasts as a measure of air pollution1

Leaf yeasts are microscopic fungi that live on the surfaces of deciduous tree leaves and the leaves of grasses, shrubs and herbs. They are very sensitive to toxins present in the air. They have an advantage over lichens for studying air pollution because of their faster response. Lichens take years to grow whilst leaf yeasts develop in about a week. Their numbers vary considerably according to the air quality and enable you to be able to map the effects of pollution with a discrimination of about 1 km2.

Procedure for mapping air pollution using leaf yeasts

In this procedure the leaves of trees from different places in your study area are collected. In the laboratory, the yeast cells on the leaves are allowed to fall onto a special nutritious medium which enable them to grow into small visible colonies. The colonies are counted and their numbers related to sources of air pollution.

First draw a map of the area you wish to study or else use an Ordnance Survey map: the series 1:25,000 is particularly useful for this study. Your study area should be based on 1 km square grids.

For each of the squares on the map, visit the area and look for suitable trees. Ash, sycamore and limes are very common in towns. Select between three and five trees of the species you have decided to sample in each 1 km square, one in the middle and the others nearer the edge. Choose trees where you can collect leaves by reaching up from the ground. Record their location on the map and note particularly whether they are beside a busy or quiet road, or in a field or park.

Sample the trees on the designated day in spring or summer. Try to avoid autumn because the number of yeasts changes as autumn advances. From each tree collect enough leaves for you to cut out nine discs - about three leaves should do. Take leaves from different sides of the tree and don't collect from newly opened buds because the yeasts will not have developed on them. If you have sampled four trees in the 1 km square, you should have about twelve leaves.

When you have collected leaves from all the individual 1 km squares, return to the laboratory and prepare a dust-free area on a table or laboratory bench. Wash the area with hot water and detergent and then wipe it with a paper towel which has been dipped into 70 per cent alcohol.

Cut four to five discs from each leaf using a 1 cm cutter and place onto a clean piece of paper which has been labelled with the location, one piece of paper for each tree sampled. The discs are placed on the paper so that the lower surface is uppermost. There should now be about 50 discs for each 1 km square.

For each grid square, have five petri dishes (with their lids) prepared containing 1.5 per cent malt extract agar in them. Label each dish with a letter and assign a number for each grid square, i.e. you'll have 1A, 1B, 1C, 1D, 1E; 2A, 2B, etc. Place each dish upside down on the cleaned bench, lift the base containing the medium off the lid and place it upside down next to the lid. Do not turn the dish over or touch the medium otherwise it may pick up yeasts from the air or from your fingers.

On the inside of each lid put nine small blobs of Vaseline using a sterile piece of glass rod or similar implement. Using a pair of forceps, place the nine discs from each individual tree onto the lid so that their smooth upper sides are stuck to the Vaseline. Replace the lid over the base containing the agar. Repeat for the other trees in the 1 km square. Pile the Petri dishes for each 1 km square on top of each other and secure them together with tape.

Invert the pile so that the leaves are uppermost and are suspended over the agar. The leaf yeasts will fall off the leaves and land on the agar where they will grow.

Place the individual piles of dishes onto a tray and put them in an area where they will not receive any direct sunlight. After 24 hours, invert the piles of dishes so that no more yeasts will drop off the leaf discs.

Leave the stacks for a further 48 to 72 hours (it doesn't matter what time interval you choose as long as you use the same interval for each test). Examine the agar medium and you will find pink colonies of yeasts have developed from each individual yeast that has fallen off the disc. Ignore white glistening colonies or fluffy white or grey ones as these are caused by other organisms.

Using a magnifying glass, count the number of each pink colony under the individual discs and record the results. If the colonies have coalesced because they have been incubated for too long, then reject the result for that disc. If the number of colonies under a disc is >50 then estimate the number.

From all the individual dishes for each 1 km square, record the median value and classify the results according to this scale:

30+ = very good 14-29 = good 6-13 = moderate 2-5 = poor 0-1 = very poor

Transfer the readings you have obtained to the map of your sample area and try and relate the results to sources of pollution. Remember to take into account the wind direction on the days before the sampling took place.

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  • james
    What are biological reason that lichen is sensitive to air pollution?
    6 years ago

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