Science: Plants that tell a sad tale of pollution

Lichens can reveal much about air quality. Sanjida O'Connell reports
Professor Mark Seaward, of the environmental sciences department at Bradford University, has a list of every lichen spotted in the British Isles in the past 300 years. Amateur and professional lichenologists scour the country and send sightings to Professor Seaward, who enters the location of each finding on his computer. He likens it to train-spotting. Esoteric though they seem, lichens carry an important message - they are perfect monitors of air pollution.

William Nylander, a Scandinavian, was the first to suggest that lichens might indicate air quality, in 1861. His observation was confirmed in 1896, when he noticed that all lichens had disappeared from the Jardin du Luxembourg in Paris due to air pollution.

Professor Seaward was the first to notice the reappearance of lichens in the Jardin in 1990. They were a few millimetres wide and had taken about two years to reach that size.

Because of their sensitivity to air pollutants, lichens can chart the extent of pollution. Maps showing sightings of lichens sensitive to sulphur dioxide display a coffin pattern between Liverpool and London where none survives. The Clean Air Acts of 1956 and 1968 helped to reduce the intensity of air pollution in our cities, but the policy resulted in widespread blanket pollution, since gases are dispersed more efficiently from factories and power stations, many of which are outside cities.

Professor Seaward seems to know many of his 1,900 maps off by heart. He can rapidly call up on his computer the distribution of a lichen which grows only on gravestones, while another map has a scattering of white dots indicating extinction. "Some lichens are tolerating pollution, but only because they are thriving in the absence of competition," he says.

Lichens can incorporate heavy metals into their bodies. Several years ago, there was a lichen that was only found in Scotland. It grew on stones rich in heavy metals, including lead. Now, because of car emissions, it has moved south to colonise most of Britain.

The train-spotting mentality pays off. In the late l970s, Professor Seaward examined the level and type of radioactive elements in lichens in Poland. The amount was four times higher than normal because of the Poles' habit of burning lignite (brown coal).

In April 1986, the Chernobyl disaster released radioactive material for 10 days, depositing radionuclides throughout Europe. Professor Seaward was the only person who knew what the background levels of radiation were in Poland. He returned to find that the lichens he had measured previously had absorbed radioactive caesium extremely efficiently - caesium is chemically similar to potassium, which is an essential nutrient. Levels of radioactivity had increased 165 times.

Professor Seaward had thought lichens were harmless, but he was proved wrong. He is now working on lichens attacking Italian frescoes and monuments, in collaboration with Dr Howell Edwards from the department of chemical technology at Bradford.

Professor Seaward says frescoes at the Palazzo Farnese near Rome will be obliterated within 15 years. They were painted in the 1600s, so why are the lichens attacking them now? It may be due to changes in the type of pollution in Italy, and the fact that maintenance has not been as intensive as it was before the war. Also, the frescoes have been repainted, but the paint formula was different from the original one, which had kept the lichens at bay.

Lichens are two organisms living together - a fungus and an alga. The alga photosynthesises, making carbohydrate from sunlight, and the fungus acts like a placenta. On the frescoes, the fungus forms a crust by reacting with chemicals in the paint - to remove the lichen, the whole paintwork will also be lifted off. Using a microprobe, a laser is beamed at the lichen and its chemical composition is analysed, without disrupting the lichen and consequently damaging the painting.

Professor Seaward and Dr Edwards are testing how much of this crust the lichens produce under different conditions in the lab. "If we can find the answer," Professor Seaward said, "we can start to consider ways of controlling the environment around the frescoes."