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Like small signposts, these curious organisms can tell us a lot about the air we are breathing.
Fresh, clean air is wonderful to breathe in. Without the health risks of air pollution, fresh air feels great for our lungs.
Lichens love clean air too - in fact, their sensitivity to air pollution means they make good air quality indicators.
Lichens are able to tell us the effects of air pollution on ecosystems, not just of the number of pollution particles in the air. This is handy if you want to know how air pollution is changing ecological communities and what that means for the people who rely on them.
If you don't yet know what lichens look like, once you start searching you might see them everywhere, even in the unlikeliest of places.
Lichens are miniature ecosystems made of fungus and an algae and/or cyanobacteria. These different life forms work so closely together that the algae or cyanobacteria actually sits inside the fungus. The algae or cyanobacteria provide the fungus with sugars made from sunlight, and the fungus provides the home.
Lichens look like spots or clumps of colour, like someone has splashed paint onto a branch of a tree. Their colours range from green to brown to white to russet red. Even in these colours, lichens can be understated additions to tree trunks and rocks, and you might miss them at first glance.
There are many types of lichen but they only grow in three ways. They can be branched like tiny shrubs, they can have crinkly little leaves or they can grow like a crust on a surface.
The Museum has a lichen identification guide to help you look for and find different types of lichen in your area.
To grow, lichens get their nutrients from the air. Because lichens have no roots or protective surface, they cannot filter what they absorb, so anything in the air is taken straight inside. If there are pollutants, it can accumulate in the lichen and can become toxic very quickly.
Museum lichenologist Gothamie Weerakoon says that lichens make great air pollution indicators. She says, 'they are very sensitive and respond to pollution in short time frames.'
Butterflies, nematodes, frogs and toads are very good indicators of environmental pollutants, but lichens are easier to study and are quicker to respond to environmental change.
Next time you are on a walk, you can look around for the types of lichen that grow in your area. As a rule of thumb, the smaller the size and less variety of lichens in an area, the more polluted is the environment.
Two main air pollutants that affect lichen growth are nitrogen and sulphur dioxide.
We breathe in harmless nitrogen gas all the time - in fact it makes up a large part of Earth's atmosphere. But when nitrogren is heated and combined with oxygen (as it is in a car engine), nitrogen oxides are created.
Nitrogen dioxide in the air can be a powerful polluter and becomes harmful for human health in high concentrations. In UK cities, around half the nitrogen dioxide air pollution comes from road traffic. Farms also emit nitrogen pollutants from fertilizers, farm machinery and livestock waste.
Nitrogen dioxide gas can inflame the lining of the lungs and can cause respiratory symptoms such as shortness of breath and a cough. Nitrogen dioxide can also decrease your body's immune response to lung infections. For people with asthma, nitrogen dioxide air pollution may mean more frequent attacks.
Some lichens will die in the presence of nitrogen, while others will thrive. By learning a few common lichen species, you can judge the nitrogen pollution levels in your area.
Sulphur dioxide pollution comes from coal burning and industry. This pollutant has killed many lichens around the UK in the past, but now because we burn less coal, they are beginning to return.
In high concentrations, sulphur dioxide can irritate the mucus lining of the eyes, nose, throat and lungs. Exposure to sulphur dioxide may cause coughing and tightness in your chest. People with asthma are more sensitive to sulphur dioxide pollution.
You can find out more about surveying lichens to assess air pollution as part of an earlier Open Air Laboratories (OPAL) citizen science project that mapped lichens in the UK.
Humans have many uses for lichens, including as a source of potential antibiotic, anti-fungal and anti-cancer drugs. Lichens also can be used in perfumes and incense and as a clothes dye.
Lichens are also important for other plants and animals. Their ability to grow in some barren places, like on the surface of a rock, allows for other things to grow in the organic material that lichens leave behind when they break down.
They are also food for many animals, including reindeer, squirrels, snails and insects.
Researchers at the Museum are part of a global team investigating nitrogen pollution from tea estates in Sri Lanka and in the forests of the Himalayas.
Tea growers in Sri Lanka use fertilizers that contain nitrogen in the form of ammonia. This ammonia does not stay on the tea farm, and some of it inevitably flows into the surrounding ecosystems.
The nitrogen is carried into the atmosphere through winds and sits in the clouds, ready to rain on the nearby mountains and cloud forests.
Gothamie is part of a team setting up permanent plots in Sri Lanka to monitor the effects of nitrogen air pollution. They are using lichens as indicators.
These plots sit high up in the mountain cloud forests, and the researchers are recording how the lichen community in these plots change over time.
To set up the experiment, they consider the local climate and known atmospheric nitrogen levels. They then release nitrogen in a controlled way onto the plots to test the effects of different nitrogen concentrations.
Gothamie says, 'Every year the amount of nitrogen increases in the atmosphere, from the increase of fertilisers being used.'
This research will help them understand how increases in fertiliser use on tea farms will change the ecology of southern Asia.