Lichen: Nature's Air Pollution Indicator

Lichens are miniature ecosystems made of fungus and algae and/or cyanobacteria. They are highly sensitive to air pollution and can act as bio-indicators of air quality. Their ability to react to air pollutants at different levels, their low growth rate, and their longevity make them ideal for monitoring the quality of the environments in which they live. Scientists have used lichens to classify levels of atmospheric pollution and monitor urban pollution levels.

Lichen sensitivity to nitrogen pollution

Lichens are miniature ecosystems made of fungi and algae and/or cyanobacteria. They are very sensitive to air pollution and respond to it in short time frames, which makes them excellent bio-indicators of air quality. They are particularly sensitive to nitrogen pollution.

Nitrogen deposition can increase the load of nutrients, and too much nitrogen can harm and kill the algae's chlorophyll, which is used to produce sugars to feed it and the fungi. Some lichens will die in the presence of nitrogen, while others will thrive. The golden shield lichen (Xanthoria parietina), for example, can live in areas with high levels of nitrogen, especially ammonia. It is common on trees and buildings near farmland and on sea cliffs where seabirds' droppings provide nitrogen.

The two main nitrogen-containing air pollutants that affect lichen growth are nitrogen and sulphur dioxide. Sulphur dioxide pollution comes from coal burning and industry. This pollutant has killed many lichens around the UK in the past, but now that less coal is being burned, they are beginning to return.

Lichenologists like Gothamie Weerakoon are using lichens as indicators of nitrogen air pollution. They are setting up permanent plots in Sri Lanka's mountain cloud forests to monitor how the lichen community in these plots changes over time. Researchers are also studying the impact of nitrogen pollution from tea estates in Sri Lanka and in the forests of the Himalayas.

In Europe and the UK, the lichen flora of oak trees growing in agricultural areas has changed from communities dominated by species preferring acid bark to species that tolerate and benefit from nitrogen. In the Netherlands, high levels of ammonia have led to the complete disappearance of acid-preferring species, resulting in communities dominated by nitrophytic species.

Sulphur dioxide pollution and lichen

Sulphur dioxide is one of the two main air pollutants that affect lichen growth, the other being nitrogen. Sulphur dioxide dissolves in water to produce acidic ions, which are readily absorbed through the lichen thalli, disrupting photosynthesis. It also inhibits the activity of nitrogenase, which is used by cyanobacterial photobionts to fix atmospheric nitrogen.

Sulphur dioxide pollution comes from coal burning and industry. In the 1970s and 1980s, SO2 and fluoride pollution dominated air pollution chemistry. High concentrations of sulphur dioxide can irritate the mucus lining of the eyes, nose, throat, and lungs. Exposure to sulphur dioxide may cause coughing and tightness in the chest. People with asthma are more sensitive to sulphur dioxide pollution.

Usnea lichens, also called old man's beard, do not grow in areas where there is sulphur dioxide pollution. If you see one of these on your walk, it probably means that coal has not been burnt in the area for some time. Other lichen species that are sensitive to sulphur dioxide include Chaenotheca chrysocephala, Cladonia digitata, Evernia prunastri, Lecanora pulicaris, and Phlyctis argena.

In contrast, Lecanora conizaeoides and Lepraria incana are crusty lichens that can tolerate poor air quality with respect to SO2. Leafy lichens such as Parmelia caperata or Evernia prunastri can survive in moderate to good air.

By observing the types of lichen that grow in an area, it is possible to judge the sulphur dioxide pollution levels in that area. Lichens are sensitive to air pollution and make good indicators of air quality. They are miniature ecosystems made of fungus and algae and/or cyanobacteria.

Lichen as bio-indicators

Lichens are miniature ecosystems made of fungus and algae and/or cyanobacteria. They are highly sensitive to air pollution and are therefore used as bio-indicators. They are especially sensitive to sulphur, nitrogen, acidity, halogens, heavy metals, and ozone.

Lichens are used as bio-indicators because they are directly exposed to their environment. They have no protective physical structures, so rainwater and air enter the organism directly, and dust is trapped between mycelium filaments. Lichens can fix pollutants, and their ability to react to air pollutants at different levels makes them ideal for monitoring the quality of the environment in which they live.

Lichens are used to classify levels of pollution in various areas. They are also used to monitor the effects of anthropogenic pollution on natural systems. Scientists monitor lichen communities and pair this data with atmospheric deposition data to determine the sources and levels of pollution causing detrimental effects.

The presence of different lichen species in an area can indicate the level of air pollution. For example, the presence of leafy lichens such as Parmelia caperata or Evernia prunastri indicates moderate to good air quality, while rare species such as Usnea articulata or Teloschistes flavicans may grow in areas with very clean air. In contrast, the presence of only crusty lichens, such as Lecanora conizaeoides or Lepraria incana, indicates poor air quality.

The health of lichen communities can also indicate the potential beginning of ecosystem decline due to atmospheric nitrogen deposition. An increase in nitrogen-tolerant lichen species, combined with a decrease in nitrogen-sensitive species, may signal rising nitrogen deposition levels.

Lichen biodiversity and air quality

Lichens are miniature ecosystems that grow on tree trunks and rocks and are made up of a symbiotic association of fungi and algae. They are very sensitive to air pollution and are therefore good indicators of air quality. They are particularly sensitive to sulphur, nitrogen, acidity, halogens (e.g. fluoride), heavy metals, and ozone.

The presence of certain lichen species can indicate typical sulphur dioxide and nitrogen dioxide levels. For example, the oakmoss lichen is sensitive to nitrogen in the air and can be found on woodland branches where the air is clean. In contrast, the golden shield lichen can live in areas with high levels of nitrogen, especially ammonia, and is commonly found near farmland. The absence of lichens indicates very poor air quality, while the presence of crusty lichens like Lecanora conizaeoides or Lepraria incana indicates poor air quality. Leafy lichens like Parmelia caperata or Evernia prunastri can survive in moderate to good air, and rare species like Usnea articulata or Teloschistes flavicans may grow in areas with very clean air.

The biodiversity of lichens in a given region indicates good air and environmental quality, while scarce lichen vegetation indicates poor air and environmental quality linked to the presence of atmospheric pollutants. Scientists have drawn up an index of atmospheric purity (IAP) using lichens to classify levels of pollution in various areas. They monitor the health of lichens and pair this data with atmospheric deposition data to determine the sources and levels of pollution. Lichen observations submitted by the public while hiking or walking in urban neighbourhoods can also inform researchers and scientists about potential changes in ecosystem processes and services in different environments.

In addition to their sensitivity to air pollution, lichens are also used to monitor the effects of nitrogen pollution on ecosystems. For example, in Sri Lanka, tea growers use fertilisers containing nitrogen in the form of ammonia, which ends up in the surrounding ecosystems. Researchers are setting up permanent plots in the mountain cloud forests to monitor the effects of nitrogen pollution by releasing nitrogen in a controlled way onto the plots to test the effects of different nitrogen concentrations.

Lichen and human health

Lichens are sensitive to air pollution and are therefore used as indicators of air quality. They are particularly sensitive to sulphur, nitrogen, acidity, halogens (e.g. fluoride), heavy metals, and ozone. They can also absorb smaller pollutants like sulphur, mercury, and nitrogen. This means that in areas with high levels of these pollutants, lichens may be slow-growing or even absent altogether.

Lichen play an important role in keeping both people and nature healthy. They act as an air scrubber, cleaning the air that we breathe by trapping particulate matter and absorbing smaller pollutants. This results in cleaner, healthier air for us, lowering the risk of respiratory problems like asthma or chronic obstructive pulmonary disease (COPD).

Lichen have also been used by humans for thousands of years. They were used by early human cultures in dyes, spices, perfumes, teas, brandies, and medicine. They produce thousands of chemicals not found elsewhere in nature, and their extracts and active substances have been shown to have multiple health benefits, including anti-cancer, anti-inflammation, anti-oxidative stress, and anti-diabetes properties.

In addition, lichens are a keystone species in many ecosystems, playing an important role in the health and survival of these ecosystems. They are ecologically important to many national parks, and their presence or absence can indicate the health of the surrounding environment.

Finally, lichens can provide a window into the past, with the oldest lichen fossils dating back 300-400 million years. They have survived or re-evolved after several of Earth's mass global extinctions, and their long lifespan and ability to thrive in all extremes on Earth can provide a humbling perspective on human existence.

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