Lichen: Pollution's Natural Enemy?

Lichens are miniature ecosystems made of fungus and algae and/or cyanobacteria. They grow in all environments except the high seas, on the tissues of live animals, and in highly polluted areas. They are sensitive to air pollution because they have no roots or protective surface to filter what they absorb from the air, and so they make excellent indicators of air quality. Scientists monitor the health of lichens and pair this data with atmospheric deposition data to determine the sources and levels of pollution. Lichens are especially sensitive to sulphur dioxide and nitrogen, and certain species are more sensitive than others.

Lichen as bio-indicators of air pollution

Lichens are miniature ecosystems composed of a fungus and an algae and/or cyanobacteria. They are found in a variety of natural and human-made environments, including rocks, trees, barren earth, metal, and concrete. Lichens do not have any physical means of defence against their environment, such as plant cuticles, so they absorb all their nutrients and water from the atmosphere, including pollutants. This makes them excellent bio-indicators of air quality.

Lichens are sensitive to a number of air pollutants, including sulphur dioxide, nitrogen oxides, and heavy metals. They are especially vulnerable to sulphur dioxide, which is emitted by coal combustion and industrial development. In the past, sulphur dioxide pollution has killed many lichens, but due to decreased coal burning, they are now returning. Lichens are also affected by nitrogen deposition, which can increase the load of nutrients and harm or kill the algae's chlorophyll, depriving the fungi of necessary sugars for survival.

The presence or absence of lichens in an area can indicate the level of air pollution. If there are no lichens present, the air quality is very poor. Only certain crusty lichens, such as Lecanora conizaeoides or Lepraria incana, can tolerate poor air quality. In moderate to good air quality, leafy lichens such as Parmelia caperata or Evernia prunastri can survive. In areas with very clean air, rare species such as Usnea articulata or Teloschistes flavicans may be found.

The variety and health of lichen species in an area can provide valuable information about the sources and levels of pollution. Scientists monitor lichen communities, and if there is an increase in nitrogen-tolerant species and a decrease in nitrogen-sensitive species, it may indicate rising nitrogen deposition. Lichens can also accumulate pollutants such as metals, organic elements, and radioelements, which can be measured to assess environmental and health risks.

Lichen observation has been used to detect and map the effects of air pollution. For example, a study of lichens from Plummers Island, Maryland, showed a dramatic rise in atmospheric lead deposition prior to the early 1980s, coinciding with the completion of a bridge and the phasing out of leaded gasoline in the US. Lichen communities today contain fewer sensitive species and lower species diversity than in the past, indicating a continued presence of air pollution.

Lichen growth on trees

Lichens are miniature ecosystems composed of a fungus and an alga and/or cyanobacteria living in a symbiotic relationship. The algae in lichens photosynthesize to create food from sunlight energy, and both the algae and fungus absorb water, minerals, and pollutants from the air, through rain and dust.

Lichens are extremely sensitive to air pollution and can be used as indicators of air quality. They are also used in perfumes and incense and as a clothes dye. They grow on healthy trees, as well as stressed or otherwise unhealthy ones. Epiphytic lichens, which grow on trees (dead or alive), have been widely studied with respect to pollutants such as SO2, nitrogenous gases, and acidity. The growth of lichen on tree bark depends more on the physical surface of the bark than on the kind of tree. Young trees generally have smoother bark that tends to attract crustose lichen species. As trees get older, the bark usually develops uneven broken surfaces that permit the foliose and fruticose lichens to attach to the tree. Lichens found on trees are often circular or oblong in shape with leaf-like lobes (foliose) that are parallel with or slightly higher than the bark.

Some species of lichen require alkaline conditions and are only found growing on old bark. Bark can become more alkaline with age, so species such as ash—which has a relatively high pH (alkalinity) of bark—are home to a lot of species. Some lichens are sensitive to nitrogen pollution, such as the monk's hood lichen, which is widespread and found on trees with acidic bark. Other lichens, such as the golden shield lichen (Xanthoria parietina), 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.

In the past, sulphur dioxide pollution from coal burning and industry has killed many lichens. However, as coal burning has decreased, these lichens are beginning to return. Usnea lichens, also called old man's beard, do not grow in areas where there is sulphur dioxide pollution. Lichens can also indicate the presence of other pollutants. For example, lichens from Plummers Island, Maryland, show the dramatic rise in atmospheric lead (Pb) deposition prior to the early 1980s, coinciding with the completion of the American Legion Memorial Bridge.

Lichen and nitrogen deposition

Lichens are miniature ecosystems composed of fungi and algae and/or cyanobacteria. They can absorb nitrogen in multiple forms from soil, rock, and air, and play a role in the carbon cycle. Lichens are sensitive to air pollution, making them excellent indicators of air quality. Nitrogen deposition, for instance, can have a range of effects on lichens.

Nitrogen deposition can cause direct toxicity to lichens, particularly from Nr gases and aerosols. The symbiotic relationship between the fungi and algae or cyanobacteria in lichens means that their responses to increasing nitrogen deposition can differ. For example, as nitrogen deposition increases, the photobiont (algae or cyanobacteria) becomes less dependent on the fungal nutrient supply as it can access its own nitrogen. This stimulates the photobiont, causing it to build up and increase photosynthesis and carbon input. However, if the lichen cannot increase its photobiont growth, a high nitrogen uptake may result in a higher concentration than it physiologically requires, negatively impacting the lichen and its host plant.

The ability of lichens to fix nitrogen helps them spread and survive in harsh environments. However, only a small fraction of lichen species can fix nitrogen. Nitrogen deposition can also cause soil acidification, reducing plant biodiversity. In the Netherlands, for instance, high levels of ammonia have led to the disappearance of acid-preferring lichen species, resulting in communities dominated by nitrophytic species.

Lichens have been used as bioindicators of air quality for over a century in Europe. They can provide information about atmospheric deposition levels, such as the dramatic rise in atmospheric lead deposition observed in lichens from Plummers Island, Maryland, in the early 1980s. Similarly, studies in Utah and New Mexico have shown that lichen nitrogen and sulfur concentrations can be correlated with throughfall deposition levels, indicating that lichens can be effective bioindicators of nitrogen and sulfur deposition.

The presence or absence of certain lichen species in an area can indicate the levels of air pollution, particularly sulfur dioxide (SO2). For example, the presence of crusty lichens like Lecanora conizaeoides or Lepraria incana indicates poor air quality with high SO2 levels, while leafy lichens like Parmelia caperata or Evernia prunastri can survive in moderate to good air quality. In areas with very clean air, rare species like Usnea articulata or Teloschistes flavicans may be found.

Lichen and sulphur dioxide

Lichens are miniature ecosystems made of fungus and algae and/or cyanobacteria. They are durable enough to grow on tree bark and bare rock, yet sensitive to pollution and air quality. They absorb nutrients, water, minerals, and pollutants from the air, through rain and dust. Sulphur dioxide, a pollutant that comes from coal burning and industry, has killed many lichens in the past. In high concentrations, it can irritate the mucus lining of the eyes, nose, throat, and lungs, and may cause coughing and tightness in the chest. People with asthma are more vulnerable to sulphur dioxide pollution.

Some lichen species, such as Usnea lichens (also called old man's beard), do not grow in areas with sulphur dioxide pollution. However, other species, such as Lecanora conizaeoides and Lepraria incana, can tolerate poor air quality with high levels of sulphur dioxide. The presence or absence of lichens, as well as the specific species found in an area, can be used to estimate the level of sulphur dioxide pollution in that location. This is known as lichenometry.

Lichenometry has been used to study the impact of sulphur dioxide pollution in various regions. For example, in the National Capital Region of the United States, samples of the pollution-tolerant lichen Flavoparmelia caperata were analysed for sulphur content. In the UK, the decline of lichens due to sulphur dioxide pollution has been observed, particularly in urban areas. However, with the reduction of coal burning, lichens are beginning to return to these areas.

The sensitivity of lichens to sulphur dioxide pollution is influenced by various factors, including the presence of antioxidants and the activity of antioxidant enzymes. Lichens play an important role in ecosystems by contributing to biogeochemical cycling and carbon storage. They are also a source of potential antibiotics, anti-fungal agents, and anti-cancer drugs.

Lichen and other pollutants

Lichens are miniature ecosystems made of a symbiotic relationship between a fungus and a chlorophyll-containing partner, either algae or cyanobacteria. The algae or cyanobacteria photosynthesize and provide the fungus with sugars made from sunlight, and the fungus provides a home. Lichens can be found in a variety of environments, including on rocks, trees, barren earth, metal, and concrete.

Lichens are sensitive to air pollution and can be used as bioindicators of air quality. They have no roots or protective surface, so they absorb whatever is in the air or water, including pollutants. This makes them valuable indicator species, as they can provide information on the condition of their environment. Scientists can monitor the health of lichens and pair this data with atmospheric deposition data to determine the sources and levels of pollution.

The two pollutants that most affect lichens are nitrogen (N) and sulfur dioxide (SO2). Nitrogen deposition can increase the load of nutrients, and too much nitrogen can harm and kill the algae's chlorophyll, depriving the fungi of necessary sugars for survival. Sulfur dioxide is emitted by coal combustion and industrial development and can irritate the mucous membranes of the eyes, nose, throat, and lungs.

The presence or absence of lichens, as well as the specific species of lichens in an area, can indicate the level of air pollution. For example, in areas with poor air quality, only crusty lichens may be present, while leafy lichens may survive in moderate to good air, and rare species may grow in very clean air. The golden shield lichen (Xanthoria parietina) can live in areas with high levels of nitrogen, especially ammonia. It is commonly found on trees and buildings near farmland and on sea cliffs where seabird droppings provide nitrogen.

Lichens can also accumulate various pollutants such as metals, organic elements, and radioelements. They can develop structural changes in response to air pollution, including reduced photosynthesis and bleaching, discoloration, and reduced growth. Over time, sensitive species may be replaced by pollution-tolerant species. For example, in the Netherlands, high levels of ammonia have led to the disappearance of acid-preferring species, resulting in communities dominated by nitrophytic species.

Frequently asked questions