Air Pollution's Impact: The Lichen's Story

Lichens are composite organisms that combine fungi and algae or cyanobacteria. They are highly sensitive to air pollution and can act as bio-indicators of air quality. Lichens obtain their nutrients from the atmosphere, and as they have no roots or protective surface, they cannot filter what they absorb. This means that pollutants can quickly accumulate in lichens and become toxic. The two main air pollutants that affect lichen growth are nitrogen and sulphur dioxide. Nitrogen deposition can harm and kill the algae's chlorophyll, which is used to produce sugars that feed the lichen. Sulphur dioxide, on the other hand, interferes with the cyanobacteria's ability to fix nitrogen and destroys the chlorophyll of the algae, inhibiting photosynthesis.

Nitrogen pollution can cause lichens to die by killing the chlorophyll of the algae they depend on

Lichens are a composite organism made up of a fungus and an algae species. The fungus provides structure and protection for the algae, and the algae provide food for the fungus in the form of sugars made from sunlight through photosynthesis. Lichens are highly sensitive to changes in atmospheric chemistry and deposition, and so they are excellent indicators of air pollution.

Lichens obtain their nutrients from the air. They have no roots or protective surface, so they cannot filter what they absorb. If there are pollutants in the air, they can accumulate in the lichen and become toxic. Lichens are especially sensitive to nitrogen and sulphur dioxide.

In the presence of nitrogen, some lichens will die, while others will thrive. For example, 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 seabird droppings provide nitrogen.

Sulphur dioxide is harmful to lichens and can cause them to stop growing in an area

Sulphur dioxide is a gas that dissolves in water to produce highly reactive acidic ions. Lichens, which are miniature ecosystems made of fungus and algae and/or cyanobacteria, have no roots or protective surface, so they absorb anything in the air straight into their system. Sulphur dioxide is therefore very harmful to lichens and can cause them to stop growing in an area.

Sulphur dioxide disrupts important physiological processes in lichens by impairing photosynthesis and respiration. It is easily absorbed by lichens and, once dissolved in their cellular cytoplasm, it has an acidifying effect. Enzyme-mediated processes such as photosynthesis and respiration are very sensitive to pH changes, and are impaired by added acidity. All lichens are thus sensitive to sulphur dioxide, although some are more so than others.

The net effect of sulphur dioxide on lichen communities is always a decrease in diversity as sensitive species are lost and no new species replace them. Sulphur dioxide emissions have historically been a significant industrial pollutant, being a by-product of the use of high-sulphur fuels. While sulphur-removal technology has reduced sulphur dioxide pollution levels in many countries, it is still a significant pollutant in many areas.

Sulphur dioxide can also inhibit lichen reproduction (both sexual and asexual) as well as spore germination, though the effects are not uniform across species. Sulphur dioxide has also been shown to inhibit the activity of nitrogenase, which is used by cyanobacterial photobionts to fix atmospheric nitrogen.

Some lichens are highly tolerant of sulphur dioxide, such as the crustose lichen Lecanora conizaeoides. The superhydrophobicity of the thallus surface in one of the most sulphur-dioxide-tolerant lichen species, Lecanora conizaeoides, suggests that surface hydrophobicity could be a general feature of lichen symbioses controlling their tolerance to sulphur dioxide.

Lichens can be used as bio-indicators of air pollution, showing the presence of harmful pollutants

Lichens are composite organisms that combine fungi and algae or cyanobacteria. They are highly sensitive to air pollution because they absorb all their nutrients from the atmosphere. This makes them valuable bio-indicators of air quality.

Lichens have been used as bio-indicators of air pollution for over a century in Europe. They are especially sensitive to sulphur, nitrogen, acidity, halogens (e.g. fluoride), heavy metals and ozone. The presence or absence of certain lichen species can indicate the level of air pollution in an area. For example, the presence of Usnea lichens, also called old man's beard, indicates that coal has not been burnt in the area for a long time.

Lichens are also used to indicate the levels of specific pollutants. Nitrogen deposition, for example, can increase the load of nutrients, and too much nitrogen can harm and kill the algae's chlorophyll, which is used to produce sugars that feed the lichen. Sulphur dioxide, on the other hand, interferes with the cyanobacteria's ability to fix nitrogen and destroys the chlorophyll of the algae, inhibiting photosynthesis.

The variety of lichen species is taken into account when calculating and representing the air quality index. This is done by determining the degree of cover and vitality, as different lichen species have different sensitivities to air pollutants.

Air pollution can cause a decline in lichen diversity, with species responding according to their sensitivity

Lichens are highly sensitive to air pollution. They are composite organisms, combining fungi and algae or cyanobacteria. They obtain their nutrients from the atmosphere, meaning they cannot filter the pollutants they absorb. This makes them excellent bioindicators of air quality.

Lichen diversity is affected by air pollution, specifically sulphur and nitrogen dioxide. Sulphur dioxide is released through coal burning and industry, and nitrogen dioxide is created when nitrogen is heated and combined with oxygen, as in car engines.

Sulphur dioxide easily dissolves in water, which lichens then absorb. It also interferes with the cyanobacteria's ability to fix nitrogen and destroys the chlorophyll of the algae, inhibiting photosynthesis. This can cause the decline of certain lichen species, as they respond according to their sensitivity to the pollutant.

Nitrogen dioxide is a powerful pollutant, affecting human health and lichen diversity. Lichens need nitrogen to survive, as it supports the production of necessary proteins and organic acids. However, too much nitrogen can over-fertilise certain lichen species, causing them to die. Excess nitrogen can also kill the chlorophyll of the algae, which deprives the fungi of necessary sugars.

The presence of certain lichen species can indicate the levels of these pollutants in the air. For example, the presence of Usnea lichens indicates an absence of sulphur dioxide pollution, as they do not grow in areas with this pollutant. The variety and size of lichens in an area can also indicate its pollution levels: the smaller the lichens and the less variety, the more polluted the environment.

Some lichens can thrive in nitrogen pollution, particularly those that favour metal-enriched substrata

Lichens are miniature ecosystems that arise from the symbiotic relationship between a fungus and a chlorophyll-containing partner, either algae or cyanobacteria. They are highly sensitive to air pollution, particularly nitrogen (N) and sulphur dioxide (SO2) pollution, as they absorb all their nutrients from the atmosphere.

While some lichens cannot tolerate the presence of nitrogen and will die, others can thrive in nitrogen-rich environments. The golden shield lichen (*Xanthoria parietina*), for instance, can survive 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 are a source of nitrogen.

Nitrogen-tolerant lichens, also known as "nitrophyte" species, favour environments with high nitrogen levels and tree bark with a high pH. Their preference for metal-enriched substrata is linked to the impact of nitrogen on bark pH. Nitrogen pollution, particularly in the form of ammonia, can increase bark pH, making it more alkaline. Lichens that thrive in these conditions include the *Candelaria concolor*, *Physcia adscendens*, and *Xanthoria polycarpa*.

The presence of these nitrophyte lichens indicates elevated nitrogen deposition, either from natural or anthropogenic sources. They are often found in agricultural settings, where ammonia emissions from livestock and fertiliser use contribute to nitrogen pollution. The Dutch have developed a classification system for lichens based on their nitrogen preferences, distinguishing between "nitrophyte" and "acidophyte" species. This classification method has been instrumental in mapping and monitoring nitrogen and ammonia pollution patterns across Europe.

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