Lichen's Role In Air Pollution Monitoring

Lichens are sensitive to air pollution and are used as bio-indicators to monitor air quality. They are indicator species, which are organisms that provide information on the condition of their environment. Lichens are used to determine the sources and levels of pollution and their effects on ecosystems. They are especially sensitive to nitrogen and sulphur dioxide, and their health and distribution can be used to monitor air quality. Lichens are also used to study the effects of heavy metal pollution. While lichens are useful, they are limited in that they can only monitor certain types of pollutants and do not provide real-time feedback.

Lichens as bio-indicators

Lichens are sensitive to air pollution and are, therefore, useful bio-indicators of air quality. Lichens are miniature ecosystems that consist of a symbiotic relationship between a fungus and a chlorophyll-containing partner, either algae or cyanobacteria. They have no roots, so they absorb all their nutrients directly from the air. This means that if there are pollutants in the air, they will accumulate in the lichen and can become toxic very quickly.

Lichens are used in the field of biomonitoring, which involves monitoring environmental pollution using living organisms. Lichen biomonitoring involves measuring the concentrations of pollutants in lichen tissue and using that data to assess the levels of pollution in the surrounding environment. The health of lichens within a particular area is negatively affected by the level of environmental pollution in that area.

The Dutch developed a method of classifying lichens: “nitrophyte” lichen species thrive in high-nitrogen environments and on tree bark with high pH, while “acidophyte” lichen species prefer the opposite. Different lichen species have different sensitivities to air pollutants, so the variety of lichen species must be taken into account when calculating and representing the air quality index.

Lichens are also useful bio-indicators because they are easier to study and quicker to respond to environmental change than other bio-indicators such as butterflies, nematodes, frogs, and toads. They are also versatile in highlighting the existence and the rise and fall of levels of new pollution regimes over wide geographical areas.

However, it is important to note that the information lichens provide as bio-indicators is limited. They can only be used to monitor nitrogen and sulfur dioxide levels, while there are many other pollutants that are ecological and health threats. They are also unable to give immediate feedback on exact pollution levels, only averages over a longer period of time.

The effects of nitrogen pollution

Nitrogen pollution is one of the biggest contributors to biodiversity loss on Earth, alongside habitat destruction and climate change. It is caused by human activities, including industrial and agricultural processes, and has a range of detrimental effects on the environment and human health.

Nitrogen is the most abundant element in our atmosphere, making up about 78% of the air we breathe. However, most of this nitrogen is unusable by organisms. Through natural and artificial processes, atmospheric nitrogen can be converted into a "reactive" form that is more accessible to living things. This reactive nitrogen is essential for life, playing a role in the creation of amino acids, proteins, and enzymes. It is also commonly used as a fertiliser, leading to a significant increase in global food production.

However, too much reactive nitrogen is harmful to the environment. It can be lost to the environment in various ways, such as through leaching into soil, rivers, and lakes, or being emitted into the air. This excess nitrogen can have a range of negative consequences, including the pollution of water and air, degradation of soils, and the creation of "'dead zones'" in the ocean. It can also cause toxic algal blooms, which can be harmful to humans and other organisms, and contribute to ozone depletion and climate change.

In the context of air pollution, nitrogen dioxide (NO2) is a significant pollutant. It is created when nitrogen is heated and combined with oxygen, such as in car engines. Nitrogen dioxide is harmful to human health, particularly the respiratory system, and can cause issues such as shortness of breath, coughing, and decreased immune response to lung infections. It is estimated that 77% of people breathe annual average concentrations of nitrogen dioxide beyond safe levels, with traffic and agricultural emissions being major contributors.

Sulphur dioxide and lichens

Sulphur dioxide is a major air pollutant and one of the two main air pollutants that affect lichen growth, the other being nitrogen. Sulphur dioxide dissolves in water to produce acidic ions that are readily absorbed through the lichen thalli, disrupting photosynthesis. Sulphur dioxide has also been shown to inhibit the activity of nitrogenase, which is used by cyanobacterial photobionts to fix atmospheric nitrogen. Sulphur dioxide fumigation of the lichens Evernia prunastri and Ramalina fraxinea resulted in changes in net photosynthesis, dark respiration and chlorophyll content in relation to both concentration and duration of exposure. Net photosynthesis was the most sensitive response variable, with a significant reduction in chlorophyll content found when no recovery in net photosynthesis occurred after two weeks. A reduction in dark respiration was only found at high SO2 concentrations.

Lichens are miniature ecosystems made of fungus and an algae and/or cyanobacteria. They are sensitive to air pollution because they have no roots or protective surface, so they cannot filter what they absorb. Instead, they receive all their nutrients from the atmosphere, which makes them valuable as indicator species. Lichens are used by scientists to determine the air quality in a given area. They are very sensitive to pollution and respond to it in short time frames.

The Dutch developed a method of classifying lichens: “nitrophyte” lichen species thrive in high-nitrogen environments and on tree bark with high pH, while “acidophyte” lichen species prefer the opposite. Bark pH has been found to be affected by sulphur dioxide concentrations. This method has been used to map and monitor nitrogen and ammonia pollution patterns across countries in Europe.

Lichenologists like Gothamie Weerakoon at the Natural History Museum are using lichens as indicators to monitor the effects of nitrogen air pollution from tea estates in Sri Lanka and in the forests of the Himalayas. They are setting up permanent plots in Sri Lanka to monitor the effects of nitrogen air pollution, recording how the lichen community in these plots change over time.

Heavy metal pollution

Lichens are widely recognised as bioindicators of environmental pollution due to their sensitivity to various pollutants, including heavy metals. Their health and distribution can be used to monitor air quality. Heavy metal pollution is a significant environmental problem, with detrimental effects on ecosystems and human health. Lichens can accumulate heavy metals, which can cause multiple physiological changes and lead to adverse effects on their health.

The surface of lichens and the intercellular spaces of their medulla can serve as deposition sites for fine particles containing heavy metals, which can remain unaltered for prolonged periods. This ability to accumulate and retain significant quantities of heavy metals makes lichens valuable bioindicators for assessing the health of an ecosystem. By studying lichens, scientists can gain insights into the impact of human activities on the environment and develop effective strategies for reducing heavy metal pollution.

Lichen biomonitoring involves measuring the concentrations of pollutants, including heavy metals, in lichen tissue. This data is then used to assess the levels of pollution in the surrounding environment. One common approach is to study the physiological responses of lichens to heavy metal pollution, such as the degradation of chlorophyll, which can lead to decreased photosynthetic efficiency and overall health.

Lichens have been used in Germany as bioindicators for air pollution, and their health is monitored and paired with atmospheric deposition data to identify critical sources and overall levels of pollution. This includes studying the accumulation of heavy metals and their effects on lichen physiology. Additionally, the Dutch have developed a method of classifying lichens into "nitrophyte" and "acidophyte" species, which has helped map and monitor nitrogen and ammonia pollution patterns across Europe.

In summary, lichens are valuable tools for monitoring heavy metal pollution due to their sensitivity and ability to accumulate these pollutants. By studying lichens, scientists can gain insights into the presence and impact of heavy metal pollution, helping to develop strategies for environmental protection and sustainability.

Lichen distribution and air quality

Lichens are sensitive to air pollution and are, therefore, good indicators of air quality. They absorb nutrients from the air and do not have a protective surface or roots, so they cannot filter what they absorb. This means that pollutants can accumulate in the lichen and become toxic. The two main air pollutants that affect lichen growth are nitrogen and sulphur dioxide.

Lichens are miniature ecosystems made of fungus and an algae and/or cyanobacteria. The fungus provides shelter for the algae, and the algae provide food for the fungus. This symbiotic relationship means that lichens do not need roots. They can be found in both nature and human-made environments, including rocks, trees, barren earth, metal and concrete.

The presence of different lichen species in a location can tell us a lot about the air quality. Some lichens will die in the presence of nitrogen, while others will thrive. The oakmoss lichen, for example, is sensitive to nitrogen and can be found on woodland branches where the air is clean. In contrast, the golden shield lichen (Xanthoria parietina) can live in areas with high levels of nitrogen, especially ammonia. It is often found near farmland, on trees and buildings, and on sea cliffs where seabird droppings provide nitrogen.

The variety of lichen species is taken into account when calculating and representing the air quality index. The Dutch developed a method of classifying lichens: "nitrophyte" lichen species thrive in high-nitrogen environments and on tree bark with a high pH. "Acidophyte" lichen species prefer the opposite. This method has been used to map and monitor nitrogen and ammonia pollution patterns across Europe.

Lichen distribution and abundance can be used to map air pollution levels. For example, a study in the UK used citizen science to investigate patterns in the distribution and abundance of selected lichen species on tree trunks and branches and relate these to air pollution and climate. Data was collected on nine lichen indicators on 19,334 deciduous trees. The data confirmed the relationships between levels of nitrogenous air pollutants and the distribution and abundance of lichens.

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