Lichen Growth: Pollution's Impact And Influence

Lichens are highly prevalent in approximately 8% of terrestrial ecosystems and are commonly found in environments with extreme temperatures, limited water availability, and low nutrient levels. They are key components of many ecosystems and contribute significantly to the sustainability of natural systems. They are also widely recognised as bioindicators of environmental pollution because of their sensitivity to various pollutants, including nitrogen, sulphur dioxide, heavy metals, ozone, and ammonia. Therefore, their health and distribution can be used to monitor air quality. This raises the question: does pollution inhibit lichen growth?

Nitrogen dioxide and sulphur dioxide

Lichens are sensitive to atmospheric pollution and can be used as indicators of air quality. Two of the main air pollutants that affect lichen growth are nitrogen and sulphur dioxide.

Nitrogen dioxide is a powerful pollutant that is harmful to human health in high concentrations. It is produced when nitrogen is heated and combined with oxygen, as happens in car engines. Farms also emit nitrogen pollutants from fertilisers, farm machinery, and livestock waste. Nitrogen dioxide can inflame the lining of the lungs, causing respiratory symptoms such as shortness of breath and coughing. It can also decrease the body's immune response to lung infections and make asthma worse. 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 commonly found near farmland and on sea cliffs where seabird droppings provide nitrogen.

Sulphur dioxide is a significant industrial pollutant and a by-product of burning high-sulphur fuels. It is also produced by coal burning and industrial activities. Sulphur dioxide molecules are strongly polar and are readily deposited onto surfaces of all kinds. This pollutant has killed many lichens, but the reduction in coal burning in recent years has allowed some lichen species to return. Sulphur dioxide dissolves easily in water to form highly reactive acidic ions, which are readily absorbed through lichen thalli. This disrupts photosynthesis and inhibits both sexual and asexual lichen reproduction, as well as spore germination. The effects of sulphur dioxide vary across lichen species, with some species such as *Lecanora conizaeoides* being able to tolerate high concentrations.

Heavy metal pollution

Lichens are highly prevalent in approximately 8% of terrestrial ecosystems and are commonly found in environments with extreme temperatures, limited water availability, and low nutrient levels. They are key components of many ecosystems and play essential roles in soil formation, habitat creation, nutrient cycling, and food sources.

Lichens are also widely recognized as bioindicators of environmental pollution due to their sensitivity to various pollutants, including heavy metals. Heavy metal pollution is a significant environmental problem with detrimental effects on ecosystems and human health. Lichens can accumulate heavy metals, which cause multiple physiological changes and can remain unaltered for prolonged periods. The accumulation of heavy metals in lichen thalli can disrupt metabolic pathways and inhibit photosynthesis by degrading chlorophyll, reducing photosynthetic efficiency, and changing the content of photosynthetic pigments.

Despite the potential toxicity of heavy metals, certain lichen species are effective colonizers of polluted sites or are even confined to metal-enriched habitats. The structure and physiology of lichens determine their success in colonizing extremely hostile environments. Some lichen species exhibit high plasticity in growth form and high intraspecific genetic variation, allowing them to adapt to heavy metal-enriched sites.

Metal accumulation studies of epilithic lichens are limited due to their low biomass and intimate association with the substrate. However, research has shown that the substrate is the main source of heavy metals in the thalli of epilithic lichens, and growth form plays a significant role in the accumulation of metal by lichens. Crustose lichens, in particular, have been found to possess the ability to hyperaccumulate heavy metals.

In summary, heavy metal pollution can inhibit lichen growth by disrupting their metabolic pathways and inhibiting their photosynthetic processes. However, some lichen species exhibit remarkable resilience and adaptation strategies, allowing them to colonize even heavily polluted environments.

Lichen as bio-indicators

Lichens are highly prevalent in approximately 8% of terrestrial ecosystems and are commonly found thriving in environments with extreme temperatures, limited water availability, and low nutrient levels. They are key components of many ecosystems and play essential roles in soil formation, habitat creation for plants and insects, as a food source, and nutrient cycling.

Lichens are sensitive to atmospheric pollution such as nitrogen (N) because they receive all their nutrients and water from wet and dry atmospheric deposition. Nitrogen deposition can increase the load of nutrients, and too much N can harm and kill the algae’s chlorophyll, which is used to produce sugars to feed the lichen. Certain species of lichen are more tolerant of N than others. Scientists monitor lichen communities, and if there is an increase in N-tolerant species and a decrease in N-sensitive species, this indicates a potential increase in atmospheric N deposition and the beginning of ecosystem decline.

Lichens are widely recognized as bioindicators of environmental pollution because of their sensitivity to various pollutants, including heavy metals. Their health and distribution can be used to monitor air quality. Lichens are used in the field of biomonitoring, which involves measuring the concentrations of pollutants in lichen tissue and using that data to assess the levels of pollution in the surrounding environment. The accumulation of heavy metals inside lichen thalli can lead to the disruption of metabolic pathways and the inhibition of photosynthesis.

Sulphur dioxide (SO2) is another common pollutant that has been studied extensively in relation to lichens. SO2 is a significant industrial pollutant and a by-product of using high-sulphur fuels. While sulphur-removal technology has reduced SO2 pollution levels, it is still a problem in many areas. SO2 can inhibit lichen reproduction and spore germination, and its effects are not uniform across species. Some lichens, such as the golden shield lichen, can live in areas with high levels of nitrogen, while others will die in the presence of nitrogen. Usnea lichens, also called old man's beard, do not grow in areas with sulphur dioxide pollution.

Air pollution and local habitat stresses

Lichens are highly prevalent in approximately 8% of terrestrial ecosystems and are commonly found in environments with extreme temperatures, limited water availability, and low nutrient levels. They are key components of many ecosystems and contribute significantly to the sustainability of natural systems. They play essential roles in soil formation, habitat creation for plants and insects, nutrient cycling, and as a food source.

However, lichens are also highly sensitive to air pollution. They are used in the field of biomonitoring, which involves measuring the concentrations of pollutants in lichen tissue to assess the levels of pollution in the surrounding environment. Lichens are especially sensitive to nitrogen and sulphur dioxide. The two primary sources of sulphur dioxide pollution are coal burning and industry. While we burn less coal now, and sulphur-removal technology has reduced sulphur dioxide pollution in many countries, it remains a significant pollutant in many areas. Sulphur dioxide dissolves readily in water to produce highly reactive acidic ions, which are absorbed through lichen thalli and disrupt photosynthesis. Sulphur dioxide also inhibits lichen reproduction and spore germination.

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 for the lichen. Lichens that are sensitive to nitrogen include oakmoss and Usnea lichens, also called old man's beard. However, some lichens can thrive in the presence of nitrogen, such as the golden shield lichen, which can live in areas with high levels of nitrogen, especially ammonia.

Lichens are also affected by heavy metal pollutants, which can lead to the disruption of metabolic pathways and the inhibition of photosynthesis. Heavy metals can accumulate in lichen thalli and cause multiple physiological changes.

The impact of ammonia

Lichens are sensitive to air pollution and can therefore be used as indicators of air quality. They are particularly susceptible to nitrogen pollution, which can harm and kill the algae's chlorophyll, which is used to produce sugars to feed the lichen.

Ammonia (NH3) is a form of reactive nitrogen that is becoming a significant pollutant due to the increase in intensive animal rearing. It is released from composting plants and livestock farming, and can also come from agricultural chemicals such as fertilisers. Ammonia emissions from agriculture and other sources are a form of airborne reduced nitrogen.

Studies have shown that ammonia can have a detrimental impact on lichen photosynthesis. Lichens exposed to ammonia pollution showed physiological damage, with dry deposition of NH3 causing the most harm. The photosystem II of the two lichens exposed to NH3 was susceptible to this pollutant in the gas phase. However, some lichens are more tolerant of ammonia than others. The golden shield lichen (Xanthoria parietina) can live in areas with high levels of nitrogen, especially ammonia, and is common on trees and buildings near farmland.

The effects of ammonia on lichens can be complex, with some studies finding evidence of declines in lichen diversity due to ammonia, while others found no effect. The susceptibility of lichens to ammonia toxicity may be linked to their photosynthetic capacity. Lichens can avoid the buildup of ammonia to toxic levels by converting it to amino acids, which relies on carbon from photosynthesis. Therefore, species with decreased photosynthetic capacity may be more susceptible to ammonia toxicity.

In summary, ammonia is a significant pollutant that can have detrimental effects on lichens, particularly their photosynthetic performance. However, the impact of ammonia on lichens can vary depending on the species and their photosynthetic capacity.

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