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Lichens are sensitive organisms that can be used as bioindicators of air quality. They are particularly susceptible to nitrogen and sulphur dioxide, with the latter causing the disappearance of lichens in the Jardin du Luxembourg in Paris between 1866 and 1896. Sulphur dioxide is produced by burning coal and through industrial processes. Nitrogen, on the other hand, is present in the Earth's atmosphere and becomes harmful when heated and combined with oxygen, forming nitrogen oxides. Lichens are also vulnerable to ammonia, which is a gaseous air pollutant resulting from agricultural activities, and heavy metal pollution. Their sensitivity to these pollutants makes lichens valuable indicators of air quality and ecological health.
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What You'll Learn
Nitrogen oxides
Lichens are sensitive to various atmospheric pollutants, including nitrogen oxides. They are composite organisms comprising a fungus (mycobiont) and one or more algal species (phycobionts). Lichens derive their water and essential nutrients from the atmosphere, making them highly susceptible to air pollution.
Lichens, particularly shrubby and leafy varieties, are sensitive to nitrogen oxides due to their dependence on atmospheric nutrients. Nitrogen deposition increases the nutrient load, and excessive nitrogen can harm and kill the algae's chlorophyll, disrupting the symbiotic relationship with the fungus. This disruption can lead to a decline in the lichen's overall health and population.
Scientists have studied the impact of nitrogen oxides on lichen communities, observing that increased nitrogen oxide pollution leads to a decrease in nitrogen-sensitive lichen species and a concurrent increase in nitrogen-tolerant species. This shift in species composition and health can indicate potential ecological decline due to excessive nitrogen deposition.
Lichen monitoring, such as the Open Air Laboratories (OPAL) citizen science project, is essential for understanding the effects of nitrogen oxide pollution on lichens and the broader ecosystem. By studying lichens, scientists can gain insights into the impact of human activities and develop strategies to mitigate pollution and promote environmental sustainability.
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Sulphur dioxide
Lichens are composite organisms consisting of a symbiotic partnership between a fungus and an alga. The fungus provides structure and protection for the alga, which reciprocates by providing energy through photosynthesis. Lichens do not have any physical means of defence against the environment, so they absorb pollutants present in the air, rainwater, and dust. This makes them effective bioaccumulators of pollutants, including sulphur dioxide.
The sensitivity of lichens to sulphur dioxide varies depending on the species. For instance, the Usnea lichen, also known as old man's beard, does not grow in areas with sulphur dioxide pollution. Other lichens, like the golden shield lichen (Xanthoria parietina), can tolerate higher levels of nitrogen pollution and are more resistant to sulphur dioxide.
The presence or absence of different lichen species in an area can provide valuable information about the levels of sulphur dioxide pollution in that environment. By studying the distribution and health of lichens, scientists can assess the impact of sulphur dioxide pollution on ecosystems and develop strategies to mitigate its effects.
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Ammonia
Lichens are miniature ecosystems made of fungus and algae and/or cyanobacteria. They are very sensitive to air pollution and are therefore good indicators of air quality. They are particularly sensitive to nitrogen and sulphur dioxide.
The golden shield lichen (Xanthoria parietina) is one example of a lichen that 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.
In summary, ammonia is a pollutant gas that lichens are particularly sensitive to, and it can have detrimental effects on their growth and health.
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Nitrogen dioxide
Lichens are sensitive to various atmospheric pollutants, including nitrogen, sulphur dioxide, ammonia, and heavy metals. They are well-suited for monitoring air quality due to their ability to absorb pollutants through their entire thallus, which is essential for gas exchange during photosynthesis and respiration.
Lichens absorb nitrogen dioxide from the air, and high concentrations can be detrimental to their health. Nitrogen deposition increases the load of nutrients, and excessive nitrogen can harm and kill the algae's chlorophyll, which is essential for producing sugars for the lichen symbiosis. This disruption can lead to a decline in the lichen's overall health and even local ecosystem decline.
The presence and health of lichens are used as bioindicators of nitrogen dioxide pollution. Scientists monitor lichen communities, and a decrease in nitrogen-sensitive lichen species, such as Cetraria pinastri and Usnea hirta, indicates an increase in nitrogen deposition. This information helps identify areas with potentially harmful levels of nitrogen dioxide and allows for the development of strategies to reduce nitrogen pollution.
In addition to nitrogen dioxide, lichens are also sensitive to other nitrogenous compounds, such as ammonium and nitrate. The concentration of nitrogen, especially as ammonium, may be more critical in determining the tolerance or sensitivity of lichens to nitrogen pollution.
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Heavy metals
Lichens are highly prevalent in around 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. Lichens are sensitive to atmospheric pollution, such as nitrogen, as they receive all their nutrients and water from atmospheric deposition. 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 the fungi. Lichens can also absorb heavy metals from the soil, helping to detoxify polluted areas.
Lichens have been used to monitor the quality of the environments in which they live, as they are true "sponges" that recover compounds present in the atmosphere throughout the year and throughout their life cycle. They are particularly well-adapted to the study of gaseous or particulate air pollution due to their anatomical and physiological characteristics. For example, rainwater and air enter the entire thallus, and dust is trapped between mycelium filaments, while lichenic acids fix pollutants. Fruticose lichens, with their bushy structure, have a higher surface-to-volume ratio than other types of lichens and can better recover air pollutants present as aerosols.
Some lichens can tolerate high concentrations of metals by sequestering them extracellularly. Metal accumulation depends on the element and its abundance in the environment. For example, Zn, Pb, Cd, and Ni were accumulated mostly extracellularly, while Cu and Ni were accumulated intracellularly. Lichens can also hyperaccumulate heavy metals, with higher concentrations of metals in their thalli than in the corresponding substrates.
Studies have shown that lichens from South Korea's Gangwon Province and Jeju Island were able to detect slight differences in ambient heavy metal concentrations through physiological variables, including chlorophyll damage, lipid oxidation, and protein content. Arsenic, in particular, had a significant impact on chlorophyll degradation and protein content. Another study on epilithic lichens colonising artificial post-smelting wastes found that the crustose lichens Candelariella aurella, Lecanora muralis, and Lecidea fuscoatra, and the fruticose lichen Stereocaulon nanodes, had the ability to hyperaccumulate heavy metals.
In conclusion, lichens are important bioindicators of environmental quality, especially regarding nitrogen deposition and heavy metal pollution. Their ability to tolerate and accumulate pollutants makes them essential components of ecosystems, contributing to the sustainability and health of natural systems.
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Frequently asked questions
Lichens are particularly sensitive to sulphur dioxide (SO2) pollution.
Sulphur dioxide is a pollutant produced when coal is burned for heating or in power plants. It can irritate the mucus lining of the eyes, nose, throat and lungs, causing coughing and tightness in the chest. Lichens absorb sulphur dioxide rapidly, leading to an accumulation of sulphur in their tissues, which inhibits their growth.
Lichens are also sensitive to nitrogen oxides, which are created when nitrogen is heated and combined with oxygen, as in a car engine. Nitrogen dioxide can irritate the lungs, causing respiratory symptoms.
Yes, the golden shield lichen (Xanthoria parietina) can live in areas with high levels of nitrogen, especially ammonia, which often comes from agricultural activities.
Yes, some species of Cladonia have been shown to be tolerant of high levels of nitrogen.
