Lichen's Resilience: Unveiling Air Pollution Secrets

Lichens are highly sensitive to air pollution and are therefore used as bio-indicators of air quality. They are miniature ecosystems made of a fungus and an alga and/or cyanobacteria. Lichens absorb water, minerals, and pollutants from the air through rain and dust. They are particularly sensitive to sulphur, nitrogen, acidity, halogens, heavy metals, and ozone. The presence or absence of certain lichen species can indicate the level of air pollution in an environment. For example, leafy lichens such as Evernia prunastri can survive in moderate to good air, while rare species such as Usnea articulata may grow in very clean air. Lichens are also used to monitor nitrogen levels in tea farms in Sri Lanka and the Himalayan forests.

Lichens absorb pollutants from the air, through rain and dust

Lichens are miniature ecosystems made up of a fungus and an alga and/or cyanobacteria. The alga in lichens photosynthesizes (creates food from sunlight energy), and both the alga and fungus absorb water, minerals, and pollutants from the air, through rain and dust. Lichens do not have roots or a protective surface, so they cannot filter what they absorb; as a result, anything in the air is taken straight inside. If there are pollutants, they can accumulate in the lichen and become toxic very quickly.

Lichens are very sensitive to air pollution and respond to it in short time frames. They are especially sensitive to sulphur, nitrogen, acidity, halogens (e.g. fluoride), heavy metals, and ozone. 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 to feed it and the fungi. Lichens can develop structural changes in response to air pollution, including reduced photosynthesis and bleaching. Pollution can also cause the death of the lichen alga, discoloration, and reduced growth of the lichen fungus, or it can kill a lichen completely.

Scientists monitor lichen communities to determine the sources and levels of pollution causing detrimental effects. They pair bio-monitoring data with atmospheric deposition data from the National Atmospheric Deposition Association (NADP). The NADP National Trends Network (NTN) measures nitrate (NO3-) and ammonium (NH4+) weekly in rain and snow samples over 250 U.S. sites. The US Environmental Protection Agency Clean Air Status and Trends Network (CASTNET) measures gaseous HNO3 and dry deposition particulate NH4+ and NO3- across 94 U.S. sites.

Lichens are considered "canaries in the coal mine" for chemically sensitive vascular plants. They are easier to study than other indicators of environmental pollutants, such as butterflies, nematodes, frogs, and toads, and they are quicker to respond to environmental change. As a rule of thumb, the smaller the size and less variety of lichens in an area, the more polluted the environment.

They are sensitive to nitrogen, sulphur dioxide and heavy metals

Lichens are sensitive to air pollution and can, therefore, be used as indicators of air quality. They absorb water, minerals, and pollutants from the air, through rain and dust. Some sensitive lichen species develop structural changes in response to air pollution, including reduced photosynthesis and bleaching.

Lichens are sensitive to nitrogen in the air. 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 lichen. Certain species of lichen are more tolerant of nitrogen than others. Scientists monitor lichen communities, and if there is an increase in nitrogen-tolerant species, along with a decrease in nitrogen-sensitive species, this indicates a rise in atmospheric nitrogen deposition.

Nitrogen dioxide is a harmful pollutant that can cause respiratory issues such as coughing and a reduced immune response to lung infections. It is produced when nitrogen is heated and combined with oxygen, as happens in car engines. Farms, especially those using fertilisers, are a significant source of nitrogen pollution, along with machinery and livestock waste.

Lichens are also sensitive to sulphur dioxide. In high concentrations, it can irritate the mucus lining of the eyes, nose, throat, and lungs. Sulphur dioxide pollution comes from burning coal and industrial activities. Usnea lichens, also called old man's beard, do not grow in areas with sulphur dioxide pollution.

Lichens are also effective bio-indicators of heavy metal pollution. Several studies have shown that sulphur, nitrogen, and metal concentrations in lichen tissue indicate air quality within a region.

Lichens are used to monitor nitrogen levels at tea farms in Sri Lanka and the Himalayas

Lichens are composite organisms consisting of a symbiotic relationship between a fungus and a chlorophyll-containing partner, either algae or cyanobacteria. They are sensitive to air pollution because they absorb nutrients directly from the atmosphere, which makes them valuable as indicators of air quality.

Tea growers in Sri Lanka and the Himalayas use fertilisers that contain nitrogen in the form of ammonia. This ammonia does not stay on the tea farm, and some of it inevitably flows into the surrounding ecosystems. The nitrogen is carried into the atmosphere through winds and sits in the clouds, ready to rain on the nearby mountains and cloud forests.

Gothamie Weerakoon, a lichenologist and senior curator of lichens and slime moulds at the Natural History Museum of London, is part of a team setting up permanent plots in Sri Lanka to monitor the effects of nitrogen air pollution. They are using lichens as indicators. These plots sit high up in the mountain cloud forests, and the researchers are recording how the lichen community in these plots change over time. To set up the experiment, they consider the local climate and known atmospheric nitrogen levels. They then release nitrogen in a controlled way onto the plots to test the effects of different nitrogen concentrations.

Weerakoon states that "lichens are perfect bioindicators to assess the impacts of atmospheric nitrogen pollution as the organism directly absorbs the nutrients required for its growth from the atmosphere." This research will help them understand how increases in fertiliser use on tea farms will change the ecology of southern Asia.

Lichens can indicate the typical sulphur dioxide levels in large towns and cities

Lichens are miniature ecosystems composed of a fungus and an alga and/or cyanobacteria. They are sensitive to air pollution and can be used as indicators of air quality. They absorb water, minerals, and pollutants from the air through rain and dust. Some sensitive lichen species develop structural changes in response to air pollution, including reduced photosynthesis and bleaching.

Lichens are excellent bio-indicators of sulphur dioxide levels in the lower troposphere. Sulphur dioxide is a major pollutant in large towns and cities, due to coal-burning and industrial activities. Lichens respond predictably to changes in air quality, and their presence or absence can indicate the typical sulphur dioxide levels in an area. For example, Usnea lichens, also called old man's beard, do not grow in areas where there is sulphur dioxide pollution. Their presence indicates that coal has not been burnt in the area for some time.

The use of lichens as indicators of atmospheric quality dates back to the 1860s when independent observations in Europe showed that lichen declines were caused by air pollution. Since then, studies have established that lichens respond predictably to changes in air quality. Over 1500 papers have been published on the topic.

Scientists can monitor the health of lichens and pair this bio-monitoring data with atmospheric deposition data to determine the sources and levels of pollution causing detrimental effects. This information can be used to develop policies and interventions to improve air quality and protect sensitive ecosystems.

In addition to sulphur dioxide, lichens are also sensitive to nitrogen in the air. 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. Scientists can monitor lichen communities, and a shift in species composition or their health may indicate an increase in atmospheric nitrogen deposition.

They are used as bio-indicators of air quality in Europe for over a century

Lichens have been used as bio-indicators of air quality in Europe for over a century. They are sensitive to a number of air pollutants, including sulphur dioxide, nitrogen dioxide, and heavy metals. This sensitivity is due to the fact that lichens absorb water, minerals, and pollutants from the air through rain and dust, and have no roots or protective surface to filter what they absorb. As a result, pollutants can accumulate in lichens and become toxic.

The presence or absence of certain lichen species, as well as changes in their health and distribution, can indicate the overall air quality of an environment. For example, in areas with poor air quality, only crusty lichens such as Lecanora conizaeoides or Lepraria incana can typically tolerate the high levels of sulphur dioxide. In moderate to good air quality, leafy lichens such as Parmelia caperata or Evernia prunastri can survive, while rare species such as Usnea articulata or Teloschistes flavicans may be found in areas with very clean air.

Scientists can monitor the health and species composition of lichen communities to detect potential increases in atmospheric nitrogen deposition. For example, an increase in nitrogen-tolerant species alongside a decrease in nitrogen-sensitive species may indicate rising nitrogen levels in the atmosphere. This information can then be used to inform policies and actions to improve air quality and protect sensitive ecosystems.

In addition to nitrogen and sulphur dioxide levels, lichens can also be used to monitor ammonia pollution. For example, in the Netherlands, high levels of ammonia have led to the disappearance of acid-preferring lichen species, with communities now dominated by nitrophytic species. Lichens have also been used to study the effects of nitrogen pollution in Sri Lanka and the Himalayan forests.

While lichens are valuable bio-indicators, they have some limitations. For instance, lichens can only monitor certain pollutants like nitrogen and sulphur dioxide, while there are numerous other pollutants that pose ecological and health threats. Furthermore, lichens provide average pollution data over a long period rather than immediate feedback on exact pollution levels, which can make it challenging to source, control, manage, and improve air quality in real-time.

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