Lichen's Sensitivity: Pollution's Impact And Effects Explored

Lichens are composite organisms, a symbiotic relationship between a fungus and an algae or cyanobacteria. They are highly sensitive to air pollution and have been used as indicators of air quality for over a century. They absorb nutrients from the atmosphere and do not have a protective surface, so they are vulnerable to pollutants in the air. Sulphur dioxide and nitrogen are two of the main pollutants that affect lichens, and they can cause structural changes, reduced growth, discolouration, and even death. Lichens can be early warning signs of chemically sensitive vascular plants, and their presence or absence can indicate the level of pollution in an area.

Nitrogen pollution

Lichens are highly sensitive to nitrogen pollution. As lichens absorb all their nutrients from the atmosphere, they are vulnerable to absorbing pollutants, which can quickly become toxic. Lichens are used as bio-indicators of nitrogen pollution, as they can be used to monitor the levels of nitrogen pollution in a given area.

Lichens are composed of a symbiotic relationship between algae and fungi. The algae in lichens photosynthesise, creating food from sunlight, and the fungi provide shelter. Nitrogen deposition can increase the load of nutrients, which can harm and kill the algae's chlorophyll, which is used to produce sugars to feed the lichen. This can cause the death of the lichen algae, which deprives the fungi of the necessary sugars for survival.

Some lichens are more sensitive to nitrogen than others. Scientists monitor lichen communities, and an increase in nitrogen-tolerant species, in combination with a decrease in nitrogen-sensitive species, may indicate an increase in atmospheric nitrogen deposition. Lichens are considered the "canaries in the coal mine" of nitrogen deposition, as a shift in their species composition and health can indicate the potential beginning of ecosystem decline due to nitrogen deposition.

Nitrogen-loving, pollution-tolerant lichen communities have been found in areas of poor air quality, such as urban parks. On the other hand, pollution-sensitive lichen species are found infrequently in these areas.

Sulphur dioxide pollution

Sulphur dioxide is a significant industrial pollutant, being a by-product of burning coal and other high-sulphur fuels. While sulphur-removal technology has reduced sulphur dioxide pollution in many countries, it is still a notable pollutant in many areas. Sulphur dioxide is the most intensively studied pollutant when it comes to lichens.

Sulphur dioxide is a gas that dissolves in water to produce highly reactive acidic ions, which are readily absorbed through lichen thalli. Once absorbed, sulphur dioxide disrupts the process of photosynthesis and can cause structural changes in lichens, including reduced photosynthesis and bleaching. It can also inhibit lichen reproduction and 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.

In high concentrations, sulphur dioxide can irritate the mucus lining of the eyes, nose, throat, and lungs. Exposure may cause coughing and tightness in the chest. People with asthma are more susceptible to sulphur dioxide pollution.

Usnea lichens, also called old man's beard, do not grow in areas where there is sulphur dioxide pollution. Sulphur dioxide has killed many lichens in the past, but because we burn less coal now, their populations are beginning to recover.

Heavy metals

Lichens are highly susceptible to changes in atmospheric chemistry and deposition, and as such, they are very sensitive indicators of such changes. Lichens are composite organisms made up of a single species of fungus (mycobiont) and one or more species of algae (phycobionts). The fungus provides structure and protection for the algae, which in turn provide energy and nutrients through photosynthesis. Lichens absorb almost all their nutrients from the atmosphere, and due to their lack of a cuticle or other means of controlling nutrient uptake, they are particularly vulnerable to heavy metal pollution.

Lichens can be used as bioindicators of heavy metal pollution in the environment, providing a cost-effective and non-invasive way to monitor environmental health. For example, a study in Loja, Ecuador, found that the lichens Parmotrema arnoldii and Tillandsia usneoides were suitable indicators of heavy metal deposition in the city, with higher concentrations of cadmium, copper, manganese, lead, and zinc found in urban areas than in nearby forest controls.

Heavy metal pollution can cause multiple physiological changes in lichens, including the disruption of metabolic pathways and the inhibition of photosynthesis. For example, heavy metal accumulation can cause oxidative stress in lichens, leading to the production of reactive oxygen species and subsequent lipid peroxidation. Lichens can also absorb heavy metals from the soil, helping to detoxify polluted areas.

The impact of heavy metal pollution on lichens depends on the particular species as well as the concentration of a given element. For instance, the lichen species Cladonia cariosa, Cladonia rei, and Diploschistes muscorum are prominent examples of lichens that spontaneously colonise post-industrial wastes. These lichens have been found to accumulate high concentrations of zinc, lead, cadmium, arsenic, copper, and nickel.

The accumulation patterns of heavy metals in lichens differ depending on the specific metal and the lichen species. For example, in the lichen species Candelariella aurella, Lecanora muralis, and Lecidea fuscoatra, bioaccumulation factors showed a propensity to hyperaccumulate heavy metals. In contrast, the fruticose lichen Stereocaulon nanodes had lower concentrations of metals in its thalli than in the corresponding substrates.

The growth form of lichens also plays a significant role in the accumulation of heavy metals. Crustose lichens, which adhere closely to their substrates, tend to accumulate higher amounts of heavy metals than fruticose lichens, which grow like small shrubs or tangles hanging down.

In conclusion, lichens are highly sensitive to heavy metal pollution and can be used as effective bioindicators of heavy metal pollution in the environment. The impact of heavy metal pollution on lichens depends on various factors, including the specific lichen species, the concentration and type of heavy metal, and the growth form of the lichen.

Ozone

Lichens are highly sensitive to changes in atmospheric chemistry and deposition, and their sensitivity to pollutants makes them good bioindicators for air quality. They absorb anything in the air, and pollutants can quickly become toxic.

While fumigation studies have shown that lichen physiology is detrimentally affected by ozone, the most sensitive regional indicators of this pollutant will probably continue to be vascular plants.

Particulate pollutants

Lichens are highly sensitive to particulate pollutants, which can have detrimental effects on their growth and survival. Lichens are miniature ecosystems composed of a symbiotic relationship between fungus and algae and/or cyanobacteria. They absorb all their nutrients from the atmosphere, including pollutants, as they lack roots or a protective surface to filter what they take in.

One of the most well-studied particulate pollutants regarding its impact on lichens is sulphur dioxide (SO2). SO2 is a significant industrial pollutant, produced as a by-product of burning high-sulphur fuels, such as coal. While the use of coal has decreased in many parts of the world, leading to a reduction in SO2 pollution, it still poses a threat to lichens in many areas. SO2 dissolves in water to form highly reactive acidic ions, which are readily absorbed by lichen thalli. Once absorbed, these ions disrupt the lichen's photosynthesis process, inhibiting their growth and reproduction. Additionally, SO2 can prevent spore germination and reduce the activity of nitrogenase, an enzyme used by cyanobacterial photobionts to fix atmospheric nitrogen.

The sensitivity of lichens to SO2 has been observed since the 19th century, with botanists noting the scarcity of lichens in urban areas due to industrial smoke. Studies have shown that certain species, such as Usnea lichens, do not grow in regions with SO2 pollution. The presence or absence of specific lichen species can, therefore, indicate the level of SO2 pollution in an area.

Another source of particulate pollutants is metal plaques, which are often made of bronze or copper and fixed to boulders or other structures. The toxic compounds leach out of these plaques when washed by rain, killing lichens in the flow path or inhibiting their growth. This results in a lichen-free zone below the plaque, while lichens thrive in areas untouched by the runoff.

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