Monitoring anthropogenic air pollution is a very complex problem because of the high number of potentially dangerous substances, the difficulty of estimating their synergistic effects, the large spatial and temporal variation of pollution phenomena, and the high costs of recording instruments. However, biological monitoring can be very useful in the assessment of trace element air pollution.

Lichens are widely used as bioaccumulators of trace elements, due to their resistance to heavy metals and to their metabolism strictly dependent on atmospheric exchanges (Sarret et al., 1998). They receive nutrients directly from the atmosphere and lack roots, waxy cuticle and stomata (Garty, 2001). As a consequence, lichens are able to accumulate trace elements to very high levels, far above their physiological requirements (Nieboer and Richardson, 1981).

Dr. Richard leBrasseur of the Green Infrastructure Performance Lab at Dalhousie University studies the many unique ecosystem services within peri-rural landscapes.   As part of his ongoing research, he visually analysed the spatial representation of a poor air quality bioindicator lichen on the Dalhousie University Agricultural Campus in Truro, Nova Scotia. 



What is a Lichen

Even if you don't know what a lichen is, you have probably seen one before.  They grow just about everywhere and there are over 3000 species of lichens in North America.   Lichens resemble strange-shaped moss or mold growing on trees and rocks.    Even though lichens may look like moss, they aren't plants.


Lichens are actually a combination of organisms living together in what is called a symbiotic relationship.  A lichen is a partnership between a fungus and algae or, in some cases, cyanobacteria.  The algae cells live surrounded by a network of fungus cells where they are protected.  They work together as a team to help each other survive and each brings specific benefits to the partnership.  


The Role of the Fungus

The fungus or mycobiont of the lichen relationship provides structure and support.  It is the fungus that gives the lichen its unique shape and some of its color.  The bright yellows, reds and oranges of some lichens (see image on the left) are due to pigments that the fungus produces.  Since the fungus surrounds the alga, it protects it from the environment.   Without the fungus, the alga would be exposed to the elements.  The fungal coating allows the agla to live in environments where it normally could not. 


The Role of the Algae

The algae in lichens also has an important role.  Unlike plants and algae, a fungus cannot make its own food.  The alga (or cynanobacteria) in a lichen has chlorophyll and is essentially a food factory.  As it completes photosynthesis, the alga produces a lot of sugar.  The fungus feeds off this sugar as an energy source.  The green color present in some lichens is due to the chlorophyll of the alga.



Although there are thousands of species of lichens, there are really three main types of lichens.  Lichens are classified by their growth form or shape of their body.  The three types of lichens include many subgroups due to the many variations of lichen body types.  There are three main types of lichens.

Crustose Lichens

The most important characteristic about Crustose lichens is that they grow right up against their surfaces.  These lichens form a "crust" on the rock or other material that they grow on and cling very tightly.  It would be very hard to pull a crustose lichen off a rock that it is growing on because, as the lichen grows, it actually grows into the rock and becomes embedded in it. Crustose lichens often look like colored scaly or flaky patches on rock.  Other times they form very small spheres that can take on a pebbly appearance when many of them are together

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Foliose Lichens

The identifying characteristic of foliose lichens is that they are leaf-like.  Unlike crustose lichens, the body of a foliose lichen is not fully attached to its substrate.  While the base of the lichen is attached, most of the body is not.  A person would be able to pull most of a foliose lichen off of a rock or branch that it is growing on. The word "foliose" means leafy.  Many foliose lichens have flat, frilly sections.  They often look like tufts of odd-shaped and odd-colored lettuce growing on a branch or rock. 

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Fruticose Lichens

Fruticose lichens have the most unique shapes.  While crustose lichens cling very close to their substrate and foliose lichens are mostly two-dimensional structures, fruticose lichens are very three-dimensional.  They can be bushy or hang from trees and are often described as shrub-like or mossy and the fastest growing type of lichen. Fruticose lichens are easily identified by the fact that they are not flat but actually "stick out" from their substrate.  They often grow branched structures that can sometimes be inches long. 

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An Important Food Source

Lichens are an important part of the food web in several ecosystems.  Since they take care of their own food production, they can be classified as producers and provide food for other organisms.  Many small animals such as squirrels and birds eat lichens.  Larger mammals such as deer, mountain goats and caribou also use lichens as a significant food source, especially in the winter when food is scarce.  Some bears have even been known to graze on lichens.


Soil Formation

Soil is the foundation for most ecosystems and lichens are critical in creating soil.  Many lichens are considered colonizing organisms.  This means they are some of the first organisms that can live in an ecosystem that is either new or starting over.  They can live on rocks and, as they grow, the acids they secrete break down the rocks.  This makes a significant contribution in the formation of soil.  Once soil is formed, other organisms are able to live in the area and the ecosystem can continue to develop.


Habitat for Other Organisms

Lichens provide important habitat for many other organisms.  They are an excellent home for insects, arthropods, and other small invertebrates.  Lichens are also used by organisms to help build and insulate their homes.  Birds, for example, use various species of lichens (especially the fruticose types) to build their nests.  Lichens are absorbent, flexible and soft and are therefore great additions to borrows and other small animal homes.  Lichens also provide excellent camouflage for small insects and other invertebrates. 

Bioindicators for Pollution

One of the most important uses for lichens has to do with their sensitivity to pollution.  Some species of lichens are extremely sensitive to specific pollutants.  Scientists use these species as indicators in various parts of the world to track the amount of pollution in those areas.  In this way, lichens can act much like a canary in a coal mine.  They warn when chemicals like heavy metals or sulfur are present at low levels so that steps can be taken to eliminate the pollution before it can reach levels that can affect other organisms. The photo below shows some examples of lichens that are used as bioindicators. 



Although lichens have been used in folk medicines for centuries, science is just beginning to investigate the medicinal properties of lichens.  Studies have shown that acids from various lichen species may be useful in killing bacteria.  Other information suggests that lichens may be helpful in fighting off viral infections and certain types of cancers.  


Environmental Lichenology

Good air quality is essential for our health and for the wellbeing of our environment. Lichen are very sensitive to pollution. Scientists have learned to use this variability as an indicator of the air quality of any given site. Also, because the presence or absence of certain lichen species is easily recorded, and has in some cases been recorded at different times in the past, lichens can be a record through time of changing patterns of air quality 1


Xanthoria parietina is considered one of the most indicative species of trace-elements within air pollution; it is a biomonitoring organism.  In Truro and many parts of the Atlantic Provinces, there is a concern for the air quality as impacted by the increasing intensification of agricultural land through the application of nitrogen (N) fertilizer. Future food production requirements and urban expansion is expected to further exacerbate this problem, making the monitoring and control of nitrogen emissions a high priority for Nova Scotia’s environmental science.

1   Boltersdorf, S. H., Pesch, R., & Werner, W. (2014). Comparative use of lichens, mosses and tree bark to evaluate

     nitrogen deposition in Germany. Environmental Pollution, 189, 43-53.


Xanthoria parietina

It is often associated with high level of nitrogen and favored by eutrophication, and can be often found near farmland and around livestock 1. The species is widespread, and has been reported from Australia, Africa, Asia, North America and throughout much of Europe. In eastern North America and Europe, it is found more frequently near coastal locations. The increases in nitrate deposition as a result of industrial and agricultural developments in southern Ontario, Canada in the 20th century are thought to be responsible for the reappearance of this species in the local lichen flora 2.


The photosynthetic symbionts, or photobionts, associated with X. parietina are from the green algal genus Trebouxia.


Xanthoria parietina is a very pollution-tolerant species particularly to Nitrogen (i.e. a nitrophytic species). In laboratory experiments, this species can tolerate exposure to air contaminants and bisulphite ions with little or no damaging effect 3. It is also tolerant of heavy metal contamination 4. For these reasons, this species has found use as a biomonitor for measuring levels of toxic elements 5.

1 Frati, L.; Santoni, S.; Nicolardi, V.; Gaggi, C.; Brunialti, G.; Guttova, A.; Gaudino, S.; Pati, A.; Pirintsos, S.A.; Loppi, S. (March 2007). "Lichen

   biomonitoring of ammonia emission and nitrogen deposition around a pig stockfarm". Environmental Pollution. 146 (2): 311–316.

2  Brodo, Irwin M.; Lewis, Chris; Craig, Brian (June 2007). "Xanthoria parietina, a Coastal Lichen, Rediscovered in Ontario". Northeastern Naturalist.

     14 (2): 300–306.

3  Silberstein, L.; Siegel, B. Z.; Siegel, S. M.; Mukhtar, A.; Galun, M. (28 March 2007). "Comparative Studies on Xanthoria Parietina, a Pollution Resistant

     Lichen, and Ramalina Duriaei, a Sensitive Species. I. Effects of Air Pollution on Physiological Processes". The Lichenologist. 28 (4): 355–365.

4  Backor, M.; Fahselt, D.; Davidson, R. D.; Wu, C.T. (1 August 2003). "Effects of Copper on Wild and Tolerant Strains of the Lichen Photobiont

     Trebouxia erici (Chlorophyta) and Possible Tolerance Mechanisms". Archives of Environmental Contamination and Toxicology. 45 (2): 159–167.

5  Loppi, Stefano; Paoli, Luca; Gaggi, Carlo (17 March 2006). "Diversity of Epiphytic Lichens and Hg Contents of Xanthoria parietina Thalli as Monitors

     of Geothermal Air Pollution in the Mt. Amiata Area (Central Italy)". Journal of Atmospheric Chemistry. 53 (2): 93–105.

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Nimis, P. L., Scheidegger, C., & Wolseley, P. A. (2002). Monitoring with lichens—monitoring lichens.

In Monitoring with Lichens—Monitoring Lichens (pp. 1-4). Springer, Dordrecht.


Students in the Landscape Architecture Program, working with Dalhousie’s Green Infrastructure Performance Lab, wanted to find out how much Xanthoria parietina is present on the walls of the Langille Athletic Centre’s exterior aggregate walls and what information about air quality and air pollution within the Agricultural Campus may be inferred.


Systematic sampling was operationalised with a 12” x 12” quadrat at a vertical transect height of 48” and centered within the panels of the Athletic Centre’s exterior walls. Every other accessible precast concrete panel with exposed aggregate (n=xx) on all major building faces (n=4) had a hi-resolution photographic image recorded of that quadrat.  Images were processed through a digital pixel analysis which quantified the number of pixels considered to include the presence of the Xanthoria parietina lichen.

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The quadrat images were taken April 26, 2020 at 10:45 am (southern facades) and 2:45 pm (northern facades). Images were uploaded to the on-line software package - Color Summarizer 0.76. This analysis program was developed by Martin Krzywinski at the University of British Columbia.

Quadrat #12 Pixel Analysis showing pixelation to achieve image clustering data of the Xanthoria parietina lichen.

The color summarizer reports a summary of colours in an image using clustering, to group similar colours together and derive a set of colours that are representative of the image and descriptive statistics for components in each of the colour space.

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The clustering requires that you specify the number of clusters prior to analysis. You can set 2 to 10 clusters. Trial and error determined 6 clusters were the most effective in achieving the representative colours for the Xanthoria parietina lichen in this context.


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Quadrat #12 Pixel Analysis Result showing 9.45% of image contained the Xanthoria parietina lichen.

This analysis used colour clustering to report the representative colours of the quadrat image. Colours were clustered into 6 groups (k-means).  Briefly, k-means clustering partitions the pixels into k-sets as to minimize the within-cluster sum of squares, calculated by the Euclidian distance to the center of the cluster, which is taken to be the average of the set.

The color summarizer reports a summary of the colour components, HEX and RGB.  A HEX color is expressed as a six-digit combination of numbers and letters defined by its mix of red, green and blue (RGB) but basically, a RGB is the process by which colors are rendered onscreen by using combinations of red, green and blue. RGB is specific to digital applications only, print and other applications utilise CMYK or PMS. RGB colours appear vibrant because they are illuminated and there is a larger range in colour gamut than on the printed page. The RGB histogram illustrates the it distribution of each primary color’s brightness level in the image of the red, green, or blue colours.

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The percentage of potential Xanthoria parietina pixels, overall, on all 38 panels (n=38) totals 1.94%. The highest percentage and lowest percentage panels are shown below (#7 & #21), indicating range. The standard deviation of 0.92 is indicates the results are skewed towards the lower percentages. No other analyses were competed.

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Quadrat Sampling of Pixel Analysis Results.


The simple summary statistics provided some insight. Results suggest that, overall, on average, 1.94% of the Langille Athletic Centre’s exterior aggregate walls may be covered with the Xanthoria parietina lichen. This number is lower than expected when the experiment began.  Though the presence of this species of lichen itself indicates air pollution, the low percentage suggests the air pollution level in and around the building is not at a high enough percentage to confirm presence of nitrogen due to agricultural practices in the area.

GIPL's Mission is to advance the understanding of green infrastructure planning and design to positively impact the challenges our everyday landscapes face.  GIPL seeks to build partnerships, combining research & practice which generate innovative solutions and ideas toward healthy communities.


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The Green Infrastructure Performance Laboratory

Director, Richard leBrasseur, PhD

Dalhousie University

Department of Plant, Food, and Environmental Sciences

20 Rock Garden Road, EE Building, Room 223

Truro, Nova Scotia, Canada  B2N 5E3