Lichens as Environmental Monitors

For those with a modicum of familiarity with lichens, it may come as a bit of a surprise that they are widely used to monitor environmental quality. Their use as so-called bio-indicators belies their well-earned reputation for stolid tenacity under Stygian conditions. Lichens are globally distributed with about 14,000 species that extend across a broad range of sizes and shapes and inhabit every niche from the frozen polar extremities to the equatorial tropics, favoring places where nothing else will grow; they are the dominant form of vegetation for about 8 percent of the earth’s surface. The seeming oxymoron of lichen sensitivity deserves some consideration that begins with a few fundamentals of lichenology.

A lichen is "an association of a fungus and a photosynthetic symbiont resulting in a stable vegetative body having a specific structure" according to the definition accepted by the International Association of Lichenologists. In other words, it is not really a singular entity but a mutualistic association of two species from different kingdoms; the fungus from Kingdom Fungi and the alga from Kingdom Plantae. Since fungi are heterotrophic like animals, they must get their nutrition from an autotrophic plant, the alga. The fungus provides structural stability, water and minerals and the alga provides complex carbohydrates. Lichens have been anthropomorphically cited as “fungi that discovered agriculture." The basic functional structure of a lichen is relatively simple.  The fungal portion, called the mycobiont, constitutes the bulk of the vegetative body; the algal component, or photobiont provides photosynthetic derived nutrients.   This relationship is generally characterized as benign mutualism, a type of symbiosis in which both constituents share the benefits of the association. However, it is probably better to characterize the relationship as at least partially parasitic, for in virtually every case the fungus penetrates the alga and absorbs about half of its nutrients; the survival of the lichen depends on the alga replenishing lost cells through photosynthesis.

Lichens are sensitive to environmental pollutants for two very fundamental reasons. First and foremost is the epiphytic nature of their physiology; they ingest air and water directly from the atmosphere and do not therefore benefit from the filtering and concomitant cleansing effects of an intermediary such as soil. Air and water are exchanged over the entire external surface of the fungal body, which is called a thallus.  Lichens have no natural means to retain water (lacking protective cells) and are thus hydrated and dehydrated several times over the course of a day.  The end result is that lichens are immediately and directly sensitive to any anomalous additives to the air and to the water, concentrating trace contaminants with cyclic celerity. The second reason for their sensitivity is their mutualistic association, which is a somewhat tenuous balance of nutrition and resources. If an environmental pollutant has a deleterious effect on either the alga or on the fungus, then the union is threatened to a greater extent than would be the case with a single organism. Either the alga or the fungus may be affected individually or the dynamic of their resource sharing may be; in any of the three possible impacts, lichen debility results. Lichens are therefore well suited natural environmental monitors.

The conflated composition of the lichen led to some understandable confusion in the nascent sciences of the Age of Reason that transcended medieval superstitions. The French botanist Joseph Tournefort first placed the lichens in a separate genus of the plant kingdom in the late 17^th Century. Carolus Linnaeus, in establishing the taxonomic rules of orders, families, genera and species, classified them as algae. His student, Erik Archarius, who is known as the father of lichenology, placed them with the fungi (then part of the Kingdom Plantae in the Phylum Thallophyta). However, it was not until the middle of the 19^th Century that William Nylander, a Finnish botanist who had emigrated from Helsinki to Paris, correctly identified the duality of the fungal-algal lichen. He is also credited with discovering chemotaxonomy, using chemical reagents such as calcium hypochlorite in the analysis of lichen speciation, and with establishing a relationship between the lichen population and prevailing environmental conditions, the latter published in the Bulletin of the Botanical Society of France in 1866 as Les Lichens du Jardin du Luxembourg.  Nylander noted that the lichens in the verdant Paris garden were notably richer in quality and quantity than in any other part of the city. He may accordingly not be mistakenly referred to as the father of lichen environmental monitoring.

In the most generalized sense, lichens are divided into five categories according to their basic morphology: crustose, foliose, fruticose, squalmulose and leprose.  Crustose lichens are the most familiar, as they are the tightly adherent crust-like growths typically found on any relatively old stone surface like a grave marker. Foliose and fruticose are the lichens that look like little leaves (folium is the Latin word for leaf) or little ramified shrubs (frutex is the Latin word for shrub and has nothing to do with fruit). The last two are essentially variants of the crustose variety; squamulose lichens are crust-like with upturned scales (a squama is a scale) and leprose lichens are crust-like with a loose powdery surface. On a macroscopic perspective, the environmental health of an ecosystem can be ascertained according to the types  and distribution of lichens; their absence in an otherwise appropriate location indicating that air quality is exceptionally poor. Only crustose lichens are found in the most polluted areas; as air quality improves, the more complex lichen types such as squamulose and leprose become apparent. At the other end of the scale, fruticose and foliose lichen are indicators of clean air. In our area, the extensive colonies of the leafy foliose Rock Tripe (Umbilicaria mammulata) and the branchedfruticose Reindeer Lichen (Cladina rangiferina) at the upper elevations are indicative of salubrity. The structure is evident in the photographs of each above.

The practical use of lichens as environmental indicators is more specialized in that it depends on the reaction of an individual species of lichen to a specific pollutant and on the absorption of different air pollutants by the lichen as an inherent consequence of the photosynthetic respiration process.  The primary pollutant of concern is sulphur dioxide (SO₂), a strongly polar molecule that is introduced into the atmosphere primarily as a by-product of the combustion of fossil fuels which frequently have sulphureus composition. Sulphur dioxide adheres readily to the external surfaces which results in a reduction in respiration and concomitant photosynthesis of the algal component on which the lichen relies for sustaining energy; it is the primary agent for lichen mortality, particularly those of fruticose structure due to their larger surface areas. Natural background SO₂ ranges over one order of magnitude (0.28 to 2.8 milligrams per cubic meter – mg/m³) whereas excessive combustion of high sulphur fuels can raise this up to about 200 mg/m³; lichens are essentially exterminated above levels of 60 mg/m³. The second major lichenous pollutant is the result of the hydrolytic reaction of sulphur dioxide to sulphuric acid (H₂SO₄) and of nitrogen oxide (NO₂), also a product of fuel combustion, to nitric acid (HNO₃). The end result is acid rain, defined as atmospheric water (rain or snow) with a PH of less than 5.6 (PH stands for pouvoir hydrogèn which is French for power of hydrogen and is a measure of acidity; 7 is neutral and <7 is acidic). Lichens subject to a PH of less than 2.5 exhibit reduced photosynthesis and a decrease in weight. The third and final pollutant category affecting lichen population diversity and mortality are metals in their ionic state. Silver (Ag⁺), mercury (Hg⁺) and copper (Cu⁺) are noted for their lethal toxicity while lead (Pb⁺), zinc (Zn⁺²) and nickel (Ni⁺²) are of intermediate toxicity.

Lichen environmental monitoring has come a long way since Nylander perambulated the pathways of Parisian parks and observed that they were more lichenous than their adjacent urban environs. The first extension of the general view of lichens as indicators was undertaken by the Swedish botanist Rutger Sernander, who systematized Nylander’s lichen and non-lichen dichotomy into a tripartite of a “lichen desert” zone, a transition or struggle zone, and a normal or lichen growth zone which he applied to the city of Stockholm in the 1920’s; detailed lichen maps of Helsinki and Oslo were diagrammed in the 1930’s.  The increased use of fossil fuel that ultimately resulted in the coining of the word smog from smoke and fog to describe the atmospheric effects incubated resurgence in monitoring with lichens in the 1970’s. A ten zone system based on lichen sensitivity was devised in the U. K. by David Hawksworth and Francis Rose to map the extent of sulphur dioxide damage in Ireland, Wales and England. At about the same time, the Canadians Fabius LeBlanc and Jacques DeSloover developed the Index of Atmospheric Purity (IAP), a quantitative score based on the relative plenitude of various lichen species, and used it to map Montreal. More recently, the availability of sophisticated laboratory protocols such as atomic absorption spectrophotometry and X-ray fluorescence spectrometry has resulted in the capability to monitor elemental atoms that comprise the lichen thallus. The analysis generally includes sulphur, nitrogen and fluorine in addition to a wide range of metals from copper to zinc including lead and chromium. For example, in 1993, the U. S. Forest Service in conjunction with the Environmental Protection Agency initiated a lichen indicator section in the extant Environmental Monitoring and Assessment Program/Forest Health Monitoring (EMAP/FHM) Program at a number of test sites. The assessment consists of the determination of 27 different contaminants in 10 different species of lichen and includes a complete survey of epiphytic lichens in selected areas. Lichen analysis was used to determine the extent of radioactive contamination following the reactor accident at Chernobyl near Kiev in Ukraine.

So why do we use lichens to conduct atmospheric monitoring? It’s really quite simple - because they do it for us. Were it not for lichens, each sampling location would need to have an air monitor with sophisticated electronics necessary for the requisite accuracy and precision in addition to a power supply and a suction device to siphon air across a collection membrane of some sort. This would be prohibitively expensive – and unnecessary.  The extent to which lichens provide effective and affordable environmental monitoring is manifest in the practice of transplanting lichens into areas lacking sufficient populations to function as natural air monitors. The bottom line is that lichens instantiate environmental measurement systems.