Lichen Sensitivity to Pollution
Writer’s comment: When the teacher of my BioSci 10 class started telling the class how to do a research paper, I chuckled to myself. Hmm, an introduction. A methods section. Results. Conclusions. It would have sounded threatening if I had not done a research paper before, but I had done three or four full-fledged ones in my previous two years at Davis. I expected to do an experiment on my own and write up the results. While sitting at the bus stop at the Silo, I noticed a patch of lichen growing on a tree, and began to wonder about it. I decided to study this for my research project. To my surprise, we were assigned a simple read-and-summarize-style paper, which started out sounding fairly dull. I decided to stick with studying the lichens, because, even if I couldn't run experiments, I could still learn a bit about them. And so I ended up with this paper, which turned out quite a bit more interesting than I’d thought. I didn’t get to play with polluting the lichens, but I did find out what others had found. Pollution does indeed kill.
Instructor’s comment: The paper "Lichen Sensitivity to Pollution" was submitted to fulfill the GE writing requirement for Biological Science 10. The intention of this assignment was not to teach students how to write a scientific paper (an appropriate goal for this type of course, in my opinion), but rather the paper was a tool to initiate an in-depth analysis of a biological subject of the student's own choosing and interest. Clearly David Putzolu has accomplished this goal. In this paper David demonstrates his knowledge of what a lichen is, its sensitivity to environmental pollution, how environmental monitoring programs utilizing lichens are designed, and how the data are interpreted. What is most important, though, is David's ability to communicate this information in a clear and concise manner to the reader. Interestingly, David effectively utilizes a scientific format in organizing this paper, although it was not necessary and is often not the most efficient organizational format for this type of paper. From the instructor's perspective this paper accomplishes the major goal of Biological Science 10, which was to get the students to become aware of the biological world in which they live, initiate an interest in how man interacts with the environment, and to develop (to some degree) the ability to understand and analyze scientific data. It is hoped that this approach will prepare individuals to make better, more educated decisions concerning the many biological and environmental issues facing our society.
—Kurt Maier, Zoology
What are commonly called lichens are not single organisms but two different organisms, consisting of a symbiotic relationship between a fungus and an alga. Both are relatively simply organisms apart but taken together form a relatively complex system which is able to survive in more severe habitats of the earth than any other type of creature or multi-creature symbiote that is multicellular. Despite this adaptation that different species of lichens have achieved, each individual lichen is still a very sensitive organism that has narrow environmental parameters that it can prosper in. Toxic effects of pollutants on lichens as well as methods of lichen use as air pollution indicators will be discussed in this paper.
The term lichen denotes an organism that is actually a symbiosis of two simpler creatures. This symbiosis is between an alga, in most cases green but sometimes blue-green, and a fungus of the class ascomycete. Lichens can be found in many different terrains, from frozen tundras to hot deserts. This seems a bit strange, given the two organisms that a lichen consists of. Most algae are water dwelling, and the watery medium they exist in is necessary for their reproduction, nutrient delivery, and structural support. Fungi also generally prefer moist surroundings and are especially dependent on other creatures, as they are almost all saprobes, relying on the organic matter of other creatures for their nutrients. It seems that neither of these creatures could survive on a bare rock, considering the fact that water and organic matter are mostly absent and are the things most important to algae and fungi, respectively. Yet it is a very common sight to see a large boulder with nothing but a lichen covering. The reason that these two organisms, which are from different kingdoms, are able to survive together is precisely because they are very different from each other, and are able to fulfill each other’s requirements for survival.
The fungus part of a lichen is composed of a thick hyphal mat which surrounds the alga while still permitting light to pass through to allow photosynthesis in the alga. Thus the fungus protects the alga from the environment. It also traps available water in itself, usually from sources such as rain and dew. This water is trapped inside with the alga, giving it the liquid it needs. The alga, in turn, gives the fungus part of the results of the photosynthesis it carries on. Thus, the alga gets the liquid environment it requires from rainwater which contains the basic inorganic chemicals necessary for the alga, and the fungus gets the organic chemicals it needs from the alga. It is this special system which allows these two organisms to survive together in a place neither could survive alone.
It would seem from the minimal environmental requirements that a lichen has that it should be a much more dominant life form. It can be found in many places, but nearly all of them are places which are too harsh for other organisms to survive in. The reason that lichen is not found in the richer, more agreeable environments that other organisms are found in is twofold. One reason is simple competition, and the other is that the delicate balance of the lichen requires a desolate environment.
The competition for environments more agreeable than those on which lichen is found is very fierce, and lichen is generally not very successful in this competition compared to higher plants. Lichens are adapted to surviving with very little help from the environment, and so organisms which make more use of available benefits are more successful. The lichen does not have the structures necessary to efficiently absorb nutrients from its substrate as well as many other organisms. Furthermore, lichens have a very slow growth rate, and so other plants tend to proliferate.
The lichen requires a desolate environment to prevent it from destroying itself. With too many available nutrients, the fungal part of the lichen grows too quickly, soon leaving the algal partner behind. What results is a fungus that is adapted to be part of a lichen. When other organisms, such as simple fungi, compete, the lichen fungi usually loses because it is not as well adapted to a generous environment. With too much available water, again the fungal partner grows too quickly, but this time it turns to the algal partner for nutrients, digesting it completely. Once the algal partner is gone, the fungus has water but no nutrients, and quickly dies from this lack. Thus, the efficient balancing system that allows a lichen to survive in very harsh environments is also the system that prevents it from being found anywhere but those environments.
The delicate system of the lichen has some other faults. The water surroundings of normal alga allow it to rid itself of wastes and harmful chemicals by simply allowing them to diffuse into the water and be carried away or reduced in concentration. The alga in the lichen, on the other hand, cannot excrete such contaminants. This is because the water that surrounds it does not flow away, but evaporates, leaving any harmful wastes behind. It is this property that makes a lichen very sensitive to environmental contaminants, much more so than mosses and higher plants, which usually have alternative sources of nutrition, such as dirt, to avoid contaminants, as well as ways of ridding themselves of poisonous wastes much more easily than lichens. The lichen population of an area is noticeably damaged long before pollution shows itself by adversely affecting animals or higher plants.
The lichen is especially vulnerable to toxins in rainwater, due to the method of water absorption it utilizes. When rained on, the lichen can absorb from three to twenty times its weight in water. The elements in this water stay in the thallus, or main growth, of the lichen permanently. If these elements are toxic, their concentration will increase rapidly, soon killing the lichen.
Experiments involving lichen sensitivity to various pollutants will be discussed below, as well as several methods of examining lichen populations to determine pollution levels.
“Physiological Responses of Lichens to Air Pollutant Fumigations” (Fields 1988) is a comprehensive study of how lichens react to various pollutants. Three different methods of exposures were used in the many experiments listed, these being (1) gaseous pollutants being injected into closed vessels, (2) pollutant exposure in aqueous solutions, and (3) pollutants being passed over lichens in the laboratory in continuous flow chambers. Sulfur dioxide received the most use in experiments, while other pollutants that lichens were exposed to included hydrogen fluoride, ozone, nitrogen dioxide, and peroxyacetyl nitrate. To determine the effects of these pollutants on the lichens, the quantities of photosynthesis inhibition, respiration rate, potassium emission, pigment degradation, and electrolyte leakage were measured. The concentrations of various pollutants varied from experiment to experiment and taken together encompassed a much wider range of pollution levels than would be found in any natural setting.
Three primary methods of air-quality monitoring via lichens are discussed in the paper “Mapping Air Quality with Lichens, the North American Experience” (Showman 1988). These methods are put into the categories distribution mapping, IAP mapping, and the “other” kinds of maps, which are usually just variations of the two preceding types.
Distribution mapping is the simplest form of mapping. There are several steps to a distribution map. The first is to determine the area of study. The map should usually be centered around either a point source, such as a paper mill, or a general source area, such as a city. It is also possible to center the map on an area where future construction will occur to be able to do a comparative study later. Other questions affecting the choice of area to be studied are prevailing winds, pollution types, and any other data the researcher considers relevant. The next step is reconnaissance, which is essentially familiarizing oneself with the terrain and any factors which might affect lichen growth apart from the selected pollutant source, perhaps re-choosing the mapping area because of them. Next, specific sites within the map can be chosen. These sites should be chosen on the basis of a good lichen flora. Sites should also be standardized for tree size and species. Data collection is the next step. This consists of marking down a site by location, specific lichen flora found, and extraneous local factors which might affect lichen growth, such as trash burning, fumigation, etc. Any lichen found which cannot be identified should be taken back to the lab for identification, and all lichen found should be sampled and kept for future rechecking of species type. The distribution map is finally made by blacking in the sites where each species was present, so by examining voids in the map of specific species a comparative level of pollution can be found. The areas lacking the most sensitive species will be those of highest pollution, and those with the most species of lichen present will have almost no airborne toxins.
IAP (Index of Atmospheric Purity) mapping is a more complex mapping that has some advantages over distribution mapping. In distribution mapping a species must be absent (having disappeared) for pollution effects to be noticeable. IAP mapping does not require such substantial changes. Instead, it measures the relative abundance of lichen of each species type. The steps to this more complex mapping are detailed below.
The first two steps, which are determining study area and reconnaissance of study area, are essentially the same as in distribution mapping, except that special attention should be paid to standardizing tree species and age. Variation in this is allowed only if the bark of all trees studied is very similar in all qualities which would affect lichens, such as acidity, wetness, density, etc. Site selection is also more precise than in distribution mapping. Sites selected should be as similar as possible. To achieve a maximum of standardization, at least ten similar trees to be studied should be present at each site.
Data collection for this method involves careful scrutiny of the boles on the bottom two meters of ten trees. All species of lichen present should be recorded using the following ordinal scale, which was developed by LeBlanc & DeSloover (1970):
5 - an epiphyte which is very frequent and has a high degree of coverage on most trees;
4 - a species which is infrequent or has a high degree of coverage on some trees;
3 - a species which is infrequent or has a medium degree of coverage on some trees;
2 - a species which is very infrequent or has a low degree of coverage;
1 - a species which is very rare and has a very low degree of coverage.
Also valuable in this form of mapping is taking a collection of samples of all species found in the study area. Finally, the IAP value of each site is calculated. This is done by the following formula, again originally given by LeBlanc and DeSloover (1970): IAP = SN (Q * F) / 10 The N in this formula is the number of species on the site; Q is determined by adding the number of species together and then averaging the sums of all the sites where that species was found; F is the frequency-coverage value assigned to each species at the site. The division by 10 is simply there to result in more manageable numbers. Finally, these values are mapped out into isoplanes.
The other mapping methods that are mentioned are similar in choice of where to study, specific site selection, and data presentation. The main difference is what quality or quantity of lichens is measured. Some examples include richness, which is number of lichen species per site, injury to lichens, frequency, total lichen covering, and luxuriance-density, a scale developed by Skorepa & Vitt (1976). Sulfur content is also a commonly-used method, as lichens tend to concentrate environmental sulfur as well as other chemicals in themselves.
In nearly all of the studies mentioned in “Physiological Responses of Lichens to Air Pollutant Fumigations” (Fields 1988), there was damage to lichens by all of the pollutants used. Some species were much more resistant that others, the heartiest being crustose and macrolichens. Damage occurred to nearly all facets of the lichen, including nitrogen fixation, electrolyte leakage, respiration, and photosynthesis. The most sensitive measure was nitrogen fixation, followed by electrolyte leakage; next was photosynthesis and respiration, and least indicatory of the presence of pollutants was pigment status.
The results of many lichen mappings are described in “Mapping Air Quality with Lichens, the North American Experience” (Showman 1988). In the majority of them the lichen growth does show a decline near to pollution centers. An interesting phenomenon discovered was that the same species of lichen would have differing sensitivity depending on the tree species it was on to a great extent. For instance, the species Punctelia rudecta was found to be resistant, meaning not affected by pollutants, on two species of tree, but sensitive on another. Lichen distribution mapping has been found to be less sensitive than the more complex IAP measures, but at the same time it is also less error-prone.
The aforementioned varying sensitivity of lichen to pollutants, specifically nitrogen fixation > electrolyte leakage > photosynthesis and respiration > pigment status, reflects the actual damaging potential of the atmosphere to the lichens. Thus, when a lichen is exposed to pollutants, the first thing to begin experiencing difficulty is nitrogen fixation, which correspondingly slows growth rate. Photosynthesis and respiration, which are damaged with higher concentrations of pollutants than nitrogen fixation, are more vital to the lichen, and damage to these systems is serious. By the time pigment status is altered, the lichen is in very bad shape, if not dead. This progression of damage fits well with the distribution mapping and IAP mapping studies. IAP mapping takes into account the relative abundance of lichen at the sites selected and is very sensitive to pollution levels. Lichen distribution mapping, which maps the existence or non-existence of lichen of each species, is not as sensitive. This would be expected because lower levels of pollution only inhibit lichen growth, which would only be reflected by the IAP method. Higher levels of pollution are required to kill a lichen, thus the lesser sensitivity of the lichen distribution mapping method.
The high sensitivity of lichen to pollution levels is very useful. In areas of higher contamination, the simple distribution mapping method can be used to determine locales with highest concentrations of pollutants. For less polluted regions, the more sensitive IAP method can be used. Both can be used repeatedly over the years to watch variation in pollution levels over time. Lichen mappings are much cheaper than instrumental monitoring, and, in the case of distribution mapping, nearly as reliable. Furthermore, lichens are so widespread that almost any place on the earth could be examined for pollution levels by the methods described above. This has a definite advantage over having to place an array of instruments in any area where pollution data are needed. Hopefully, in the future, lichen will be utilized to the fullest extent possible to help detect pollution levels and thereby control them.
DeSloover, J. and F. LeBlanc. “Mapping Atmospheric Pollution of the Basis of Lichen Sensitivity.” Proceedings of the Symposium on Recent Advances on Tropical Ecology. Ed. R. Misra and B. Gopal. Varanasi, India: 1968. 42-56.
Fields, R. F. “Physiological Responses of Lichens to Air Pollutant Fumigations.” Bibliotheca Lichenologica 30 (1988): 175-200.
Showman, R. E. “Mapping Air Quality with Lichens, the North American Experience.” Bibliotheca Lichenologica 30 (1988): 67-89.
Skorepa, A. C. and D. H. Vitt. “A Quantitative Study of Epiphytic Lichen Vegetation in Relation to SO2 Pollution in Western Alberta.” Northern Forest Research Centre. Information Rept. NOR-X-161. 1976.