Lichens and pollution monitoring D. H. S. Richardson and E. Nieboer

Lichens are traditionally cited as the prime example of mutual symbiosis in the plant kingdom. Scientifically fascinating, they are now receiving increasing attention by applied biologists for pollution monitoring. The present article explains the reasons why lichens are suitable for this role and reviews the techniques currently employed.

Nature and physiological characteristics of lichens Lichens are formed by an association of a fungus and an alga. The fungus, with a few exceptions, belongs to the cup- fungi group (Ascomycotina) while the algae may be blue- green or green. The composite plant may be fruticose (shrubby), foliose (leafy), or crustose (crust-like) (figures 1 (cover), 2, 3). The algae in most lichens form a layer enclosed on both sides by fungal tissue which has no protective cuticle so that the water content of the whole lichen thallus rapidly equilibrates with the surrounding environment. Such plants are termed poikilohydric. They dry quickly under bright sunny conditions, assuming a state of inactivity, reviving physiologically when remoistened / 1 I. Water uptake occurs very rapidly and lichens may hold several hundred percent of their dry weight of water. As a consequence, under damp conditions toxic substances dissolved in rain water have ready access to cells within the thallus. The presence of pollutants may be reflected in a reduced capacity of lichens for photosynthesis, or for nitrogen fixation in the case of those species containing blue-green algae [ 2,3 1. However, when dry, lichens are less susceptible to the harmful effects of gaseous pollutants.

A second characteristic of lichens is their capacity to accumulate substances rapidly from their environment by processes which include the active uptake of anions, the

David H S. Richardson, M.Sc., D. Phil.

Was born in 1942 in Cornwall and is a graduate of Nottingham and Oxford Universities. He lectured at Exeter University from 1967-l 969 and then moved to Laurentian University, Ontario, Canada, but spent a year at the Umversity of Victoria, British Columbia. After a recent period back in Oxford and as an Honorary Research Associate at the Western Australian Herbarium, Perth, he was appointed University Professor of Botany at Trinity College, Dublin. in 1980. His research interests involve aspects of the physiology and biology of lichens and mosses.

Evert Nieboer, M.Sc., Ph.D.

Was born in 1940 in Holland and is a graduate of McMaster and Waterloo Universities (Canada). He spent two years as a Post-Doctoral Fellow with Professor R. J P Williams at Oxford and subsequently joined the Chemistry Department at Laurentian University. Recently he has been appointed as Professor of Toxicology in the Biochemistry Department, Faculty of Health Sciences, McMaster University. During his ten yeartenure at Laurentian, he worked in close collaboration with D. H. S. Richardson on lichens. HIS current research includes the application of trace-element analysis to biology and medicine, as well as the human toxicology of inorganic substances

This IS a Euro-article, sponsored by the Commission of the European Communities through its Directorate-General for Scientific and TechnIcal Information and Information Management, which arranges the necessary translations. Such articles are published concurrently in some or all of the participating journals: Umschau in Wissenschaft und Technik in the Federal Republic of Germany, La Recherche in France, Naruur en Techniek in Holland and Technologylrelandin the republic of Ireland.

Endeavour, NewSeriesVolume5 No. 3,198l (0 Pergamon Press, Printed in Great Britain)

passive adsorption of cations by an ion-exchange process and the trapping of particulates. Collectively, these mechanisms result in lichens from urban or industrial areas having elevated levels of sulphur and various metals as compared with samples collected in rural regions. Lichens have a considerable uptake capacity for metal ions due to the presence of metal-binding sites on both the alga1 and fungal partners. The evidence is that the sites are largely extracellular, and function in a comparable way to those of synthetic cation-exchange resins. Thus

M2+ + 2HA ~MA++A~+ 2H+,

where M+* denotes the entering metal ions and HA the fixed protonated binding groups; MA+ and A- are respectively the fixed metal complex and anionic binding group; and H + represents the released hydrogen ions. Laboratory studies have shown that lichens exhibit molar-exchange ratios close to those expected for a synthetic ion-exchange resin (Table 1) [4, 51. Particulate trapping is a second major mechanism by which lichens accumulate substances. Scanning electron microscope studies have shown that the surfaces of many lichens have numerous tiny holes through which particles can gain access to the interior, where they lodge among the less dense fungal strands of the medulla [ 61. It is often found in lichens from unpolluted areas, that the ratio of iron (Fe) to titanium (Ti) contents is rather constant due to the fact that Fe and Ti oxides tend to crystallise together during rock formation. Thus, if a lichen takes up tiny particles of rock, the ratio of the two elements will not alter much. However, around industrial plants emitting one or other of these two elements, the content ratio will change and this may be used as a parameter for monitoring the extent of fallout (Table 2) [ 71.

Lichen distribution in urban/industrial areas Even in the last century, naturalists noted that the industrial

TABLE 1. MOLAR EXCHANGE RATIOS OBSERVED IN EXPERIMENTS USINGTHE LICHENS

UMEILICARIA MUHLENBERGII 6 CLADONIA RANGIFERINA -

Exchange Metal taken up Cation released Sr:Metal during treatment by treatment Molar ratio p moles/g p moles/g

Umbilicaria Ni ‘2forSr*2 11.8*0 12.2 + 0.4 1 : 0.97 + 0.03 Sr’ ’ forTi+ 4.1 f 0.3 8.6 & 1 .O 1:2.1 io.2 Sr ‘2forH’ 6.3 + 0.7 13.2 & 0.8 1:2.1 10.2

Cladonia Ni*+ forSIZ+ 28.8 29.7 1: 1.0 CL? for SIZ’ 31.9 28.5 1:l.l TI - for SrZ’ 13.8 6.8 1:2.0

(From 14,51)

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Figure 2 Foliose (leafy) lichens belonging to the genus Peltigera. Note the difference in morphology from those illustrated in figure 1

(see cover). Epiphytic foliose lichens are popular in pollution studies.

(Photo: D. H. S. Richardson)

Figure 3 A mozaic of crustose (crust) lichens including Rhizocarpongeographicum which can be removed only with the aid of hammer and chisel.

(Photo: D. H. S. Richardson)

Figure 6 Morphological changes in thalli of Ever collected from standard ash trees along a transect industrial city of Newcastle upon Tyne, U.K. Note t of the thalli improves with increasing distance (qul (From j 121).

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nia prunastri from the :hat the luxuriant oted in miles).

:e

Figure 7 Fluorescence exhibited in the pollution free contra is diagnostic of algae in healthy thalli. The progressive change fluorescence observed in transplants 7b to 7d correlated with pollution-induced damage (from [ 131).

Is (7a) in

TABLEZ. IRONANDTITANIUM LEVELSAND Fe/Ti RATIOS IN CLADONIASPP. NEAR IRON-ANDTITANIUM-EMITTING

SOURCES,AS COMPAREDWITH CONTROLS

Collection Site METAL CONTENT

Iron Titanium Average Fe/Ti (range, ugg-‘) (range, ugg-‘) ratio (_+ SD)

New Brunswick (control site)

132-610 15-83 7.9 + 0.7

New Brunswick (urban site)

345-555 47-93 6.4 + 0.2

Sudbury (control site) 240-365 22-40 9.9 + 0.6

Sudbury 450-I 150 25-57 19+1 (25 km from smelter)

The New Brunswick collection sites were in the vicinity of the City of St John where there is an oil-fired power station that emits Ti. The Sudbury sampleswere collected 25 km from the Copper Cliff nickel smelter which emits significant quantities of Fe, Cu, Ni, and S, but not Ti. The control sites were respectively 50 (New Brunswick) and 100 km (Sudbury) from the industrial centres. (From 171)

revolution caused changes in the lichen flora near cities 181. Sernander in the 1920s introduced the term ‘lichen desert’ to describe situations where lichens were absent, as in city centres. Outside such barren zones the lichen flora is much reduced in diversity and abundance, and he referred to this as the ‘struggle zone’. Beyond this, where lichens thrived in unpolluted air, was the ‘normal zone’.

Subsequently, lichenologists established qualitative scales for estimating air quality by relating the occurrence of indicator lichen species with known sulphur dioxide levels [21. Using a simplified scale, 15,000 school children were able to map air quality over the whole of Britain in a single year (figure 4) 191. A more sophisticated approach has been to calculate indices of atmospheric purity (IAP) based on the number and abundance of epiphytic lichen and moss species. The IAP method assesses the degree of air pollution using the formula:

IAP= r;” (Q xf)/lO i=l

when i is a running species number with values of 1 to n, n being the total number of epiphytic species present at a particular site; Q is the ecological index of a given species expressed by the average number of other epiphytes occurring with it at all the investigated sites; and f is the frequency-coverage of each species at the site. The frequency is the proportion of trees on which an epiphytic species is physically present at a given site and the coverage is the surface area colonized by the species at the site. Both frequency and cover are assessed as percentages during data collection. From the frequency and cover data, f is assigned a numerical value on a scale of l-5 for each species. A value of 1 represents a species that is very rare and has a very low coverage; 5 represents a species which is very frequent, with a very high coverage [ 101.

IAP values are calculated for each site and may be plotted on a map of the area under study. Zones are delimited by IAP values which fall in the ranges l-24, 2549, 50-74, and >74, and these are drawn by isometric lines. The IAP method makes possible the determination of areas of high and low atmospheric purity around pollution sources. This technique has been used, for example, to delineate pollution zones around a copper smelter in

Heavily polluted air

Figure 4 Air quality assessed by examining the distribution of

lichens over the whole of Britain in 1971 with the help of 15,000

school children (from 191).

Quebec, Canada (figure 5) [ 101, an iron works in Finland, and an oil-extraction plant in the tar sands of Alberta, Canada. As illustrated in figure 5, IAP zones may be correlated with specific atmospheric SO, concentrations. This is not surprising, as laboratory studies have demonstrated that lichens are easily damaged by this pollutant 111 I. The IAP method is also of considerable value in environmental impact studies which seek to establish baselines prior to the establishment or expansion of an industry in a rural area. Arrangements are made for the re-examination of the plots at intervals after start-up. If changes are revealed in the epiphyte communities it is possible that pollution control standards are not high enough.

Morphologicaland anatomical changes Because lichens rapidly accumulate toxic substances present in the air, they often exhibit morphological changes following an increase in pollution levels. Abnormal thalli are also seen in collections made progressively closer to urban centres, as illustrated in figure 6 [ 121. In addition, anatomical changes may be apparent. For example, Hypogymnia physodes is smooth, grey, and has large lobes when growing in rural areas whereas surface cracks, brown or black thallus discolourations, and small lobes are found in thalli from polluted sites. Such changes in appearance

129

$ COP

. IAP

\ 5 km

ZONE I: ZONE II: ZONE III: ZONE Ip: ZONE P:

Figure 5 Air quality assessed by the IAP-index method around the copper-mining and smelting centre at Murdochville, Quebec (Canada) (from [lo]).

have been found in England, Finland, and Canada. Recently, the value of infra-red photography has been demonstrated for assessing the health of lichens. The tips of healthy thalli appear red when photographed using infra- red colour film, while central areas where the algae are more dispersed show up blue. Damaged thalli also exhibit a blue colour with this film. It has been found that water content of the thalli has much less effect on infra-red film colour than it does on normal colour film (e.g. ectachrome). Infrared film is hence more suitable for demonstrating pigment changes in lichen thalli induced by pollution 1131.

extraction and quantitative estimation of chlorophyll by spectrophotometry as it is easier and not subject to difficulties induced by the lichen acids during extraction 1141.

Contamination by radionuclides

The algal partner in lichens is known to be specially sensitive to the effects of air pollutants such as sulphur dioxide. Indeed, the percent of unhealthy cells has been found to reflect the degree of pollution in the areas from which the lichen was collected. Under extremely polluted conditions, brown spots of phaeophytin or white spots of bleached chlorophyll may even be apparent within the algal cells. In the laboratory, potassium efflux and photosynthetic rate reduction are common measurable responses [ 11 J. A rapid assessment of the condition of the algal cells may be made by examining fresh sections of lichen thalli with a fluorescence microscope. Healthy cells excited by an appropriate form of short wave radiation emit a powerful red radiation due to the emission capacity of chorophyll. Increasing damage results in the algae exhibiting red through brown and orange to yellow, and finally white, fluorescence. By calculating the percentage of cells exhibiting the various types of fluorescence (figure 7) it is possible to determine the degree of damage in a particular lichen sample [ 131. This technique is likely to supersede the 130

In 1958 it was reported that there were unusually high levels of radioactivity in the tissues of Norwegian reindeer 1151. Subsequent studies revealed that Alaskan reindeer had more than 20 times the levels of radioisotopes observed in cows grazing near the Nevada nuclear test site. The radioactivity in the reindeer was mainly due to the isotopes 13’Cs and 90Sr, derived from lichen forage. Further confirmation came from the observation that, when atmospheric testing was at its height, the 90Sr content of cows’ milk from Lapp farms in Finland, where lichens were used to supplement the feed, was 3-4 times higher than milk from farms in control areas [ 161. Studies on the 13’Cs levels in lichens since the early 1960s have shown that there has been an overall decline in the last decade which correlates with a reduced frequency of atmospheric nuclear testing (figure 8) 1171. Concomitant changes have been observed in the i3’Cs body burdens of reindeer and reindeer herders. Lichens have also been employed to monitor systematically the deposition rates and distribution of radioisotopes of elements beyond uranium in the Periodic Table. Interest in this group, which includes plutonium, americium, and curium, is related to their extreme radiation hazard [ 181. Of current concern is the nuclear contamination associated with the crashing of nuclear-powered satellites. Studies on lichens in the crash-zone of Cosmos 945. which extended

1960 I I

1965 1970 I ,>

1975 1980 year

Figure 8 ‘37Cs content of the lichen Cladonia stellaris collected between 1961 and 1977 at remote sites in Northern Finland. The range of values is indicated by vertical bars and the

numbers in parentheses denote the number of replicate samples analysed. Note that levels peaked during the period of wide-spread atmospheric nuclear testing (from [I 7)).

over 124,000 km2 of Northern Canada, revealed the presence of a range of radioisotopes 1191. However, it was concluded that most of the short-lived radioactivity observed was derived from two Chinese nuclear tests which took place shortly after satellite re-entry and which masked any radiation derived from the debris of the crashed satellite.

Accumulation of elements near urban/industrial centres Lichens have been used to monitor the atmospheric deposition of more than 30 different elements including Li, Na, K, Mg, Ca, Sr, Al, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Cd, Pb, Hg, Y, U, F, I, S, As, and Se [7,20,211. Foliose or fruticose lichens are collected at increasing distance from the supposed emission source, which may be from smelters, electricity power stations, chemical plants, or urban areas. The samples are cleaned and analysed by a suitable method such as atomic absorption spectrophotometry, X-ray fluorescence spectrometry, or neutron activation. Elevated elemental levels are observed close to the source. These fall off quickly at first and then more slowly with distance along the transect (figures 9A, 10).

In monitoring studies around a point source it has often been observed that there is a linear correlation between the elemental content of the lichen and the reciprocal of the distance from the emission source [lo,2 1 I (figure 9B). For a purely anthropogenic element, this result may be accounted for mathematically by a deposition model which considers diffusion of particulates from a point source acted upon by gravitational forces 12 11, i.e.

C(d) = Ad -i

where C(d) is the atmospheric concentration of the pollutants at any value of d which represents the distance of the collection site from the pollution source. A is a composite constant reflecting emission rate and the shape and dimension of the pollution zone. This is assumed to be a

r A

Distance From Smelter (km)

F B

3 480

z

0 I I I 1 1 1 0 4.0 8.0 12.0

Reciprocal Distance x IO’ (km-‘)

Figure 9 Iron and nickel contents of lichen samples (Stereocaulon sp.). A, as a function of distance from the Copper Cliff (Sudbury, Ontario) nickel smelter: B, as a function of the reciprocal distance along the same transect with values for the correlation coefficient (r), slope (m), and intercept (b) of r =- 0.996, m = 2563 ugg ‘/km ‘,

and b =~ -6 pg/g for nickel and r = 0.98 1, m = 34,846 ugg ‘/km ‘,

and b- 776 pg/gforiron. (Datefrom [22j).

131

E 12.1L0

2

2 9.7 w -

5 0 7.3. 0 Cladonio ranglferlna

00 0 10.8 21.6 32.1 13.2 5L.O

DISTANCE FROM ELLIOT LAKE (km1

Figure 10 Uranium content of Cladoniarangiferina as a function

of distance along a transect from the town of Elliot Lake, Ontario, Canada, a major centre for the mining and milling of uranium (from

[23J: wm = pgg-‘).

right circular cone with height small compared to the radius. In testing the linearity of the above relationship or others which have been observed (e.g. 24), it is important to employ the regression equation

C(d) = md-” + b

where b is the elemental content of the lichen at an unpolluted site [2 11. A crude estimate of b is the level observed at the site farthest from the emission source. The b-intercept value corresponds to the natural background level, which often constitutes an actual nutritional requirement.

The excellence of lichens as monitors of fallout has been confirmed by comparing their elemental content (biological monitors) with that of particulates accumulated on collectors such as high-volume samplers (mechanical monitors). Linear relationships between the two have been observed, of which an example is given in figure 11 [24]. Lichens may take up to 15 months to reach equilibrium with prevailing deposition levels though markedly elevated levels develop in less than half this time. Falling pollution levels are reflected more slowly by lichens because the mean residence time for most elements is about four years [7,2 11.

Lichens as monitors of ameliorating environments Re-invasion of urban areas of England by tolerant lichen species following enforcement of the Clean Air Acts 1956 and 1968 has been closely followed. Extensions in the distribution of species such as Lecanora muralis, L. conizaeoides, and Xanthoria elegans have been mapped (figure 12). It has been found that lichens can be used to evaluate ameliorating environmental conditions, although an interval of about 5 years may be required for the lichen distribution to reflect the improvement 1251. Where lichens are not totally killed by the prevailing contaminants, the effects of additional environmental controls may be monitored by assaying for decreasing elemental contents, though, as explained above, a year or two may elapse before significant changes may be detected.

Conclusions A combination of the approaches discussed above allows assessment of environmental conditions around a wide variety of urban or industrial centres. By employing transplant techniques, lichens may even be used for monitoring where naturally occurring thalli are absent due 132

L Fe-o

-0.4 0.4 1.2 2.0 2.8

loglOlbulk precipitation,mg m-2)

Figure 11 A comparison of the metals in samples of the lichen

Hypogymniaphysodestransplantedfor 7 months with that in bulk precipitation fallout during the same period at a site 1.4 km from a

Danish steelworks. The logarithmic format of the plot is used for

convenience as good linearity exists for the direct plot of metal content versus bulk precipitation for which r = 0.993, m = 8.5 pg

g’/mgmmZ,and b=-89pgg-’ (Data from 1241).

to existing or former pollution levels. In such situations, twigs or bark cores carrying lichens are moved from a rural collecting site to the desired sampling location. There they are implanted into trees or nailed to boards erected on wooden posts. Alternatively, fruticose lichens such as Cladonia stellaris may be placed on the ground in small plastic flower pots covered with nylon netting or suspended in such netting from trees. After the requisite time interval of a few weeks or months, the lichens are examined for increased elemental content, and anatomical or physiological changes 112,241.

The use of lichens as air pollution monitors has been recognized for some time. However, only recently have those in industry concerned with the environment, e.g. engineers, begun to appreciate the value of these plants in defining the zones of influence around emission sources. Elucidation of the fundamental physiology explaining the observed responses of lichens to air pollutants may be expected to lead to a general acceptance of these plants in biological monitoring programmes. Lately we have begun to understand the uptake mechanisms, the location, and the toxicity of various elements. Much remains to be done. There is little doubt that today’s sophistication in data collection, analytical techniques, and computer simulation 125 1, is likely to encourage the use ofthese unusual plants to help man prevent unnecessary contamination of his environment.

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Edward Arnold, London. 1976. 13 j Stewart, W. D. P. Endeavour New Series, 2,170, 1978. 141 Nieboer, E.. Lavoie, P. Sasseville, R. L. P., Puckett, K. J.. and

Richardson, D. H. S. Can.J. Bat., 54,720, 1976. 151 Burton, M. A. S., LeSueur, P., and Puckett, K. J. Can. J. Bat. 59,

91,1981. 161 Garty. J.Galun,M.,andKessel,M. New Phytol. 82, 159. 1979. 171 Nicboer, E., Richardson, D. H. S., and Tomassini, F. D.

Bryologist, 81,226, 1978

I I I I I I 1969 1971

Figure 12 Digitized field data plotted on a map of theWest Yorkshire conurbation, U.K. The black 1 km2 units indicate those

areas which have been re-invaded by Lecanora muralis since the implementation of the Clean Air Acts 1956 and 1968. (from [25]).

18 Ferry. B. W., Baddeley, M. S., and Hawksworth. D. L. ‘Air pollution and lichens’. Athlone Press. London. 1973.

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the foodchain lichen-reindeer-man during 1976-1978. In Radioactive foodchains in the subarctic environment’. Paper No. 97. Technical progress report. U.S. Energy Research and Development Administration. 1979.

181 Helm. E. and Persson. R. B. R. Nature(Lond). 273,289, 1978.

I 191 Taylor, H. W., Hutchison, E. A., McInnes, K. L., and Svoboda, J. Science, 205, 1383. 1979.

1201 Henderson, A. Lichenologist, 12, 145, 1980. 1211 Nieboer, E. and Richardson, D. H. S. Lichens as monitors of

atmospheric deposition. In ‘Atmospheric pollutants in natural waters’. (S. J. Eisenreich, ed). Ann Arbor Science Publishing, Michigan. 1981.

I221 Tomassini, F. D. ‘The measurement of photosynthetic i4C fixation rates and potassium efflux to assess the sensitivity of lichens to sulphur dioxide and the adaptation of X-ray fluorescence to determine the elemental content in lichens.’ M.Sc. Thesis, Laurentian University, Ontario, Canada. 1976.

1231 Nieboer, E., Richardson, D. H. S., Boileau, L., Beckett. P. J., and Hallman, E. D. Definition of the sphere of influence of mining activities at Elliot Lake, Ontario by assessment of the levels of uranium and other elements in lichens and mosses. Proc. 1st. Technical Transfer Seminar. Ontario Ministry OfEnvironment. Toronto,Ontario(November 25th 1980),343,1981.

1241 Pilegaard,K. WuterAirSoilPollut., 11,77, 1979. I25 j Henderson-Sellers, A. and Seaward, M. R. D. Environ. Pollut., 19,

207,1979.

Acknowledgements The authors wish to express their appreciation to the many colleagues who provided reprints and data that facilitated completion of this article. We especially thank those who providedillustrations, and also gratefully acknowledge permission ofthe publishers to reproduce the figures.

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