Cryptogamic Organisms as Biomonitors of Atmospheric Contaminants

Although lichens and mosses are completely unrelated groups of cryptogamic organisms, they have a number of features in common. They both lack root systems, have a high cation-exchange capacity (CEC), and depend largely on atmospheric deposition for their nutrient supply. Mosses and lichens therefore have elemental compositions which reflect in an integrated way gaseous, dissolved and particulate elements in atmospheric deposition (Bargagli 1998). Owing to their ability to tolerate long periods of desiccation and extreme conditions of light and temperature, lichens and mosses are the principal component of terrestrial flora in many ecosystems of continental Antarctica. Although scarcely considered by many Antarctic environmental managers, these organisms can play a very important role as biomonitors of persistent contaminant deposition around scientific stations and in field camps.

5.6.1 Accumulation of Persistent Contaminants in Antarctic Lichens

In almost all climatic regions, lichens are among the first colonisers of exposed rocks and other substrata, except snow and ice. The ecological success of these symbiotic associations of fungi and green or blue-green algae in extremely cold, dry and nutrient-poor habitats of Antarctic is the result of their ability to tolerate long periods of desiccation and to uptake available water and essential elements over the entire thallus surface, by rapid passive processes. Cations are passively bound to anionic sites on the cell wall or outer surface of the plasma membrane, and may also enter and accumulate in mycobiont and photobiont cells through slower and more selective mechanisms (Bargagli and Mikhailova 2002). Besides gaseous and soluble elements and compounds, lichens can also trap airborne particles in the loose hyphal weft of the medulla (Garty et al. 1979). Lichens are perennial and, because of their slow growth rate, persistent atmospheric pollutants accumulate in the thalli to levels well above those of atmospheric deposition. These symbiotic organisms are used worldwide as biomonitors of atmospheric deposition of trace metals, pesticides and radionuclides (e.g. Smith and Clark 1986; Bargagli 1989,1998; Muir et al. 1993; Nimis et al. 2002). Some species of macrolichens have wide geographic ranges and enable the establishment of large-scale biomonitoring networks, especially in remote regions. Many lichen surveys have been performed in the Arctic (e.g. Nash and Gries 1995; Chiarenzelli et al. 1997; Kelly and Gobas 2001); these studies generally indicate that, unlike artificial radionuclides and POPs, it is difficult to assess trace metal deposition from remote anthropogenic sources with lichens, due to the contribution of natural sources. Considering the remoteness of Antarctica, the most widespread species of macrolichens such as Umbilicaria decussata and Usnea antarctica can probably be used to detect atmospheric deposition of radionu-clides, and of several POPs from other continents in the Southern Hemisphere. An important application is in monitoring the deposition of myco-phytotoxic compounds, trace metals, PCBs and PAHs around scientific stations.

The elemental composition of Antarctic lichens shows that the marine environment is the main source of major and trace elements, both directly, through the deposition of aerosols and the melting of snow, and indirectly, through the solubilisation of salt encrustations on rock and soil surfaces (Bargagli et al. 2003). In contrast to other metals, Hg mostly occurs in the atmosphere in gaseous forms and, from several large-scale biomonitoring surveys performed in Italian regions with anthropogenic and/or natural sources of Hg (active volcanoes, geothermal fields and mineral deposits), lichens appear to be the most reliable biomonitors of atmospheric Hg (Bar-gagli 1990; Bargagli and Barghigiani 1991). Thus, owing to the inconsistency of the little available data on Hg concentrations in Antarctica in the early 1990s, a survey was performed on the distribution of Hg in surface soils and macrolichens of northern Victoria Land (Bargagli et al. 1993). Results were rather surprising, because Hg concentrations in surface soils were among the lowest ever recorded in terrestrial ecosystems (mean 0.034±0.023 |g g-1 dry wt.), while average concentrations in lichens (0.386±0.190 and 0.344±0.204 |g g-1 dry wt. in U. antarctica and U. decussata respectively) were higher than in samples of related species from other remote areas, and corresponded to average values measured in several urban and industrial areas of the Northern Hemisphere. As discussed in the previous chapter, the recent finding of photochemically driven oxidation of boundary-layer Hg° after polar sunrise, which determines a rapid deposition of oxidised Hg species in snow during the polar spring, probably contributes to the accumulation of the metal in lichen thalli. Obviously, as suggested 10 years ago, ecophysiological and environmental factors, such the very slow growth rate of Antarctic lichens (concentrations are expressed in |g per g of lichen, and are therefore affected by the growth rate of each sample specimen), and active volcanoes can also contribute to concentrations of Hg and other trace metals in Victoria Land lichens.

Predicted changes in the Antarctic climate, atmospheric deposition and environmental biogeochemistry will probably at first affect the growth rate and concentrations of major and trace elements in lichen thalli. Considering that temporal variations in the elemental composition of lichens can be used as an early warning system to detect climatic and environmental changes in Antarctic terrestrial ecosystems, concentrations of major and trace elements were determined in samples of U. decussata collected at the same sites, throughout northern Victoria Land, in January 1989 and 1999 (Bargagli et al. 1998b, 2000). Statistically significant variations were not detected, and overall average values from the two surveys are summarised in Table 6 and compared with those measured in lichens of the same genus from reference areas in Europe (Seaward et al. 1981) and the North West Territories (Canada, Chiarenzelli et al. 1997). A study on other lichen species from northern Quebec (Canada; Crête et al. 1992) reported average concentrations of 0.17, 0.09 and 4.1 |g g-1 dry wt. for Cd, Hg and Pb respectively, and 378 Bq kg-1 dry wt. for 137Cs. Although it is difficult to compare between element concentrations measured in different samples (even between those belonging to the same species and collected in the same region; Bargagli 1995), due to differences in analytical procedures, lichen growth rates and amounts of rock and soil particles adsorbed to samples, the main difference between baseline concentrations of trace metals in lichens from both hemispheres is a much lower Pb content in Antarctic samples. Concentrations of essential (e.g. Cu, Zn, Mn) and non-essential (e.g. Cd and Hg) elements in Antarctic lichens are comparable to those in samples from the Northern Hemisphere. These comparisons indicate that, in contrast to Antarctic snow, lichens probably cannot give reliable indications on metal deposition in Antarctica from anthropogenic sources in the Southern Hemisphere. However, several small-scale surveys performed around Antarctic scientific stations show that these organisms are reliable biomonitors of metal deposition from local sources. Samples belonging to the genus Usnea, for instance, were used to evaluate the impact of the construction of a crushed-rock airstrip at Rothera Point (BAS 1989), and to assess the impact of human activities at King George and Livingston islands in the areas occupied by Polish, Brazilian, Russian, Chilean, Spanish, Bulgarian and Korean stations (Poblet et al. 1997; Olech et al. 1998; Hong et al. 1999; Yurukova and Ganeva 1999).

Owing to their slow growth rates and long life cycles (probably of up to some 100 years), lichens behave as long-term integrators of persistent atmospheric pollutants; the analysis of 50- to 60-year-old specimens may yield information about total radioactive, DDT and other POP deposition since the beginning of nuclear tests, and about the large-scale production and use of pesticides. As a rule, concentrations of local and long-range transported contaminants in lichen thalli are much higher than those in the atmosphere or snow, and this makes analytical determinations easier and cheaper. The average content of HCB, HCHs, DDTs and PCBs in lichens and mosses collected in 1985 and 1988

Table 6. Baseline concentrations (mean±SD; |ig g-1 dry wt.) of major and trace elements in Umbilicaria decussata from Northern Victoria Land compared with literature data for lichens of the same genus collected in reference areas of the Northern Hemisphere

Region

Species

Al

Ca

Cd

Cr

Cu

Fe

Hg

K

Mg

Mn

Na

P

Pb

Zn

Antarctica, Victoria Land3

U. decussata

727±515

517±324

0.18±0.10

1.6±0.8

5.3±3.8

8121536

0.39+0.27

2,170+829

508+415

19+7

201+118

865+459

0.65+0.41

20+6

SW Polandb

U. cylindrica

-

-

-

4.4±0.8

7.6± 1.1

1,372+438

-

-

-

19+6

-

-

34+12

39+5

SE Ireland

U. cylindrica

-

-

-

3.6

7.7

799

-

-

-

31

-

-

41.6

63

NW Canada0

U. polyphylla

-

-

0.19±0.05

3.0±0.9

5.2±3.1

-

-

-

-

-

-

-

10.4+5.6

a Data extracted from Bargagli et al. (2000) b Data extracted from Seaward et al. (1991) c Data extracted from Chiarenzelli et al. (1997)

a Data extracted from Bargagli et al. (2000) b Data extracted from Seaward et al. (1991) c Data extracted from Chiarenzelli et al. (1997)

\o \o in the Antarctic Peninsula and northern Victoria Land ranged from 0.2-9.9 ng g-1 dry wt. (Bacci et al. 1986; Focardi et al. 1991; i.e. three orders of magnitude higher than those in Antarctic snow; Table 4). The mean content of 137Cs in samples of U. antarctica from King George Island (22.5±11.7 Bq kg-1 dry wt.) was about seven times higher than that in surface soils (3.3±2.1 Bq kg-1 dry wt.) from the same sites (Godoy et al. 1998).As lichens are temporal integrators of persistent atmospheric contaminants, and unlike expensive automatic monitoring networks, biomonitoring does not require repeated sampling to achieve significant information on atmospheric contamination. In areas where indigenous cryptogams are lacking or available species of lichens and mosses are unsuitable for biomonitoring (e.g. crustose lichens or moss with very short turfs), short-term "active" monitoring can be performed by transplants (i.e.the moss or lichen bag technique; Bargagli 1998).

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