Bryophytes are ideal candidates for direct biomonitoring because of their extreme capacity to accumulate pollutants such as heavy metals, radioisotopes and a wide range of chemical pollutants such as dioxins (Zechmeister et al. 2003b, Carballeira et al. 2006). The bioconcentration factor, defined as the ratio between the concentration in the moss (mg g_1 dry weight) and concentration in the environment, is indeed exceptionally high in bryophytes. For example, bioaccumulation factors of 8 400 000 for iron and 11 250 000 for manganese have been reported in mosses from areas highly exposed to heavy metal pollution (Ah-Peng & Rausch de Traubenberg 2004).
The reasons for this extreme intrinsic capacity of bryophytes to over-accumulate are many. Bryophytes lack the complex regulatory mechanisms of vascular plants. In bryophytes, gas exchange is not regulated by stomata and nutrients are not pumped up by the root system. Rather, bryophytes readily absorb pollutants from their immediate environment through their surfaces. The leaves, which mostly lack a protective cuticle, are most often one cell thick and therefore offer a large surface of absorption.
Pollutants accumulate both within and among cells. They enter and leave the tissues more readily if they are outside the cells than if they are inside. As a result, the extracellular fraction reflects current pollution levels in the environment, whereas the intracellular fraction is more constant and testifies to the average pollution load that is present in the environment (Mouvet & Clavieri 1999). The intracellular concentration of pollutants thus retains the 'memory' of past pollution events and may testify to former pollution peaks that are no longer detectable in the environment (Mouvet et al. 1993). This property is especially useful for monitoring effluent from nuclear power plants, for which instantaneous pollution levels remain mostly below detection levels (Beaugelin-Seiller et al. 1994).
Moss analyses make it possible to determine patterns of pollution loads at large geographical scales and to identify the most heavily polluted areas. Such analyses also make understanding of long-range pollution much faster and cheaper than using continuous chemical analyses of precipitation. In Europe, for example, the Heavy Metal Deposition Programme involves measurements of concentrations of ten heavy metals in naturally growing mosses at five-year intervals (Harmens et al. 2004). Twenty-eight countries were involved in the 2000-2001 survey with a total of about 7000 sites investigated. Heavy metal concentrations in mosses increase eastwards, which can be related to industrial emissions (Fig. 9.11). The elevated concentrations of heavy metals in areas without current emission sources, such as lead in southern Scandinavia,
suggest long-range transboundary transport of pollutants from emission sources elsewhere in Europe.
An alternative strategy to the measurement of heavy metals directly in mosses found in situ is the use of moss bags. Samples of mosses collected from clean areas are placed in nylon nets and exposed to a polluted area to enable concentrations in pollutants to be measured afterwards. The technique can be applied in environments, where mosses do not naturally occur. As an extreme example, mosses have been used as indicators of polycyclic aromatic hydrocarbon pollution resulting from the incomplete combustion of organic material, and fossil fuels in particular, in road tunnels (Zechmeister et al. 2006).
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