Role of Cryptogam in Environmental Biomonitoring
By: Vertika Shukla and D.K. Upreti*
The biomonitoring approach, based on the sensitivity of organisms, is one of the economic tools to estimate the effect of complex air pollution on biological communities. A good ecological indicator is characterized by ease of handling, sensitivity to small variations in environmental stress, applicability to extensive geographical areas in the greatest possible number of communities or ecological environments, and an ability to be quantified.
Among lower group of plants generally moss and lichens are reliable indicators of terrestrial air quality, while algae are used for water quality assessment. Lack of significant cuticle or epidermis and leaves being only single cell thick make mosses, liverworts and lichens well suited as bioindicators and biomonitors. Due to the lack of a well-developed root system, bryophytes and lichens absorb both nutrients and pollutants directly from the atmospheric fallout. Higher plants (Tracheophytes) are also employed for biomonitoring but the ability of lower group of plants especially lichens and bryophytes sequester many pollutants beyond their physiological need and remained unharmed thus makes them efficient biomonitors three times higher than the higher plants.
A number of lichen bioindicator communities have been recognized and employed for biomonitoring different forest types and habitats. The communities at disturbed habitats comprised of species of Candelaria, Chrysothrix, Graphis and Phaeophyscia. Species of lichen genera Candelaria and Phaeophyscia indicate nutrient enriched habitat whereas areas populated with Graphis species indicate an open, exposed and regenerated forest. Species of Chrysothrix are pioneer lichens to invade coniferous trees (Pinus roxburghii) in the lower Himalayan region after forest fire. On the other hand high altitude exhibits prevalence of Parmelioid and Cyanophycean communities which signify fairly good air quality and minimum human disturbance.
Owing to their higher sensitivity towards air pollution and microclimate, the epiphytic lichens are considered as one of the reliable bioindicators. The frequency of occurrence of lichen species on a defined part of tree, their diversity and dominance can be used as a parameter to estimate the degree of environmental stress. The ecological parameter related with epiphytic lichens can be applied to estimate the environmental contamination with reference to source of pollution and to identify its impact, or, on a larger scale, to detect hot-spots of environmental stress. Periodical monitoring at the same sites provides assessment of the effects of environmental change. The data quality largely depends on the uniformity of growth conditions (thallus size and meteorological condition). A high degree of standardization in sampling procedures is therefore necessary.
Lichens also show high sensitivity towards phytotoxic gases such as sulphur dioxide and oxides of nitrogen. The algal partner responsible for vital activity of photosynthesis exhibits its presence of only 5-10% in composite lichen thallus is most sensitive to sulphur dioxide and oxides of nitrogen as it reduces the magnesium present in the chlorophyll and convert it into a brown pigment, phaeophytin, resulted into inhibition of photosynthesis. Excessive level of pollutants in the atmosphere, especially S02, has detrimental effect on the physiology and morphology of sensitive species that leads to extinction of sensitive species, which ultimately results in changed lichen diversity pattern. The relationship between the presence of heavy metals and lichen species community dynamics in the natural area revealed that lichen diversity significantly negatively correlated with heavy metals like Cu, Pb and V.
Lichen biomonitoring has been successfully employed to biomonitors airborne elements emitted by power plants using fossil fuels. The majority of the investigations of power plant emissions, airborne pollutants, and lichens as monitors have been performed in Europe. Within the radius of a few kilometers of a power station revealed that lichens contained elevated concentrations of B, F, Li, and Se relative to lichens in more remote sites.
Bryophytes are amongst the most effective and reliable indicator species for monitoring changes in the ambient environment, as like lichens they lack a protective epidermis and cuticle and, hence, are more susceptible to pollutants in comparison to the vascular plants. The moss monitoring is a significant part of forest monitoring and unique for spatial resolution, elements covered, and consideration of site-specific regional factors in the statistical analyses on the performance of data management.
Bryometer developed by Taoda in the year 1973 is a bag of mosses that respond in predictable ways to various levels of air pollution. In polluted areas, standard transplantation of certain mosses has been found to be quite useful for monitoring the intensity and trend of air pollution. By employing regular survey and calculating abundance and frequency of bryophyte species, an IAP (Index of Atmospheric Purity) can be calculated. Bryophytes have not only been used to monitor airborne pollution caused by emissions from various sources but also aquatic bryophytes species like, Amblystegium riparium, Fontinalis antipyretica, F. squamosa, Eurhynchium riparioides, and Scapania undulata are used to monitor water pollution.
Symbiotic cyanobacteria – bryophyte associations on the forest floor are known to contribute significantly to nitrogen budgets through the process of biological nitrogen fixation (BNF). BNF by cyanobacterial-bryophyte associations in the canopy of coastal temperate rain forests is a unique source of ecosystem nitrogen, which is dependent on large, old trees with high epiphytic bryophyte biomass.
The use of terrestrial mosses as biomonitors in large-scale multi-element studies of heavy metal deposition from the atmosphere is a well established technique in Europe, to determine elemental concentrations in order to distinguish between different source categories. Bryophyte species Scleropodium purum has been utilized for active biomonitoring in urban environment to study metal accumulation in different sites exposed to rural, traffic, or industrial influences. The effects of the heavy metals copper (Cu), zinc (Zn), and lead (Pb) on the chlorophyll content in Thuidium delicatulum (L.) Mitt. and T. sparsifolium (Mitt.) Jaeg., Scleropodium purum as well as leafy liverwort Ptychanthus striatus (Lehm. & Linderb.) were examined which showed biomonitoring potential of different bryophytes.
Apart from heavy metals accumulation, sulfur dioxide, and acid rain the bryophytes are useful monitors for hydrogen fluoride and ozone estimation too. Orthotrichum obtusifolium is sensitive to hydrogen fluoride, whereas Polytrichum commune, Polytrichum strictum, and Racomitrium are tolerant of fluoride fumes. Sphagnum species are especially susceptible to ozone, having reduced photosynthesis, reduced growth, loss of color, and symptoms of desiccation, but that there are some remarkable reactive differences among species. Elevated ozone had no effect on germination of Polytrichum commune spores at upto 150 ppb.
Mosses are major component of Antarctic terrestrial vegetation. Mosses Ceratodon purpureus and Bryum subrotundifolium shows resistant to UV-A and that the latter species can rapidly change its protection to suit the UV environment. Both C. purpureus and B. subrotundifolium have sun and shade forms that differ markedly in colour and their protection from UV-A. It may be inferred from these studies that Antarctic mosses well protected from ambient UV, but are also as adaptable to incident UV as higher plants.
It has been observed that the uptake of radioisotopes by epigean mosses occurs not so much from substrates as directly from atmospheric fallout. It is because of cation exchange activity, Sphagnum is used to decontaminate water containing radioactive materials.
Bryophyte community as well as individual moss species characteristics on soil has been examined to determine relationships with proximity to three coal-fired power plants. It was observed that woody vascular plant communities were relatively uniform, whereas bryophyte coverage reduced to half. Three moss species which showed the most consistent relationship to distance from the power plants, Dicranum scoparium, Leucobryum albidum, and Polytrichum ohioense, mapped for presence or absence on various habitats showed that D. scoparium and L. albidum consistently were absent in the area most influenced by the emission sources, while P. ohioense was less consistent in its absence. Presence or absence of indicator moss species proved more useful than community characteristics for indicating relative air quality.
Bryophyte assemblages are also used as indicators of past climate and hydrological regimes. Sphagnum and other moss assemblages are frequently used to identify past climates. Presence of drought-tolerant species as Tortella flavovirens in subfossils indicates past dry climatic conditions in some areas of the Netherlands. Similarly holocene succession went from woodland to peat land. Peat served as a wick to draw up water and raise the water level, causing woodland roots to become water-logged.
The accumulation of heavy metals by some species of ferns, makes them ideal ecological indicators of arsenic contamination Ferns have long been recognized as potential ecological indicators of contaminated environments due to their abilities to absorb mobile arsenic through their roots. In addition, the relationship between the hyperaccumulator Pteris vittata L. and the non hyperacculumator Pteris tremula upon exposure to arsenic has been studied in order to evaluate the movement of heavy metals in soils. The discovery of ecological indicators of arsenic contaminated abandoned mines and other environments is valuable to the field of ecosystem management.
Algae are an ecologically important group in most aquatic ecosystems and have been an important component of biological monitoring programs. Algal communities possess many attributes as biological indicators of spatial and temporal environmental changes. Algal growth and taxonomic composition responds predictably and sensitively to changes in pH, conductivity, nutrients enrichments, organic contaminants and many other contaminants.
Diatoms have been used extensively in water quality monitoring as they exist in a wide range of ecological conditions, colonising almost all suitable habitats, providing multiple indicators of environmental change. Indices of water quality using diatoms provides the most precise data compared to chemical and zoological assessment.
The fluctuation of Algal Flora due to pollution shows distinct zonation pattern of their distribution. Cyanophyceae and euglenophyceae has been found higher in highly polluted station and diatoms and chlorophyceae was found higher in less polluted station. These responses justify the pollution tolerant and clean water habit of algae and presents the effectiveness of algae in water quality assessment. Thus algae can utilized successfully for monitoring aquatic ecosystem.
The oldest approach for using algae to assess stream water quality is based on the indicator species concept (Saprobien system). The saprobien system is widely used in municipal and wastewater monitoring and discriminates between polluted and clean streams. The saprobic index based on diatom species assemblages, successfully divided the sampling stations into four zones of saprobic contamination: the polysaprobic zone, the alpha-mesosaprobic zone, the beta-mesosaprobic zone and the oligosaprobic zone. The zones of saprobic contamination were characterised by the occurrence of certain groups of diatom species, namely saprobiontic species, saprophilic species, saproxenous species and saprophobous species. Chemical stress in aquatic ecosystems is known to modify the taxonomic composition of the algal population, causing a reduction of sensitive species and an increase in the number of tolerant species.
Lower group plants are thus endowed with the uniqueness to mimic the ambient environmental condition, to serve as an efficient biological proxy. Thus lower group of plants are valuable proxy for studying air pollution and climate change related studies and exhibit the relative variation (spatial as well as temporal) in the amount of pollutants between locations. The lower group plants not only successfully be used to identify areas at risk from air pollution but also provide valuable information about the screening mechanism for establishing a subset of sites where follow-up work (such as instrument monitoring) can be carried out in future. The biomonitoring approach not only provides data on the ambient environmental condition but also estimates the effect of mixed contaminants on the biological communities thus acting as early alarming biological devices /tools to predict changes in biodiversity, air pollution and climate change.
*Lichenology Laboratory, CSIR-National Botanical Research Institute, Rana Pratap Marg, Lucknow 226 001, India
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