FEATURES OF BRYOPHYTA AS BIOMONITORS
Biomonitors provide both qualitative and quantitative information about the environment. Liverworts and mosses are good bioindicators as they have a simple thalloid structure having one-cell thickness. They lack cuticle or epidermis, which causes higher absorption and accumulation of nutrients and pollutants. Some bryophytes grow in soils having specific pH and are indicators of the particular pH of the soil. Bryophytes are widely used as they are able to accumulate the pollutants and available throughout the year. The accumulated pollutants are easily measured by quantitative methods, and this provides the information about the level of pollutant deposition. Mosses and lichens are considered as the most appropriate as they are able to deposit atmospheric heavy metals in them. Electronic detectors and other methods are used to detect the level of pollutants, but it may also possible that if the level of pollutants is below a certain concentration level, then its estimation is very tough. Chemical analysis is strongly dependent on the tune and place of sampling, while bryophytes have the ability to facilitate the detection of the elements present in very low concentrations. Mosses as natural biomonitors thrive in a humid climate. Ectohydric mosses are widely used as biomonitors.
The most important feature is that mosses can be stored for several years without any deterioration, and old specimens can easily be analyzed chemically. Bryophytes have wide distribution, ability to grow on a variety of habitats, large surface area, lack of cuticle and stomata, evergreen nature, which make them unique. Bryophytes are considered as Environmental Specimen Banks as they can be grown on a variety of habitats. Bryophytes obtain nutrients from the substances dissolved in moisture. The substances are directly absorbed from the substrata by diffusion through the cells. Pollutants reached to the cells and get deposited in the form of particles or gases. During analysis, these cells show the presence of elements and their concentration gradient in the respective substrata clearly indicated the level of pollution in the environment.
BRYOPHYTES AS POLLUTION CONTROLLERS
H. splendens and P. schreberi are used for heavy metal detection in air as they have the ability to accumulate many metals in high concentrations. Marchantia polymorpha, Solenostoma crenulata, Ceratodon purpureas, and Funaria hygrometrica (Coombes and Lepp, 1974,) are metal-tolerant bryophytes. Bryophytes growing on rocks are more tolerant to pollutants as compared to bryophytes growing on tree trunks. However, epiphytic species growing on a tree base can cope with the pollution conditions better than those on tree trunks. Brachythecium rutabulum, Grimmia pulvinata, Orthotrichum diaphanum, Bryum capiUare, Bryum argenteum, Tortula muralis, Rhynchostegium confertum, and H. cupressiforme show better survival growth in a polluted environment. Different life forms of bryophytes are responding differently to the level of pollutants. Similarly, rough mats, tall turfs, wefts, large cushions, and leafy liverworts are less tolerant to pollutants than smooth mats and small cushions, while short turf and thalloid forms are highly resistant to pollutants as compared to others.
The leafy gametophores are more sensitive than the mature gametophytes (Gilbert, 1969). The reproductive potential of a species determines its degree of success in a polluted environment. The survival of B. argenteum, C. purpureas, Dicranella heteromalla, F. hygrometrica, Leptobryum pyriforme, Lunularia cruciata, M. polymorpha, and P. proligera have high reproductive capacity and fast growth. These species produce spores or gemmae on a very large scale. A few terricolous species grows best on soils at a pH of 3.4
and in SO,-polluted areas, for example, D. heteromalla, Pohlia nutans, and C. purpureus. Sulphur dioxide (SO,)-resistant bryophytes show fast growth rate; for example, Hypnum yokohamae and Glyphomitrium humillimum were able to tolerate SO, concentrations of 0.04-0.05 ppm (Gilbert, 1970).
Fluoride-resistant btryophytes are P. nutans and Aulacomnium androg- ynum, Polytrichum commune, Leucobryum glaucum, Rhytidiadelphus squar- rostis. Sphagnum and Bryum like terrestrial mosses are good indicators of SO,, NO,, fluorides, and HC1 in soil. Bryum dyffrynense is a poikilohydrous moss, and a few grasses are sensitive pollution indicators. A few aquatic mosses are found in water sources when there is good content of calcium and nutrients in water. A few mosses are specific as they grow only in copper- rich soil and are indicators of the presence of copper in the soil, for example, Merceya, Mielichhoferia elongata, and M. mielichhoferiana.
As bryophytes lack a protective layer like epidermis or cuticle, this makes them highly sensitive. Bryophytes are widely used for the measurements of heavy metal toxicity like chromium, copper, cadmium, nickel, and vanadium particularly in the areas near power stations. Cesium is a radioactive metal, and its presence in nature can be detected by bryophytes. They diy very quickly and also absorb a very small quantity of moisture present in the atmosphere in any form like mist, fog, dew, etc. Tortella lortuosa is associated with calcareous substrates, while R. lanuginosum grows only on acidic surfaces.
EFFECT OF METALS ON BRYOPHYTES
Bryophytes absorb atmospheric chemicals either soluble chemicals in wet deposition or particles from diy deposition. Bryophytes absorb heavy metals without any disturbance of the normal biological pathway. This special ability makes them unique, and that is why they are successfully used as biomonitors for environmental pollutants. The efficiency of a moss to uptake the metal varies йот species to species. Mostly, bryophytes take dissolved elements in ionic forms. The absorbed ions attached to the surface of mosses by physical and chemical forces. The cells of bryophytes have different retention capacities for different ions. A very large retention capacity indicates that both simple cation exchange on negative surface charges and complex formation with ligands on the surface of moss are involved. Mosses cells absorb strongly Си and Pd as compared to Zn and Cd. H. splendens and P schreberi are the two important species used in environmental element studies. M. polymorpha and Calymperes delessertii are good monitors of aerial lead and copper. Pottia truncata, Polytrichum ohioense, D. heteromalla, and B. argenteum are very tolerant of high tissue levels of cadmium (610 ppm), copper (2700 ppm), and zinc (55,000 ppm). H cupressiforme accumulates zinc, copper, and cadmium threefold more as compared to lichens and other plants. Metal uptake by bryophytes depends upon the level of metals and the affinity of cells of bryophytes with these metals. For example, copper and lead are absorbed more as compared to nickel. The absorption of nickel is more as compared to that of cobalt, zinc, and manganese.
SО2 AND ACID RAIN MONITORING BY BRYOPHYTES
Bryometers were not accepted by everyone so new technologies must be needed. In 1967 and 1968, Gilbert found that SO, is an important factor and decides the distribution of mosses and development of reproductive structures and capsules. He published his research article based on G. pulvinata as an SO, indicator in 1969 (Gilbert, 1969). Monitoring studies based on bryophytes developed a list of tolerant and intolerant bryophytes, which can be used as successful bioindicators. Taoda (1972), in Japan, started using epiphytic species to assess the pollution level in Tokyo city. He divided the city into five different zones, based on pollution intensity. His classification included both mosses and liverworts. This zonation is on the basis of sensitivity of bryophytes to SO,. (1) G. humillium and H. yoko- hamae; (2) Entodon compressus, Hypnum plumaeforme, Sematophyllum subhumile, and Lejeunea punctiformis; (3) Aulacopilum japonicum, B. argenteum, Fabronia matsumurae, and Venturiella sinensis; (4) Haplohy- menium sieboldii, Herpetineuron tocceae, Trocholejeunea sandvicensis, and Frullania muscicola.
SO,-exposed mosses are reduced in coverage. The damage may be due to direct exposure of SO, or by the formation of sulfuric acid. SO, reacts with water and forms sulfuric acid. Further, sulfuric acid breaks into hydrogen ions and makes water to become acidic. In plant cells, these free hydrogen ions replace magnesium of chlorophylls and further lead to the destruction of chlorophyll. Despite this simple mechanism, there are mosses that can protect their chlorophyll molecule from such destruction and such mosses tolerate the acidic environment. Dicranoweisia change S03'2 into a harmless sulfate (S04“2) salt. It is also observed that the concentration of chlorophyll is veiy high in these tolerant bryophytes. Due to acid rain, the acidification of plant barks takes place and acid-tolerant mosses grow on the surface of barks. P. schreberi grew faster in an acidic environment having pH 4.5. At pH 3.5, its growth and chlorophyll content reduced and the production of capsule decreased.
The most striking feature of bryophytes is the accumulation and retention of pollutants, which makes them so special for the interpretation of heavy metal emission pattern. There is a great need to extend the observations on mineral location and effect to a much wider range of species. More species should be recognized to inhibit the chemical environment. Other important aspect is visible responses shown by plants in a polluted environment that are different from those in an unpolluted environment. These aspects of bryophytes also attract research on their unique mechanisms at the cellular level by which they can survive in extreme acidic and polluted environments.
- • bioindicator
- • pollutants
- • nonflowering plants
- • air pollution
- • Bryophyta
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