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Vol. 13 No. 4 - October 2007

Bryophytes: A Useful Tool in Heavy Metal Monitoring

By: Vinay Sahu¹, A.K. Asthana¹, Virendra Nath^1*, M. Yunus²

The bryophytes are autotrophic cryptogams and an important component of the flora. They occur throughout the globe in different habitats, particularly on moist and shady places. They grow on soil, rocks, tree trunks, branches, leaves, buildings, old monuments etc. and in wetlands.

Environmental pollution is increasing day by day, posing a very serious problem for the flora and fauna. A large number of pollutants including heavy metals are adversely affecting our environment. Heavy metals are emitted from solid fuel combustion, vehicular emissions and in industrial processes. Bryophytes are widely used as bio-indicators for their unique and very specific responses. Some bryophyte species are extremely sensitive to pollutants and exhibit visible injury symptoms even in the presence of very minute quantities of pollutants. Such species serve as good bio-indicators and act as a "warning giver" regarding the effect on the solubirity of the environment. Some other species of bryophytes possess the capacity to absorb and retain pollutants in concentrations much higher than those absorbed and retained by the higher plants growing in the same habitat. The uptake mechanisms of elements in vascular plants and bryophytes are vastly different. Vascular plants mainly meet their nutritional requirements by absorbing them from the soil through their developed root system and their foliar systems also help in the uptake of gases (e.g. NO₂, NH­­₃ and SO₂) from the atmosphere, whereas bryophytes obtain their nutrition by absorbing the substances dissolved in air moisture from their general surface.

Metal analysis has become a frequently used and dependable yardstick in the evaluation of the environmental quality of a given site. The method was first used by Rűhling and Tyler (1968) to analyze lead in mosses to monitor roadside pollution in Sweden. Rao et al (1977) analyzed Pb, Cu, Zn, Ni, Cr and Cd of some mosses from herbarium specimens collected during 1905 to 1971 from Mount Royal in Montreal, Canada. He found significant increase in Zn concentration in all the mosses selected for studies. A study on bio-monitoring of heavy metals due to vehicular pollution with the help of Sphagnum has also been done by Saxena (2001). The high accumulation capacity of bryophytes for pollutants has led to their use for heavy metal monitoring.

Bryophytes are known as efficient accumulator of heavy metals because of their following properties:-

1. They lack true root system and depend largely on atmospheric deposition for their requirements of mineral elements.

2. They usually lack continuous cuticle layer and thus their tissues are easily permeable to water and minerals, including the gaseous pollutants in the atmosphere and the metal ions.

3. Their tissues have numerous negatively charged groups and act as an efficient cation exchangers. Their cell walls possess high exchange capacity and even their dead tissues have capacity to bind ions.

4. They generally obtain mineral nutrition from wet and dry deposition of particles and soluble salts. However, in certain bryophytes, uptake of metals from substrate occurs, mainly with rising capillary water. Such bryophyte species are less suitable for the monitoring of heavy metals.

Several experiments have been carried out to study the sensitivity of bryophytes to heavy metals. In these experiments bryophytes have been cultivated from gemmalings, spores or as transplants, for a few days to few months. In these experiments major emphasis has been given to the growth rate, however, changes in the chlorophyll contents/protein, carbohydrates, lipids have also been recorded. Lead (c. 0.003mM) inhibits the elongation of Funaria hygrometrica protonema while high concentrations of Zn (c. 1mM) caused damage to the cells on leaf margin and midrib of the younger leaves of Plagiomnium rostratum.

Transplants of bryophytes have been used in a number of studies in assessing heavy metal deposition rates. Goodman and Roberts reported that Hypnum cupressiforme transplanted in industrial area in Wales died after sometime, but it continued to accumulate heavy metals after death. In India, studies on heavy metal monitoring, has also been carried out by transplanting Marchantia polymorpha and Brachythecium populeum at vehicular polluted sites in Mussoorie. Nath and his co-workers have carried out passive monitoring of heavy metals with the help of nine taxa of bryophytes and have found that Plagiochama appendiculatum and Marchantia paleacea are good accumulators of heavy metals. Recently, Sharma and Kapila investigated 25 moss samples to monitor the lead pollution in various parts of Chandigarh city.  In Japan, bryophytes have been used as a bryometer to assess the air quality. Thus, bryophytes are being used for air quality monitoring in various countries.

Bryophytes known to accumulate and used as evaluators of specific heavy metal pollution are indicated here under.
 

Monitoring of heavy metals through bryophytes is not only cost-effective, but it also provides efficient way to assess the qualitative and quantitative differences in metal concentrations at distinct locations and on local and landscape scales.

¹Bryology Laboratory, National Botanical Research Institute, Lucknow. *E-mail : [email protected]

²School of Environmental Sciences, B.B. Ambedkar University, Lucknow

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.