Growth that inhibits

Researchers from the US have recently declared that it may be possible to reduce the degree of mercury pollution in the soil by simply growing plants in it

 
Published: Tuesday 15 October 1996

in a study published in the Proceedings of the us National Academy of Sciences , researchers from the University of Georgia report that they have developed a plant capable of absorbing highly toxic mercury ions from a growth medium and reducing them to less toxic and relatively inert metallic mercury. Once converted to its metallic state, the mercury is transferred into the atmosphere as a vapour ( Environmental Health Perspectives ; Vol 104, No 8).

Mercury, a poisonous, metallic element, is ubiquitous in the environment. Spewed by volcanoes, evaporating from water bodies and rising as gas from the Earth's crust, mercury either floats in the air as vapour or binds to particles. Eventually, it falls to the earth to settle in sediments, oceans and lakes, or re-enters the atmosphere by evaporation.

Using plants to remove pollutants from the soil, a process called phytoremediation, has been shown to work for many chemicals. The latest addition to the list is mercury. Pesticides, herbicides, explosives, solvents, radioactive cesium and strontium, and other heavy metals like nickel and lead, have all been shown in various studies as potential candidates for phytoremediation.

According to the study, the scientists have developed the mercury-eating plant by building a synthetic gene, merApe9 , and inserting it into the genome of Arabidopsis or mustard plant. The merApe9 sequence, which is an adaptation of a bacterial gene, encodes the production of mercuric ion reductase, an enzyme that converts mercury ions to their metallic state.

Mercury pollution is found to be particularly suited for phytoremediation. In the case of most other chemicals, the plants growing in the contaminated medium accumulate large amounts of toxic substances in their biomass, which must then be disposed. However, mercury's volatile nature prevents it from accumulating in plants. According to the researchers, in an actual situation, the metallic mercury vapours emitted by the plants would diffuse into the atmosphere and quickly attain non-toxic levels.

While it is feared that regulatory agencies monitoring the environment may not accept the dispersion of metallic mercury into the air as a safe remediation strategy, Richard Meagher, one of the authors of the study claims that the amount of mercury vapour released during phytoremediation would be insignificant on a global scale. "The amount of vapour coming out of a site will be 10,000-fold less than the epa 's ( environment protection agency's) emission standards," says Meagher.

Used in the manufacture of products like light bulbs, batteries, thermometers, barometers, pesticides and paints, mercury has been accumulating in the atmosphere since the industrial revolution in the late 18th century. The burning of fossil fuels in municipal and hospital incinerators, coal-combusting power plants, lead smelters and chlorine producers, also release mercury into the atmosphere. Because it can dissolve metals and particularly because it can separate gold from impurities, mercury has been heavily used for mining.

The World Health Organization estimates that approximately 10,000 tonnes of mercury are released worldwide, from both natural and manmade sources each year. Since mercury ultimately sinks into the sediments of the Earth's oceans and lakes, the micro-organisms present in them convert the inorganic mercury to methylmercury, the form potentially harmful to humans. Since methylmercury accumulates in fish, the consumption of fish becomes the primary pathway by which humans are exposed to it. Foetuses are particularly vulnerable to mercury exposure which can damage the central nervous system, cause mental retardation and result in a lack of physical development.

Scientists feel that phytoremediation of mercury is still in its infancy and that much more research is needed in this field. Though some phytoremediation schemes have been field-tested, mercury-removing plants have only been grown on agar under laboratory conditions. Arabidopsis, a common test plant, has not reduced significant amounts of mercury. Also, it has not been tried out in the field to make it a practical choice for phytoremediation. The next step, feel scientists, is clearly to introduce these genes into high biomass plants and demonstrate their effectiveness on the soil.

Subscribe to Weekly Newsletter :

Comments are moderated and will be published only after the site moderator’s approval. Please use a genuine email ID and provide your name. Selected comments may also be used in the ‘Letters’ section of the Down To Earth print edition.