Role of biotechnology in restoration of degraded lands
The urbanization and increased human activity has led to degradation of habitats. The restoration of the degraded lands can be carried out by using biotechnology which involves the manipulations of biological systems. This restoration could be carried out by the following biotechnological methods:
a) Use of micropropagation and mycorrhiza for reforestation
One of the approaches to tackle this problem is to develop strong and superior species which have the capability to grow well on degraded lands. This can be done by using mass multiplication which involves starting aseptic culture, multiplication of shoot using shoot apical meristems or buds, rooting of in vitro formed shoots, transfer, acclimatization and adaptation of micropropagated plantlets in the field. Using this methodology an estimated 500 million plants of diverse nature have been produced.
Mycorrhizae, which are symbiotic non-pathogenic associations between plant roots and fungi, improves the seedling survival and growth by enhancing uptake of nutrients and water. They also lengthen the root life and provide protection against the pathogens. A list of fungi which can efficiently form mycorrhizae has been prepared. These fungi can be used as inocula which are applied to roots of seedlings, to allow formation of mycorrhizae. The experimental infection of micropropagated plants during rooting increases their survival chances in the field, which is very important in case of plantations on degraded lands.
b) Improvement of soil infertility through the use of nitrogen fixing bacteria, Rhizobium in association with leguminous trees and Frankia in association with non leguminous species.
Biotechnological methods are being developed to help the non-leguminous plants to survive under adverse conditions such as low nutrient supply. There are about 160 species of angiosperms, which are known to form nitrogen fixing root nodules with the actinomycetes bacteria belonging to the genus Frankia which is being used for this purpose. Frankia helps in nitrogen fixation in non-leguminous plant species therefore it can be used for land reclamation through reforestation due to high biomass production
with out the need of expensive nitrogen fertilizers.
c) Development of plants tolerant to abiotic stress which can be grown on degraded lands
The techniques like tissue culture and genetic engineering are being used to develop plants resistant to abiotic stresses e.g. salinity, acidity, aluminium toxicity etc. The cell lines which exhibit resistance to salt stress are selected and then used for plantation on degraded lands. E.g. Brassica spp., Citrus aurantium, Nicotiana tabacum etc. Research is going on to understand the molecular basis of salt tolerance and to isolate genes responsible for this attribute so that salt tolerant plants can be developed using genetic engineering. In vitro selection for tolerance to abiotic stress like aluminium toxicity has been successful in certain plant species e.g. tomato, rice, barley, rice and wheat.
"Triticale" which is a man made synthetic crop has been found suitable for growing on acid soils, dry and sandy soils, on alkaline and calcareous soils and on mineral deficient and high boron soils especially in countries like Kenya, Ethiopia, Ecuador, Mexico, Brazil etc. In China, a number of new stress resistant varieties of rice, wheat and tobacco have been developed using anther culture.
e) Use of selected and engineered microbes for removal and recovery of strategic and precious metals from contaminated degraded lands.
The domestic and industrial effluents often contain harmful heavy metals. These heavy metals cause soil contamination when these effluents are used for irrigation purposes. The biotechnological methods and procedures are being developed to prevent the contamination by these heavy metals and also restore the contaminated soils. This involves the selective use of engineered microbes. Plasmids have been constructed which can enhance the recovery of gold from arsenopyrite ores, by Thiobacillus ferroxidans. Ganoderma lucidum which is a wood rotting macrofungus , is a highly potential biosorbent material for heavy metals and thus can be used to control contamination by heavy metals.
The metal pollution occurs through several processes. As the living organisms including man are constantly exposed to metals, they accumulate by a process referred to as ‘bioaccumulation.’ The continuous exposure and accumulation of a given metal leads to increase in it’s concentration which is referred to as ‘biomagnification’. Biomagnification occurs through food chain and the man gets the maximum impact due to it’s being on top of the food chain. The ‘biomethylation’ is carried out by microorganisms in the soil and water and involves the process of transfer of methyl groups from organic compounds to metals.
Some phytoplanktons (plants that float freely on water surface) and some benthics (plants attached to some substratum at the bottom of aquatic bodies) microorganisms can take up the metals from the waste water ponds. These natural bioscavengers not only control the water pollution by absorbing metals but also contributes in the recovery of industrially important metals from the effluents. The microorganisms like algae can absorb metals form the fresh water e.g. Chlorella vulgaris takes up copper, mercury, uranium. Certain fungal species like Rhizopus, Aspergillus, Pencillium, Neurospora are good absorbers of heavy metals like lead, mercury etc. Some of the bacterial species are capable of accumulating metals on cell walls such as E. coli can take up mercury while Bacilus circulans can accumulate copper.
The mechanism of metal scavenging by these microorganisms is very complex and involves multiple steps. Some of the microorganisms bioaccumulate these metals on their cell walls whereas some others have the capacity to transport these metals to intracellular and intercellular free space and cellular organelles. In certain cases some of the metals occur as immobilized metal containing crystals e.g heavy metal complexes of calcium oxalate crystals. Some of the fungal and algal species synthesize metal binding proteins or peptides. ‘Phytochelatin’ is an ubiquitious metal chelating protein present in all plants and acts as a common buffering molecule for the homeostasis of metals. It is rich in cysteine and can form salt metal complexes through sulfhydral (SH) groups. Due to this property, phytochelatin can be used as a biomarker for metal pollution detection.
The mechanisms involved in the removal of metals by microorganisms are: adsorption, complexation, precipitation and volatilization. The process of adsorption involves the binding of metal ions to the negatively charged cell surfaces of microorganisms. The process of complexation leads to production of organic acids e.g. citric acid, oxalic acid, gluconic acid, lactic acid, malic acid etc. which chelate the metal ions. In precipitation, the metals are precipitated as hydroxides or sulfates by some bacteria such as which produce ammonia, organic bases or H2S.e.g. Desulfovibrio and Desulfotomaculum transform SO4 to H2S which promotes extracellular precipitation of insoluble metal sulfides. Klebsiella aerogenes detoxifies cadmium sulphate which precipitates on cell surface. Volatilization involves bacteria that causes methylation of Hg2+ and converts to dimethyl mercury which is a volatile compound.
Whole cell of Bacillus subtilis have been shown to reduce gold from Au3+ to Au 0 through extracellular enzymatic biotransformation. Under anoxic environment, sulphate- reducing bacteria (Desulfovibrio) oxidize organic matter using sulphate as an electron acceptor. In yeast, Saccharomyces cerevisiae removal of metals is done by their precipitation as sulphides e.g. Cu2+ is precipitated as CuS.
Several technologies for metal removal have been commercialized and employed are given below:
- ATMBIOCLAIMTM process: The advance Mineral Technology (ATM) Inc. (U.S.A.) developed a waste water treatment process with Bacillus sp. immobilized and pre-treated in caustic solution. It is specific for metal cations in the order: Cu2+ > Zn2+ > Cd2+ = Ni2+ > Pb2+.
- AlgaSORBTM process: Biorecovery systems, Inc. (U.S.A) developed this proprietary based material which consists of several types of living and non living algae. The algal cultures are immobilized in silica gel in the form of beads and desorption of metals is carried out.
- Bioremediation of coal wastes through VAM fungi: Selected VAM fungi are introduced through plants in coal mine areas where it was found that VAM fungi improved the growth and survival of desirable re-vegetation species e.g. red maple, maize, alfalfa etc.
- BIO-FIXTM process: The bureau of Mines (U.S.A) developed this process that consists of biomass immobilized in polysulfone. It consists of thermally killed biomass of Sphagnum pat moss, algae, yeast, bacteria and/or aquatic flora. The beads are suitable for practical application in stirred tank reactor, fixed and fluidized-bed columns.
f) Use of biotechnology in the conservation of biodiversity
The extinction of wild species due to the destruction of habitats and ecosystems has raised serious concerns about the biodiversity in general. Biodiversity provides genes from wild species for biotechnology exercises and experiments hence biotechnology and biodiversity are interrelated. Besides taking steps to minimize and regulate the factors responsible for causing loss of biodiversity, efforts are on to develop the techniques of conservation of biodiversity. One of the methods involves the establishment of "gene banks" leading to "in situ conservation and ex situ conservation. The in situ conservation involves the conservation of plants and animals in their natural habitat and ecosystems. The ex situ conservation includes conservation of species away from their habitats. The ex situ conservation uses sample populations and establishes the “gene banks" which includes resource centers, zoos, botanical gardens, national parks, culture collection centers etc.
Biotechnology offers special methods to conserve both animal and plant genetic resources especially in the conservation of endangered plant species. The tissue culture method is being used to multiply an endangered plant species. The method of embryo transfer and artificial insemination is used for the multiplication of endangered animal species.