Biotechnology in environmental clean up process
Solid wastes account for an increasing proportion of the waste generated by urban societies. While a part of this volume consists of glass, plastics, and other non-biodegradable material, a considerable proportion of this is made of decomposable solid organic material, like food wastes from large poultry and pig farms.
In large non-urbanized communities, a common method for disposing off such biodegradable waste is the low-cost Anaerobic Landfill Technology. In this process, the solid wastes are deposited in low- lying, low value sites.
The waste deposit is compressed and covered by a layer of soil every day. These landfill areas house a wide variety of bacteria, some of which are capable of degrading different types of wastes. The only shortcoming in this process is that these bacteria take a considerably long time to degrade the waste.
However, modern biotechnology has enabled scientists to study the available bacteria, which are involved in the degradation of the waste – including hazardous substances. The most efficient strains of these bacteria can be cloned and reproduced in large quantities, and eventually applied to the specific sites. This ensures rapid degradation of the waste material.
Composting is an anaerobic microbially driven process that converts organic wastes into stable sanitary humus like material. This material can then be safely returned to the natural environment. This method is actually a low moisture, solid substrate fermentation process.
In large- scale operations using largely domestic solid wastes, the final product is mostly used for soil improvement. In the more specialised operations using raw substrates (like straw, animal manure etc.), the compost (final product) becomes the substrate for the production of mushroom.
The primary aim of a composting operation is to obtain final compost with a desired product quality in a limited time period, and within limited compost. The basic biological reaction of the composting process is the oxidation of the mixed organic substrates to produce carbon dioxide, water and other organic by-products. However, it is important to ensure that a composting plant functions under environmentally safe conditions.
Composting has long been recognised not only as a means of safely treating solid organic wastes, but also as a technique of recycling organic matter. This technique will increasingly play a significant role in future waste management schemes, since it enables the reuse of organic material derived from domestic, agriculture and food industry wastes.
Various products (chemicals) generated by the modem technologies are posing a great threat to the natural breakdown processes and the natural mechanisms of maintaining ecological balance. Many of these pollutants are complex in nature, and are hence difficult to break down. Such pollutants are accumulating in the natural environment to an alarming rate.
The application of biotechnology has helped in the environmental management of such hazardous contaminants by bioremediation. This process is also referred to as bio-restoration or bio-treatment. Bioremediation involves the use of naturally existing microorganisms to speed up the breaking down of biological substances and degradation of various materials.
This process adds substantial momentum to the process of cleaning up. The basic principle of bioremediation is the breaking down of organic contaminants into simple organic compounds like carbon dioxide, water, salts and other harmless products.
Bioremediation can help clean up the environment in two ways:
Promotion of microbial growth in situ (in the soil) can be achieved by addition of nutrients. The microbes acclimatise themselves to these toxic wastes (so called nutrients). Over a period of time, the microbes use up these compounds, thus degrading these pollutants.
Another option is to genetically engineer microorganisms, which can degrade organic pollutant molecules. For instance, bioremediation engineers from an American organisation used the ‘Flavobacterium’ species to remove pentachlorophenol from contaminated soil.
The use of microbes has also proved efficient in cleaning up toxic sites. An American microbiologist has discovered a GS-15 microbe, which can eat up uranium from the wastewater of a nuclear weapon manufacturing plant. The GS-15 microorganisms convert uranium in water into insoluble particles that precipitate and settle at the bottom.
These particles can subsequently be collected and disposed off. GS-15 bacterium also metabolizes uranium directly, thus yielding twice as much energy as it would generate normally in the presence of iron. This organism has a very fast growth rate, and can be extremely useful in waste treatment of uranium mining.
Bioremediation employs biological agents, which render hazardous wastes into non-hazardous or less hazardous compounds. Even the dead biomass houses some fungi that can trap metallic ions in aqueous solutions. This is due to their special cell wall composition. Many fermentation industries produce fungal biomass on unwanted by-products, which can be used for this purpose.
The biomass of the fungus Rhizopus arrhizus can absorb 30-130 mg of cadmium/gm of dry biomass. Fungus has ions in its cell-wall like amines, carboxyl and hydroxyl groups. 1.5kg of mycelium powder could be used to recover metals from 1 ton of water loaded with 5 grams of cadmium.
‘Algasorb’, a product patented by the Bio-recovery Systems Company, absorbs heavy metal ions from wastewater or ground water in a similar manner. Trapping dead algae in silica gel polymeric material produces Algasorb. It protects algal cells from being destroyed by other microorganisms. Algasorb functions in the same manner as commercial ion exchange resin, and heavy metals can be removed on saturation.
Controlling pollution at its source itself is an extremely effective approach towards a cleaner environment. Heavy metals like mercury, cadmium and lead are often present as pollutants in the wastewater of modem industry. The effects of mercury as pollutant have been known for quite some time now.
These metals can be accumulated by some algae and bacteria, and thus removed from the environment. For instance, ‘Pseudomonas aeruginosa ‘ can accumulate uranium and ‘Thiobacillus’ can accumulate silver. Several companies in the US sell a mixture of microbes and enzymes to clean up chemical wastes including oil, detergents, paper mill wastes and pesticides.
Biosensors are biophysical devices that can detect and measure the quantities of specific substances in a variety of environments. Biosensors include enzymes, antibodies and even microorganisms, and these can be used for clinical, immunological, genetic and other research purposes.
The biosensor probes are used to detect and monitor pollutants in the environment. These biosensors are non-destructive in nature, and can utilise whole cells or specific molecules like enzymes as biomimetic for detection. Their other advantages include rapid analysis, specificity and accurate reproducibility.
Biosensors can be created by linking one gene to another. For instance, mercury resistance gene (mer) or toluene degradation (tol) gene can be linked to the genes coding for proteins showing bioluminescence within a living bacterial cell.
The biosensor cell, when used in a. particular polluted site, can signal by emitting light – which would suggest that low levels of inorganic mercury or toluene are present at the polluted site. This can be measured further by using fibre-optic fluorimeters.
Biosensors can also be created by using enzymes, nucleic acids, antibodies or other reporter molecules attached to synthetic membranes as molecular detectors. Antibodies specific to a particular environmental contaminant can be coupled to changes in fluorescence so as to increase the sensitivity of detection.
In India, the Central Electrochemical Research Institute at Karaikudi has developed a glucose biosensor based on enzyme glucose oxidase. This enzyme is immobilised on a electrode surface acting as an electro-catalyst for the oxidation of glucose. The biosensor in turn gives a reproducible electrical signal for glucose concentration as low as 0.15 mm (milimolar), and works for several weeks with no apparent degradation of the enzyme.
Another similar application of the biosensors is ‘Bio-monitoring’, which may be defined as the measurement and assessment of toxic chemicals or their metabolites in a tissue, excreta or any other related combination. It involves the uptake, distribution, biotransformation, accumulation and removal of toxic chemicals. This helps minimise the risk to industrial workers who are directly exposed to toxic chemicals.
Biodegradation of Xenobiotic Compounds
Xenobiotics are man-made compounds of recent origin. These include dyestuffs, solvents, nitrotoluenes, benzopyrene, polystyrene, explosive oils, pesticides and surfactants. As these are unnatural substances, the microbes present in the environment do not have a specific mechanism for their degradation.
Hence, they tend to persist in the ecosystem for many years. The degradation of xenobiotic compounds depends upon the stability, size and volatility of the molecule, and the environment in which the molecule exists (like pH, susceptibility to light, weathering etc). Biotechnological tools can be used to understand their molecular properties, and help design suitable mechanisms to attack these compounds.
Oil Eating Bugs
Accidental oil spills pose a great threat to ocean environments. Such spills have a direct impact on marine organisms. To counter this problem, scientists have now developed living organisms to clean up the oil spills. The most common oil-eating microorganisms are bacteria and fungi.
Dr Anand Chakrabarty, a leading US-based scientist of Indian origin, has successfully created bacterial forms which can degrade oil into individual hydrocarbons. These bacteria include Pseudomonas aureginos’, where a gene for oil degradation has been introduced into the Pseudomonas.
Once the oil has been completely removed from the surface, these engineered oil-eating bugs eventually die, as they can no longer support their growth. Dr Chakrabarty was the first scientist to obtain a patent for such live organisms.
Penicillium species has also been found to possess oil degrading features, but its effect needs much more time than the genetically engineered bacterium. Many other microorganisms like the Alcanivorax bacteria are also capable of degrading petroleum products.
More than hundred thousand (one lakh) different chemical compounds are produced in the world every year. While some of these chemicals are biodegradable, others like chlorinated compounds are resistant to microbial degradation.
To tackle these Polychlorinated Biphenyls (PCBs), scientists have now isolated a number of PCB-degrading bacterial (Pseudomonas pseudoalkali) genes KF 707. A whole class of genes, referred to as bph-making enzymes, has also been isolated. These enzymes are responsible for the degradation of PCBs.
Other genetically engineered bacteria are also degrading different ranges of chlorinated compounds. For instance, an anaerobic bacterial strain Desulfitlobacterium sp. Y51 dechlorinates PCE (Poly chloroethylene) to cw-12-dichloroethylene (cDCE), at concentrations ranging from 01 – 160 ppm.
Japanese scientists have come up with a technology called ‘DNA shuffling’, which involves mixing the DNA of two different strains of PCB degrading bacteria. This results in the formation of chimeric bph genes, which produce enzymes capable of degrading a large range of PCBs. These genes are further introduced in the chromosome of original PCB-degrading bacteria, and the hybrid strain thus obtained is an extremely effective degrading agent.
Genes have also been isolated from bacteria that are resistant to mercury called as mer genes. These mer genes are responsible for total degradation of organic mercurial compounds. The bph genes and tod-genes for toluene degrading bacteria (pseudomonas putida Fl) have shown similar gene organisations. Both these genes code for enzymes which show a sixty per cent similarity. By exchanging the subunits of the enzymes it is possible to construct a hybrid enzyme. One such hybrid enzyme created is hybrid deoxygenase which is composed of TodCl – Bph A2 – Bph A3 – Bph A4.
This was expressed in E.coli. It was observed that this hybrid deoxygenase was capable of faster degradation for Trichloroethylene (TCE) based compounds. The todCl gene from toluene degrading bacteria has been successfully introduced, in the chromosome of bacterial strain KF707. This strain then resulted in efficient de-gradation of TCE. This KF707 strain could also be grown on toluene or benzene etc.
Among the oldest industries in the world, mining is the source of alarming levels of environmental pollution. Modem biotechnology is now being used to improve the environment surrounding mining areas through various microorganisms. For instance, a bacterium Thiobacillus ferooxidans has been used to back out copper from mine tailings. This has also helped in improving recovery.
This bacterium is naturally present in certain sulphur-containing materials, and can be used to oxidise inorganic compounds like copper sulfide minerals. This process releases acid and oxidising solutions of ferric ions that can wash out metals from the crude ore. These bacteria chew up the ore and release copper that can subsequently be collected. Such methods of bio-processing account for almost a quarter of the total copper production world-over. Bio-processing is also used to extract metals like gold from very low-grade sulfidic gold ores.
Biotechnology also offers the means of improving the efficiency of bio mining, by developing bacterial strains that can withstand high temporaries. This helps these bacteria survive the bio-processing which generates a lot of heat.