2. Future Prospects

Recombinant DNA technology allows production of character-specific efficient plants and microorganisms for bioremediation of soil, water and activated sludge by exhibiting enhanced degrading capabilities against a wide range of chemical contaminants. Genetically engineered organism can withstand adverse stressful situations and can be used as a bioremediators under various and complex environmental conditions.

Genetic engineering has led to the development of “microbial biosensors” to measure the degree of contamination in contaminated sites quickly and accurately. Various biosensors have been designed to evaluate heavy metal concentrations like Hg, Cd, Ni, Cu and As. Genetic engineering of endophytes and rhizospheric bacteria for plant-associated degradation of pollutants in soil is considered to be one of the most promising new technologies for remediation of metal contaminated sites. Bacteria like Escherichia coli and Moreaxella sp. expressing a phytochelatin have been shown to accumulate 25 times more Cd or Hg than the wild-type strains.

The main constraint of phytoremediation technology is the accumulation of pollutants or their metabolites in plant tissues, which shortens plant life and releases contaminants into the atmosphere via volatilization. This problem can be minimized by manipulation of metal tolerance, accumulation and degradation potential of plants against various inorganic pollutants. The bacterial genes responsible for metal degradation can be introduced to plants to allow degradation of metals within the plant tissues. Application of genetically engineered plant-based bioremediation for various heavy metals pollutants is in the forefront due to its eco-friendliness and reduced health hazards compared to physico-chemical based strategies, which are less eco-friendly and more hazardous to human health. Various microbial genes can be harnessed in the transgenic plant for detoxification and accumulation of inorganic contaminants. The metal-detoxifying chelators such as metallothineins and phytochetains can confer resistance to the plant by enhancing uptake, transport and accumulation of various heavy metals. Fast-growing as well as high-biomass-yielding plants like poplar, willow and Jatropa could be used for both phytoremediation and energy production. Among the fast-growing and high-biomass-yielding plants, poplar is the most commonly studied because of its rapid growth rate and potential to produce high biomass within a short period of time (5–8 years). Many of the poplar hybrid varieties have been genetically modified with microbial catabolic genes and specific transporters for increased remediation. For example, mercuric reductase and γ-glutamylcysteine synthetase genes showed increased resistance to Hg and Cd and Cu, respectively, through accumulation of higher concentrations of these metals. Plants engineered with multiple genes will facilitate complete degradation of pollutants to ensure that the harvested biomass can be utilized completely for additional benefits.

Engineered bioremediation strategies involve either the addition of growth stimulators (electron acceptors/donors) to the rhizosphere for reduction of heavy metals or addition of nutrients to the contaminated soil for enhancement of microbial growth and bioremediation properties of microorganisms or genetically modified plants. Many engineered bacteria with heavy metal reduction capacity through the expression of improved enzymes like chromate and uranyl reductase were applied in a specific rhizosphere to perform a specific function. Similarly, genetically modified plants are also known to produce specific compounds which may support the rhizospheric transformation of heavy metals.

The main drawbacks of phytoremediation technology are storage and accumulation of pollutants in the plant materials and the remediation process slowing down and often becoming inadequate when the contaminated site has multiple pollutants. The appropriate solution to these problems is to combine the microbe-plant symbiosis within the plant rhizosphere or to introduce microbes as endophytes to allow degradation of pollutants within the plant tissues. The microbial population in the rhizosphere is much higher than present in vegetation-less soil, and this is due to the facilitation provided by the plants through release of substances that are nutrients for microorganisms. This approach has been evaluated under laboratory conditions, and if it succeeds in field conditions, this technology could facilitate accelerated removal of pollutants, which in turn will support high plant biomass production for bioenergy. The major strategies for implementing bioremediation processes include biostimulation and bioaugmentation approaches guided by specific microbes in combination with plants.

Apart from the above discussed strategies, the remediation of heavy metals and trace elements can be achieved by nanotechnology. Nanoparticles enhancing microbial activity to remove toxic pollutants is called “nanobioremediation”. Nano-based technologies not only reduce the costs of cleaning up contaminated sites at a large scale, but also reduce the process time as well. “Bionanotechnology” or “nanotechnology through biotechnology” is the bio-fabrication of nano-objects or bifunctional macromolecules used as tools to construct or manipulate nano-objects. Wide physiological diversity, small size, genetic manipulability and controlled culturability make microbial cells ideal producers of nanostructures ranging from natural products, such as polymers and magnetosomes, to engineered proteins or protein constructs, such as virus-like proteins and tailored metal particles. This innovative technique would be a promising tool to address the escalating problem of heavy metal as well as organic contaminants in the environment.