Environmental Biotechnology for thr Degradation of Recalcitrant Aromatic Hydrocarbons
Jang-Young Lee and Hak-Sung Kim
We live in an era where chemicals play a very important role in our daily life. The production and use if synthetic chemicals in the United States has doubled every eight years during the last two decades and the usage of pesticides alone has increased from 1.1 billion pounds a year in 1971 to 1.4 billion pounds in 1977 and has approached more than 1.5 billion pounds by the end of the seventies. Uncontrolled production and environmental release of large amounts of synthetic chemicals have produced massive pollution problems, which had led the United States Congress to promulgate in October 1976 the Toxic Substances Control Act(TSCA), stipulating that no person may manufacture or process a chemical substance for a new use without obtaining specific approval from the U. S. Environmental Protection Agency. Specifically TSCA, FIFRA(Federal Insecticide, Fungicide and Rodenticide Act), the Clean Water Act, and several others have sought to assess the environmental hazard and impact on human health of a bewildering number of synthetic chemicals manufactured and released by the chemical industry. The assessment of environmental hazard and human health can best be done by following two key parameters, viz. the toxicological properties of the chemicals and their fate in the environment. The environmental fate of chemicals, specifically the biodegradation of natural and synthetic compounds, by natural microflora has been the subject of a number of books and reviews.The biodegradation used as a part of the clean-up effort after the EXXON VALDEZ accident in March 1989 has brought the field of biodegradation to the attention of both regulatory and scientific communities, as well as to the general public. These effort have contributed towards the acceptance of biological means for contaminant destruction as a valid alternative to physical and chemical technologies. Use of microbes in biodegradfation is as old as composting and has long been considered as a well established technology, for example, in municipal waste treatment. Recent attention, however, has been directed to treating toxic environmental pollutants. The potential for this biological technology is heightened by documentation of the diversity of organisms capable of metabolizing an increasingly wide range of toxic organic compounds. Yet it should be realized that the natural environment in which it is to be applied has complexities which cannot be readily governed nor quickly understood. Biodegradation is an extremely broad field. Some areas have only recently developed, whereas other have been under investigating for many years. As a consequence, breakthroughs can be at the level of a new paradigm, such as mineral cations serving as electron acceptors for toluene metabolism, as well as at the molecular level such as a newly constructed metabolic pathway for the breakdown of chlorinated aromatics. By definition, the newer paradigms are less well understood and in some cases may lack appropriate pure cultures with which to work. Rather than detract from the accomplishments, this indicates the richness of possibilities regarding organisms to isolate and new pathways and mechanisms to study. Studied of aerobic metabolism continue to involve the skillful construction of new pathways for degradation of chlorinated aromatic compounds both in vivo and in vitro. While the construction of new pathway is limited to the use of genetic material from isolated strains, a vast reservoir of genetic material available in nature has yet to be fully exploited. Thus new strains are sought after continoually and classic techniques foe selection on carbon sources have yielded strains with new bioldegradative capabilities. Anaerobic conditions, or those in which alternatives to oxugen serve as the electron acceptor, are receiving much attention. With anaerobiosis, there exist many conditions under which the selection can take place, effectively increasing the genera of organisms sought after. For the oxidation of toxic chemicals, important environmental electron acceptors such as nitrate, sulfate and carbonate are now augmented by the demonstration taht reduced iron and manganese also serve electron acceptors. In addition, highly chlorinated molecules such as polychlorinated biphenyls have been proposed to serve as anaerobic electron acceptors while undergoing reductive dechlorination.The primary objective of this article is to draw readers' attention to the technique and the potential of the emerging environmental biothechnology that may in future provide a means of disposing of many of the persistent, toxic chemicals from the environment through the use of specialized microorganisms, isolated from nature or newly constructed in laboratories. As a prerequisite to such an approach, the mechanisms of microbial degradation and their limitations with respect to the structure of the chemicals to be degraded must be understood. In addtion gene manipulation techniques frequently employed for construction of a novel pathway must be understood. ; these aspects will be treated in the present review with an emphasis on the aromatic compounds.