THE LIMITS OF BIOREMEDIATION, CAN THEY BE BROADENED BY THE MEANS OF GENE-TECHNOLOGY

Martin Romantschuk, Department of Biosciences, Division of General Microbiology, P.O.Box 56 (Viikinkaari 9), FIN-00014 University of Helsinki, Finland.

Bioremediation in its different forms has been used in a wide variety of environmental clean-up projects. The success has varied, depending on type and extent of soil or water contamination, site characteristics, environmental factors etc. Mostly "bioremediation" means the use of microbes to degrade xenobiotic and/or toxic compounds contaminating soil or water, but also plants are used directly or as colonization support for microbes. Among techniques used are traditional composting and various varieties of it for polluted soil, bioreactor treatment of polluted water, bioventing for providing oxygen to deep soil layers and for extraction of volatile contaminants, bioaugmentation for supplying degradation active microbes to contaminated sites, and biosorption for binding of contaminant into biological material. In all of these procedures inoculation with degradation active microbes may be used in attempts to improve degradation efficiency. The microbes used may or may not be genetically modified, that is, designed and engineered by gene-technology for a specific degradation purpose.

Composting is a form of biological waste treatment with long traditions. Commencement of efficient degradation in a composting heap can be improved by addition of a microbe starter inoculum. If the constituents of the waste are natural and easily degradable no inoculum may be needed as long as the conditions change to promote degradation. If, however, the soil contains xenobiotics, synthetic compounds, or natural but very recalcitrant compounds, the microbial population at the contaminated site may not contain microbes capable of degrading the contaminant. For some xenobiotic organic compounds no biological degradation pathway exist. Microbes with the required capacity may evolve given sufficient time, but can also been constructed in the laboratory. The use of genetically modified microbes, particularly for release in the environment is tightly restricted. While caution may be warranted to a certain point, stressing familiarity of used organisms etc., soil bioremediation using genetically modified microbes should not be unnecessarily delayed, since the alternatives may be considerably worse - no action whatsoever, due to prohibitive costs, or inefficient measures that may worsen the problem. An example: Synthetic compounds that in principle are biodegradable may prove recalcitrant due to inability of the compound to induce the appropriate operons. In these cases one option is to "contaminate" the site with a second, natural, induction active compound to turn on expression of the genes needed for degradation of both types of contaminant. This means that one contaminant is remediated by adding a second contaminant. Even if both pollutants eventually are degraded, this is not a very commendable strategy. An alternative is to use gene-technology to perform limited modifications in the promoters or relulator genes to enable induction also by the synthetic compound.

For an inoculum or any form of bioaugmentation to be efficient the organisms, whether genetically modified or not, need to reach the contaminant. This can be achieved by excavation and mixing in of the inoculum, but also spread by for example plant root colonization has been attempted. It should, however, be sufficient if at least the degradation active genes reach the contaminant. Modified laboratory organisms may not perform well when introduced into natural conditions. If, however, the modified genes, or any degradative genes to be introduced, are present on conjugative plasmids, or on other forms of mobile DNA, the degradation capacity is spread into native microbes, and perform their intended purpose. This is likely to take place whenever a set of mobile genes give the resipient microbe a selective advantage.

In summary, by learning to choose the right combinations of organisms and techniques for each case and each type of contaminant, polluted sites can in principle be biologically decontaminated. Still, the best approach is to prevent environmental contamination and spills from happening.