Prof. Joe Cummins gives us timely warnings of mad scientists developing GM trees that send mercury vapor into the air and questions the benefits of GM trees with reduced lignin content.
Soil mercury pollution can be a major chronic hazard. Most polluted sites are historical industrial sites. Gold mining, in particular, may still use primitive "quicksilver" gold extraction that pollutes soil and waterways. In many areas, soil pollution may be of geological origin rather than a result of human activity. Currently, atmospheric deposition of mercury is a leading pollution problem in the cities and wild lakes of northern countries. Most of the problem is associated with fossil fuels and medical waste incineration.
Plots of land that are polluted with high levels of mercury are planned to be phytoremediated using trees genetically modified to take up ionic mercury or organic mercury, convert it to less toxic elemental mercury, which is then expelled into the atmosphere where it will be safely diluted [1,2,3].
The environmental risk assessment submitted by the proponents of transgenic phytoremediation argued that mercury emissions from the treated sites would be below the current emission levels for elemental mercury. They said elemental mercury is retained in the atmosphere for up to two years during which time it is diluted to "non toxic" levels before precipitation . The proponents also argued that the pollution would be negligible in comparison to fossil fuel burning and hospital waste incineration. They claimed that animals fed on the GM plant would be exposed to less mercury than from conventional plants because elemental mercury was so rapidly released from the plant tissue. They also believed that the genes from mercury emission would not be transferred to non-transgenic plants.
Unfortunately, although elemental mercury does remain in the atmosphere for up to two years, it always precipitates with rain and snow. The arctic acts as a trap to condense the fallen mercury, but all of the northern communities including large North America cities on the east suffer from increasing mercury accumulation brought down with precipitation. The precipitated elemental mercury is rapidly converted to ionic and organic mercury once it is deposited.
What phytoremediation will do is to relocate soil mercury from contaminated soil sites in southern communities and disperse and redistribute the mercury to the northern communities. There are a large number of sites with mercury contaminated soil and sediment along with the geological areas of high mercury content. For example, the crude gold mining techniques used along the Amazon left high soil mercury levels. If that area were to be phytoremediated, the mercury released to the atmosphere is likely to precipitate in the cities on northern United States and Canada and impact heavily on the Arctic. Emitted mercury, condensed in the ocean, will reappear on the dinner tables of the world from bioaccumulation through the food chain.
Based on the current experience with transgenic crops, it is certain that some transgenic pollen and seed will escape. Populating expansive areas of geological mercury pollution with mercury transgenic trees could lead to a global catastrophe.
The US EPA seems rather schizophrenic in supporting the research on mercury phytoremediation by air emission on the one hand while also supporting major projects in reducing atmospheric mercury deposition. The United Nations should play a major role in regulating the global atmospheric deposition of mercury and other volatile pollutants.
Plant cell wall material is composed of three important constituents: cellulose, lignin, and hemicellulose. Lignin is particularly difficult to biodegrade, and reduces the bioavailability of the other cell wall constituents. Lignin is a complex polymer of phenylpropane units, which are cross-linked with a variety of different chemical bonds. This complexity has thus far proven as resistant to detailed chemical characterization as it is to microbial degradation.
Nonetheless, some organisms, particularly fungi, have developed the necessary enzymes to break lignin into its component chemicals, and this is crucial for nutrient recycling in the ecosystem.
Extensive efforts have been made to genetically modify trees so that they have reduced lignin to facilitate pulp production. Forage crops have also been modified to facilitate grazing and to allow animals to digest more of the forage or silage. Most of the genetic modifications included the use of anti-sense gene constructs to inhibit particular gene products in the metabolic path of lignin production.
Anti-sense modifications use the insertion of genes that produce messenger RNA with a sequence complementary to the messenger RNA of the gene, thereby binding to it to form double-stranded RNA, which is destroyed by the plant cell as part of the plants defence against viral infection.
Lignin is important to the plant, as it is implicated in plant resistance to stress and pathogens so the low lignin tree or forage crop may be too delicate to thrive in the real world (outside the green house).
Low lignin anti-sense transgenic poplars were grown for four years, and reported to produce high quality pulp without interfering with plant growth and fitness . Another low lignin anti-sense poplar was found to have a low lignin content but the structure of the lignin was altered in a manner that was less amenable to industrial lignin degradation than the normal tree . An extensive study of low lignin perennial herbaceous plants (including alfalfa, brome grass and orchard grass) that had been selected using conventional breeding identified problems including decreased winter survival and decreased biomass .
The transgenic anti-sense low lignin trees need extensive testing with exposure to environmental stress and to pests before extensive plantations are considered. In addition, the ecological impacts of such trees, including those resulting from gene flow, must also be addressed.
Article first published 05/07/02
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