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  • From: "Lawrence F. London, Jr." <venaurafarm@bellsouth.net>
  • To: permaculture <permaculture@lists.ibiblio.org>
  • Subject: Re: [permaculture] Cleaning/Containing Mercury
  • Date: Fri, 11 Jun 2010 13:02:41 -0400


This article is interesting; don't know if it offers a novel approach to
mercury remediation using transgenic trees but this statement about
mercury-resistant bacteria is interesting. Can these bacteria alone be
cultured on site to remediate mercury in soils?

Research Projects in the Merkle Lab
Development of transgenic trees for phytoremediation of contaminated
soil and water
http://www.warnell.uga.edu/research/merkle/

"Mercury resistant bacteria express merA to convert highly toxic, ionic
mercury, Hg(II), to much less toxic, elemental mercury, Hg(0).
Expression of merA in transgenic plants might provide an ecologically
compatible approach for the remediation of mercury pollution. Yellow
poplar proembryogenic masses (PEMs) were transformed with three modified
merA constructs via microprojectile bombardment."


And a Google search on "Soil mercury remediation by bacteria"
produced many results:
http://www.google.com/search?q=Soil+mercury+remediation+by+bacteria&ie=utf-8&oe=utf-8&aq=t&client=firefox-a&rlz=1R1GGLL_en___US365

And this reference:

http://www.nature.com/cr/journal/v11/n3/full/7290091a.html

Differential mercury volatilization by tobacco organs expressing a modified bacterial merA gene

Yu Ke HE1,2, Jian Ge SUN1, Xian Zhong FENG1,2, Mihaly CZAKÓ1 and László MÁRTON1

1. 1Department of Biological Sciences, University of South Carolina, 700 Sumter Street, Columbia SC29208, USA.
2. 2National Laboratory of Plant Molecular Genetics, Shanghai Institute of Plant Physiology and Ecology, Chinese Academy of Sciences, 300 Fenglin Road, Shanghai 200032, China.

Correspondence: L Márton, Fax: 001-803-7774002; E-mail: marton@biol.sc.edu; He YK, Fax: 0086-21-64042385; E-mail: heyk@iris.sipp.ac.cn

Received 17 July 2001; Revised 24 August 2001; Accepted 25 August 2001.

Abstract

Mercury pollution is a major environmental problem accompanying industrial activities. Most of the mercury released ends up and retained in the soil as complexes of the toxic ionic mercury (Hg2+), which then can be converted by microbes into the even more toxic methylmercury which tends to bioaccumulate. Mercury detoxification of the soil can also occur by microbes converting the ionic mercury into the least toxic metallic mercury (Hg0) form, which then evaporates. The remediation potential of transgenic plants carrying the MerA gene from E. coli encoding mercuric ion reductase could be evaluated. A modified version of the gene, optimized for plant codon preferences (merApe9, Rugh et al. 1996), was introduced into tobacco by Agrobacterium-mediated leaf disk transformation. Transgenic seeds were resistant to HgCl2 at 50 muM, and some of them (10-20% ) could germinate on media containing as much as 350 muM HgCl2, while the control plants were fully inhibited or died on 50 muM HgCl2. The rate of elemental mercury evolution from Hg2+ (added as HgCl2) was 5-8 times higher for transgenic plants than the control. Mercury volatilization by isolated organs standardized for fresh weight was higher (up to 5 times) in the roots than in shoots or the leaves. The data suggest that it is the root system of the transgenic plants that volatilizes most of the reduced mercury (Hg0). It also suggests that much of the mercury need not enter the vascular system to be transported to the leaves for volatilization. Transgenic plants with the merApe9 gene may be used to mercury detoxification for environmental improvement in mercury-contaminated regions more efficiently than it had been predicted based on data on volatilization of whole plants via the upper parts only (Rugh et al. 1996).

Communicated by László Márton, Department of Biological Sciences, University of South Carolina, 700 Sumter Street, Columbia, SC29208, USA
Keywords:

Environment, heavy metals, mercury, remediation, tobacco

INTRODUCTION

Environmental pollution is an increasing problem both for developing and developed countries. Mercury, both in organic and ionic form, is one of the most hazardous pollutants among the heavy metals1and its accumulation in human body results in inactivation of metabolic enzymes and structural proteins2, 3 giving rise to serious health problems (Minamata-syndrome).

Usually, mercury pollution is caused by industrial and agricultural activities, releasing mercury into air, water and land. Mercury is used in gold mining and huge amounts of mercury wastes produced in some chemical factories and fossil fuel burning. Farmers still use cheap mercury-based fungicides in certain regions. In some industrialized areas mercury in river water and soil has reached dangerous levels. The primary ionic mercury pollution is secondarily converted into organomercurials on site. Organomercurial compounds are 1-2 orders of magnitude more toxic to some eukaryotes and are more likely to biomagnify across trophic levels than ionic mercury [Hg2+]1, 4.

There are Gram-negative bacteria with mercury-resistance that is conferred on by a mercuric reductase encoded by the merA gene5. merA efficiently catalyzes the reduction of Hg2+ to elemental mercury (Hg0) which is then evaporated from the cell. The original merA gene was used to transform the model plant species Arabidopsis thaliana with the intention to develop mercury-resistant/volatilizing plants, but the transformants failed to express the mRNA and the protein6. When the merA gene was mutagenized in order to modify codons to those preferred by plants, the resultant merApe9 gene was efficiently expressed. MerA9pe gene enabled Arabidopsis plants to grow on toxic levels of Hg2+ by converting the more toxic ionic mercury to elemental mercury6, proving that an efficient new tool was created for phytoremediation technologies. Plants carrying this modified merA gene can extract, sequester, and detoxify mercury pollutants7. The mercury volatilization trait was also introduced into yellow poplar and tobacco7, 8. Mercury volatilizing transgenic plants could be incorporated into new environmental remediation technologies providing a more environmental friendly, cost effective and sustainable alternative compared to chemical and physical recovery of mercury from the polluted wastes9.

Tobacco (Nicotiana tabacum) is an important cash crop. Converting tobacco to a mercury volatilizing plant could create a multiple use crop that can be grown in mercury-contaminated soil7. Since tobacco is much larger than Arabidopsis, and has extensive root systems capable of extracting mercury, its potential for mercury evaporation is greater. Elemental mercury evolution via the upper parts of tobacco has been reported7. In this report, we describe the differential elemental mercury evolution capacities of roots, leaves and stems of tobacco transgenic for the merApe9 gene.





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