permaculture

["Lawrence F. London, Jr." <venaurafarm@bellsouth.net>]

> 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.