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Radiation-resistant bacteria that can dispose of heavy metals are being
developed to help clean up soil and water contaminated by toxic
radioactive waste. Researchers have constructed a version of the
highly radiation-resistant bacterium Deinococcus radiodurans that
converts the toxic mercury in such wastes to a less toxic form.
The United States has a major pollution problem with waste from
its nuclear weapons manufacturing program. From 1945 until the 1980s,
around three million cubic metres of radioactive waste were disposed
of by burying it in the ground. Leakage of the buried waste at the
3,000 disposal sites has contaminated surrounding soil and groundwater
with radioactive uranium-235, heavy metals like mercury and toxic
organic solvents such as toluene.
This lethal mix now affects 75 million cubic meters of soil and
two billion liters of groundwater, and the cost of cleaning it up
using purely physico-chemical technology is estimated at around
$265 billion. So the search is on for less expensive ways of detoxifying
the contaminated ground. One of these could be bioremediation. This
is the use of living organisms, usually bacteria or plants, which
can break down the pollutants to use as nutrients, or convert them
into some less harmful form.
The problem with radioactive waste, however, is that most living
organisms are highly sensitive to radiation, which kills cells and
damages DNA. Enter Deinococcus, the most radiation-resistant organism
known to man. This astounding bacterium grows happily in levels
of radioactivity of 60 grays (Gy) per hour -- about ten times the
lethal dose for a human -- that is well above those in the waste.
It is also quite tolerant of organic solvents.
But Deinococcus does not like mercury, and this, radioactivity
apart, is one of the most problematic of the pollutants. In the
radioactive waste, mercury occurs in a highly toxic ionic form,
'Hg(II)'. Researchers have genetically engineered a suite of mercury-handling
genes into Deinococcus, which enable it to convert the Hg(II) into
less toxic elemental mercury (Hg), which is volatile and so can
disperse, and is also chemically almost inert.
The genes come from the biotechnologist's friend, the bacterium
Escherichia coli, which perhaps surprisingly for a bacterium that
lives in the hospitable environment of the human gut can detoxify
ionic mercury. The Bethesda team found that they could combine the
mercury-detoxifying genes with specialized toluene-degrading genes
taken from the harmless soil bacterium Pseudomonas putida.
The end result is a strain of Deinococcus that converts mercury,
breaks down toluene to use as a source of carbon and energy, and
does all this while thriving at radiation levels that no other organism
can resist.
Nature Biotechnology 18, 85 -
90 (2000)
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