Gilding cabbages
MINING is generally seen as a dirty business, full
of grimy men and gritty landscapes. But now nature
is being coaxed into cleaning up the act.
“Phytomining”—using plants to extract valuable
metals from the earth—was once dismissed as a
modern form of alchemy. But it is gradually gaining
scientific ground as researchers begin to
understand, and enhance, the ability of some plants
to soak up such elements as nickel or gold.
Among the phytominers are Chris Anderson and
Robert Brooks at Massey University in Palmerston
North, New Zealand. In this week's issue of
Nature, they report their success in forcing a distant
relative of the cabbage, Brassica juncea, to extract
gold from the ground. A substantial amount of the
world's supply of gold is found, not as nuggets or
rich veins, but as fine dust in the soil, or among the
tons of broken rock (called tailings) left over from
mining operations. Squeezing the gold out of these
sources by conventional techniques is hard, so a
good deal of the precious metal lies ungathered.
Brassica, however, can make the most of these
rich pickings. Left to its own devices, Brassica has
a tough time absorbing gold from tailings, since it is
often bound to other materials that the plant cannot
absorb through its roots. But the New Zealand
group has borrowed a trick from the mining
industry, treating Brassica's gold-laden soil with
ammonium thiocyanate, a chemical which separates
the metal from other substances. In the laboratory
at least, Brassica's roots now seem able to soak up
the metal and carry it to the stem and leaves where
it is stored. When tested for their gold content, the
plants contained 2,000 times more metal than usual.
Dr Brooks does not yet know how easy it will be to
harvest usable gold from plant tissues. And the cost
of the process may make it unattractive to mining
companies if the price of gold falls back from its
current $300 an ounce. That said, Dr Brooks is
already testing his techniques out in the field,
growing the plants on surface mines in Western
Australia. Unfortunately, the plants can dig only so
deep, because the ammonium thiocyanate that must
be spread on the ground for them to do their job will
penetrate and release gold only up to a foot below
the surface. As a result, Dr Brooks has his eye on a
natural gold releaser, a soil bacterium called
Thiobacillus ferro-oxidans, that may be able to
help his plants go about their business without some
of the economic (and environmental) drawbacks of
his present method.
Brassica is not alone in its appetite for metal. Alan
Baker, at the University of Sheffield, has identified
over 400 species which naturally take up zinc,
cadmium and other metals in soils which would kill
common-or-garden varieties. Some of these are
already being tested on field sites in America, Italy,
Britain and New Zealand to pull nickel out of the
ground.
But it is not only gold-diggers who have an interest
in these plants. Because they also absorb and
concentrate toxic metals, such as lead, these natural
collectors could help to decontaminate industrial
waste sites, a process known as phytoremediation.
Richard Meagher and his colleagues at the
University of Georgia have been boosting the
clean-up capacity of yellow poplars, tobacco and
other crops by adding bacterial genes that convert
deadly methyl mercury into a far less toxic form.
Such genetically engineered plants soak up methyl
mercury from the ground, but avoid concentrating
the nasty substance in their own tissues by passing
the final, detoxified version out into the atmosphere
through the leaves. Mercury spills are still a problem
in the western world, and are a growing concern in
developing countries, particularly in industries such
as gold mining. Here Dr Meagher sees a two-fold
prospect for his plants: cleaning up dirty surface
mines, and, with more molecular tweaking, one day
concentrating precious metal from the ground. It
may be a long way from a gold rush, but such plants
could offer some farmers a rich harvest. |
