| On Anglesey there is a low hill,
pock-marked with craters, and strewn with multicoloured rocks. This is
Parys Mountain, and it was once the largest copper mine in the world.
Mining started here over 3500 years ago, and though the mine was
abandoned more than a century ago, almost nothing grows amongst the
boulders and heaps of gravel, for they are toxic, and the groundwater is
dilute sulphuric acid.
Here and there this has collected in pools, where nothing
lives, except a green slime, Euglena mutabilis, thriving at ph 2.3 in
the midst of poisonous concentrations of heavy metals from the
chalcopyrite, galena, and zinc blend ores. This alga is an acidophile, a
group of organisms which can tolerate very acidic conditions. In fact
they will die if placed in rainwater. The sulphuric acid results from
the digestion of pyrites and other sulphide minerals by
mineral-oxidising bacteria, such as Acidithiobacillus ferrooxidans.
Though these organisms grow at outdoor temperatures, others, which are
predominantly archaea rather than bacteria, can tolerate much hotter
conditions. One important thermophile Thermus aquaticus grows at 70 C in
hot springs of Yellowstone National Park, and is the source of the TAq
enzyme, essential for the DNA-polymerase chain reaction. Sulfobacillus
yellowstonensis, found there at the Frying Pan hot springs can
metabolise both sulphur from volcanic gases, and iron from pyrite,
producing sulphuric acid. But though these pools are boiling, the
temperature is only 80 C, for the altitude is 7000 feet.
Near sea level on the volcanic island Montserrat, organisms have been
found in pools boiling at 100 C, and nearby, the most acidophilic of all
iron-metabolising bacteria, Sulfobacillus montserratensis can survive at
a ph approaching zero. The temperature record is held by Pyrodictium
occultum, growing in the black smokers on the oceans' floors at 113 C,
though it! can survive higher temperatures. These vents eject constant
streams of very hot water - over 300 C, and are black with sulphide
minerals.
When these organisms were discovered, it was thought that they had
evolved from more familiar species which had adapted to extreme
surroundings. Now we believe the converse, that all life has evolved
from these extremophiles, which could withstand the hostile conditions
on early earth.
Acidophiles are now used in bio-mining to recover metals from low grade
sulphide ores, and many different sulphide ores can be worked with
suitable organisms. At the Kennecott Chino mine in New Mexico, a lean
copper ore is sprinkled with dilute sulphuric acid to encourage the
growth of indigenous acidophilic mineral-oxidising bacteria, and copper
is extracted from the run-off simply by adding scrap iron.
Although gold is not found as a sulphide, it does occur in "refractory"
ores in close association with pyrite and arsenopyrite. Since the 1980's
it has been extracted from these arsenic-rich ores, for an organism has
been found which digests arsenopyrite. Formerly, these ores were
worthless for they are too toxic to be worked by conventional means.
These microbial gold recovery systems, working in Ghana, Australia and
South Africa use the largest tanks of any biotechnology process.
Thiobacilli are responsible for the rust-red of the Rio Tinto, due to
ferric hydroxide forming as iron pyrite is metabolised, and for the
nuisance of pollution around abandoned mines such as the recent disaster
at Falmouth, from the flooding of the Wheal Jane tin mine. They were
also the cause of the crumbling of Parisian sewers in the 1920's, when
concrete turned to putty.
Acknowledgements
Many thanks to Dr Barrie Johnson, University of Bangor, and Dr Stuart
Shales, U.W.E.
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