Geoscience Reference
In-Depth Information
Splendid life artists. Growing on rock is an arduous business. Temperature varies greatly between
day and night, periods of drought may be prolonged and the sun's ultraviolet rays can damage
cells. But if an organism can adapt to such an environment, it is often left in peace and not out-
competed by other species. Some fungi form water-repellent substances that minimise water loss.
They also secrete acids that corrode rock so they can penetrate it with their hyphae, gaining pro-
tection. Black yeast fungi have lived in this way since prehistoric times. Four hundred million
years ago, some of them combined with algae to form lichens—one of nature's most successful
organisms. Lichens have spread to most environments on Earth and display an array of colour
and form. Here the elegant sunburst lichen forms distinct and well-delimited patches on a grave-
stone near a medieval church in V¥g¥, Norway
over a bay immersed in green in the valley below. This is Lissos, once a Roman com-
mercial centre and now an archaeological attraction with an ancient temple and ruins of
Roman houses and storerooms. The finest relics are on view in Chania Museum but the
ground is still peppered with marble fragments, perhaps the remains of an ancient col-
umn. It is hard to believe the area has not been cordoned off to the public but it makes
the excursion all the more exciting—a bit like being on a treasure hunt.
The marble shards I found were peppered with tiny black dots made by a yeast-
like and highly durable fungus. Even if washed from the surface, the fungus soon
grows out again from the inside. Black yeasts are among nature's great survivors
and can adapt to severe drought and temperature fluctuations. They are found from
the most inhospitable deserts to the coldest places on Earth, their black pigment
protecting them from ultraviolet radiation. They are extremely adept at penetrating
between the mineral granules inside a stone, where they create cavities that protect
them from drought and the rays of the sun. Their only source of nutrition is wind-
borne particles that adhere to the surface of the stone. Black yeasts are regarded
as predecessors of lichens, organisms that have successfully colonised most envi-
ronments on the planet. Lichens are a symbiosis between a fungus and an alga.
The fungus provides a protective layer that enables the alga to photosynthesise
even in the dry conditions found on rocks and other exposed places. Scientists
believe that when the Earth was in its infancy, the action of fungi caused rocks to
crumble and mingle with dead fungi and algal tissue to form the first soil. Some
soils are formed in this way even today—that is, without any intervention from
plants. In Antarctic deserts, which are too cold to sustain plant life, soil formation
is based on blue-green bacteria, fungi, algae and lichens that penetrate a millime-
tre or two into rock surfaces, thereby causing the stone to slowly fracture and join
dead microorganisms in forming a primitive soil. There are other places where one
can study soil formation. Come with me on a journey to some of them.
Soil Formation on a Glacier's Edge
A flight over northwest America en route to Seattle gives a spectacular view of Mount
Hood, which stands in the middle of the Cascade Volcanic Arc, a mountain range that
saw a major eruption less than thirty years ago when the top of Mount Saint Helens
exploded with such force that it deposited ash residues as far away as Costa Rica. We
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