Geoscience Reference
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and exoskeletons of insects, woodlice and other arthropods, are broken down only
slowly. Waxes also are degraded only slowly.
Fungi imperfecti are so-called because no sexual stages have been recognized
in them, and their precise classification, accordingly, is rather problematical. They
appear to reproduce only asexually. Many soil fungi belong to this group which in-
cludes, for example, typical moulds like Penicillium and Aspergillus species. The lat-
ter species of moulds can grow on most available substrates in soil, such as pieces
of plant litter or other organic debris and they always produce multitudes of spores.
Mucor species also colonize litter and similar material, form many spores, and from
there they often grow further into surrounding areas.
Actinomycetes, which are a large group of filamentous organisms, are somewhat
intermediate between bacteria and fungi. As individual cells, they are similar in size
to common bacteria but they can grow into long branching filaments and produce a
ramifying network resembling fungal mycelia. The actinomycete mycelium may also
later fragment into smaller elements like bacteria in size and appearance. The hyph-
al filaments are usually about one micron in diameter and may sometimes grow into
aerial filaments. Chains of spores are formed simply by the growth of cross walls.
Actinomycetes are variable in morphology and other characteristics so their identific-
ation and classification is often difficult. Some species give off an earthy odour due
to chemical compounds known as geosmins; the smell of newly dug soil, most likely,
is attributable to this. Actinomycetes have assumed great importance in recent years
because of their propensity for producing antibiotic substances. It is particularly in-
teresting that many antibiotics derived from actinomycetes or streptomycetes possess
unusual and complicated chemical constitutions.
S.A.Waksman and his colleagues discovered the first such antibiotic, streptomy-
cin, which had a significant effect against the tubercle bacillus (the causative agent
of tuberculosis). Since then much research has been devoted to the isolation and in-
vestigation of numerous soil actinomycetes. Many such antibiotics have now been ap-
plied in human and veterinary medecine, or in agriculture. Attempts to demonstrate
the presence or production of antibiotics in soil which may possibly be responsible
for antagonism between different microorganisms have met with little success. One
example is griseofulvin, but the amounts found are too small to have much import-
ance.
Among the actinomycetesa, several subgroups are known and these are classi-
fied generally on the basis of spore and spore formation characteristics. Most soil spe-
cies belong to the genera Nocardia, Streptomyces and Micromono-spora. Species of
the latter genus are widespread in soil and many are active cellulose decomposers.
Nocardia species are especially versatile in their metabolism and can degrade many
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