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materials, are generally more resistant to degradation. Nevertheless, although cellu-
lose is not a water-soluble substance, both it and hemicelluloses can be rapidly de-
graded by the hydrolytic action of enzymes called cellulases, which are produced
and secreted by various soil bacteria and also small animals, such as snails. These
enzymes break down cellulose or hemicellulose into their component sugars and, in
turn, the latter are readily digested by other microorganisms.
Lignified or woody tissues are decomposed much more slowly and often it re-
quires the combined activities of successions of different small soil animals and mi-
croorganisms to bring about their degradation. The decay and breakdown of tree
stumps and dead branches, for example, takes a long time and is influenced by many
factors, such as site and subsoil conditions, moisture content, temperature and the tree
species. W. Kühnelt described wood degradation processes from the point of view of
the various participating soil animals. Fungi are also involved in the successive stages
of wood and lignin breakdown as described later.
What has been said so far serves to emphasize the extensive range of nutritive
and metabolic activities possessed by microorganisms in general. For his part, man is
interested usually in the effects of microorganisms on their surroundings and whether
these effects are beneficial or whether they are harmful and result in undesirable spoil-
age. But from the viewpoint of the microorganisms, on the other hand, these decom-
position activities may be regarded essentially as processes that provide them with
nutrients. The nutritional requirements of microorganisms are very diverse.
Some bacteria, which are termed autotrophic, can satisfy their food requirements
from a few simple inorganic compounds. They obtain carbon from carbon dioxide, ni-
trogen from compounds like ammonia or nitrates, whilst energy for growth and meta-
bolism is acquired from the oxidation of substances such as hydrogen, sulphur, hy-
drogen sulphide, iron compounds, ammonia or nitrites. Although these bacteria do
not use solar energy for photosynthesis, as when carbon dioxide is utilized by green
plants, they are able to bring about the reduction of carbon dioxide by means of hy-
drogen produced within the cell, often from water molecules, in the course of their
intermediary metabolism.
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