Geoscience Reference
In-Depth Information
ing aerobic and the latter anaerobic. In some species the spore is seen as a swelling,
sometimes in the middle and sometimes at one end of the cell. Fungi, actinomycetes
and certain other organisms also form spores. As a rule, spores have a significantly
lower water content than vegetative cells. Fungal and other spores, like the bacterial
ones, germinate when conditions are favourable and resume active growth. All types
of spores may be blown about by wind, along with dust and fine soil particles, and so
can become distributed over distant soil areas and colonize them.
Water requirements of microorganisms vary; fungi and actinomycetes, for ex-
ample, usually make do with less water than do most bacteria, and some can survive
in quite dry environments. However, apart from spore formation, quite a number of
bacterial species including non-spore formers, seem able to withstand desiccation for
long periods, possibly because of adsorption on to the surfaces of clay minerals or
other soil colloids. Certain autotrophic nitrifying bacteria, that are not spore formers,
are known to remain viable in air-dried soil for long periods. One of us (N.W.) re-
covered nitrifying bacteria successfully from soil samples which, after drying at room
temperatures, had been stored in sealed jars at Rothamsted Experimental Station for
up to a century or more. When these soils were re-moistened, nitrification (i.e. oxid-
ation of ammonia to nitrate) by the resuscitated organisms ensued normally and the
bacteria in question could be isolated in pure culture and identified.
It is common practice, nowadays, to preserve reference cultures of microorgan-
isms by a freeze-drying process. Organisms from pure cultures are suspended in suit-
able solutions, frozen quickly and then dried in vacuum from the frozen condition.
Such dried cultures are stored at low temperatures, usually 2° to 4°C, when they re-
main viable and may be preserved for many years.
However, it is when we turn to the study of different aspects of microbial
physiology that one can appreciate their complex characteristics and behaviour, and
realize what they can do and how they affect their surroundings, in this case, the
soil. Free oxygen is essential for the growth and respiration of many microorganisms
which, accordingly, are termed 'aerobic'. There are many other species which can
flourish only in the complete absence of gaseous oxygen. These are designated 'an-
aerobic organisms' or'anaerobes', and in their respiration and metabolism they make
use of the chemically combined oxygen present in salts such as nitrates or sulphates.
Many other types of bacteria exist which, depending on conditions, can grow and
function either with or without free oxygen, and these are referred to as facultative an-
aerobes. In their metabolism, microorganisms gain energy for growth and cell main-
tenance from the oxidation of different nutrients; a sugar, for example, under aerobic
conditions is oxidized completely to carbon dioxide and water. On the other hand, un-
der anaerobic conditions (i.e. absence of free oxygen) only a partial oxidation of the
