Agriculture Reference
In-Depth Information
Physical protection in suitable sites within microaggregates allows many fragile
Gram-negative bacteria to survive unfavourable conditions. The simplest situation is
where clay particles are adsorbed onto the cell surface: under appropriate conditions of
pH, bacteria and clay particles may have opposing electrical charges which favour their
association (Dorioz and Robert, 1987). Adhesion may also be effected by a simple
attraction effect (Van der Waal's forces) or, more commonly, by the production of
a polysaccharide cover (Figure III.1).
Kilbertus and Mangenot (1981) consider that adsorption of clay particles would
constitute a protection under natural conditions. This may be more efficient than the
production of endospores because of the specific nutrients required for cells to produce
spores.
Another efficient protection system is that afforded to the Gram-negative bacteria that
live in the micropores that occur within microaggregates. These micropores are general-
ly filled with water and offer suitable physical conditions for microbial life. Hydrological
conditions are generally favourable but oxygen diffusion may be slow and promote
anaerobic conditions. This protection is very efficient and Gram-negative bacteria may
survive drastic soil treatments (Gray and Postgate, 1976). For example, most bacteria
surviving the chloroform fumigation treatment of soils are found in narrow
pores of microaggegates or enclosed in thick layers
of their own extracellular
polysaccharides (Foster, 1988).
The survival of fungal populations is mainly achieved by the translocation of living
cytoplasm towards the growing mycelial ends, and the production of resistant spores.
2.4
Community structure
The structure of microbial communities varies considerably in time and space at all scales:
micro-, meso- and macro. Such variation is related to their population characteristics,
their ability to use specific substrates, their colonisation strategies, and to external
factors including the characteristics of the physical environment and available resources.
Spatial and temporal distributions of microbial communities reflect the combined
effects of these four factors. Successions at different stages in the decomposition of
a resource especially reaffirm the major roles of resource quality and competition in
determining the balance of different groups between microbial populations at all stages
(Swift, 1982).
2.4.1
SPATIAL DISTRIBUTION
Variation on a geographic or
macro scale
has been little studied. Mishustin (1966 quoted
in Hattori, 1973) in comparing different soils from the USSR, identified a clear North-
South gradient of microbial abundance with total numbers increasing towards the South
but the relative importance of other bacteria decreasing in favour of the Actinobacteria.
Swift
et al.
(1979) confirm this pattern with hyphal lengths increasing from tundra (
ca.
dry weight) to boreal forests
temperate grasslands and forests (3000
and tropical forest
However, these are only broad approximations
