SOIL ORGANISMS - Soil Ecology - page 212

As soils dry to matric potentials less than -0.05 to -0.3 MPa, most bacterial activities
(movement, fungal lysis, chemical transformations, respiration) are substantially
reduced. Below potentials of -0.3 to -0.6 MPa, bacterial respiration declines rapidly and
becomes negligible at -2.0 MPa (Griffin, 1981b). Bacterial activity generally ceases at
potentials below -1.5 MPa. Nonetheless, the lower tolerance potentials of some bacteria
may be much more negative ( e.g., -10 MPa in Clostridium ) (Paul and Clark, 1989), and
significant activity has been recorded at potentials of-15 to -30 MPa.
The ability of bacteria to survive in dry conditions is probably due to the selective
colonisation of favourable microhabitats, and differences in the vertical and horizontal
distributions of soil moisture.
Mobility
Bacteria are normally associated with the surfaces of mineral or organic particles and
assemblages. Most are unable to move and even those which have flagellae (mostly
Gram-negative species) have very limited ranges of movement. Consequently their
dispersion is dependent on water movement or root and faunal activity.
Digestion
As with all micro-organisms, bacteria have a system of external digestion mediated
through the production of extracellular enzymes, especially in the Gram-positive
bacteria (Hattori, 1973).
Part of the metabolites released by extracellular digestion may be used by other
organisms thus creating a trophic stimulus for opportunistic or cooperating micro-
organisms (Figure III.5). Intracellular enzymes may also be released by dead cells which
can contribute to the nutrition of the colony. When active, bacteria have high levels of
metabolic activity of 50-1200 dry cells
Functional Diversity
Bacteria are able to perform an extremely wide range of chemical transformations.
They are, however, only active over a very narrow range of environmental conditions.
For example, non-symbiotic fixation of atmospheric nitrogen by Azotobacter is only
possible if easily metabolisable carbon substrates are available and N-compounds
such as ammonia or nitrates are either absent or present at extremely low concentrations.
Fixation occurs between pH levels of 6 and 9 and is at a maximum between 7.2 and 7.6.
Growth factors, vitamins and high levels of phosphorus are also required. Soil water
potentials of greater than -2.4 to -0.4 MPa, temperatures between 10 and 37 °C (with
optimal fixation at 25-30 °C) and low oxygen tensions are other necessary conditions.
Azotobacter is inhibited by a large range of toxic mineral and organic compounds but
may tolerate relatively high salinity. Colony growth and N-fixation is enhanced in the
presence of clays. Finally, Azotobacter develops synergistic relationships with a number
of other bacteria (particularly cellulolytic bacteria) which provide assimilable-carbon
(Dommergues and Mangenot, 1970).
Recent development of techniques using fluorescent rRNA-targeted oligonucleotide
probes has enabled the detection of bacteria of different phylogenetic levels, from the
subspecies to the kingdom level (Stahl and Amann, 1991).