plant surface microbiology.pdf

4 Microorganisms on the Rhizosphere Microcosm 65
plants (k strategists) to establish in the area and to form a forest in equilibrium.
The pioneer plants can survive under adverse conditions due to the presence
in the rhizosphere of microorganisms which supply nutrients for their
metabolism, and in turn, their exudates maintain these rhizosphere microorganisms.
The k strategist mycorrhizae are sufficiently stable to maintain the
required nutrient levels for this plant group. In this sense, groups of r strategist
microorganisms succeed each other, maintaining the dynamic of the system
and the reconstitution of other biogeochemical cycles until the system
equilibrium is reached with the establishment of late secondary and climax
plant groups.
10 Dynamics Among the Functional Microrganism Groups
of the Carbon, Nitrogen, Phosphorus and Sulphur Cycles
There are several stages in each biogeochemical cycle, and many microorganisms
can take part in one or more cycles depending on the diversity of their
metabolic path (Fig. 10). Microbiota metabolic versatility makes a single bacteria
species able to use various carbohydrates, such as glucose, fructose and
saccharose, as a carbon and energy source, and in their absence they can use
amino acids or other compounds.
The biosphere is composed of all living organisms which depend on matter
transformation for their maintenance. The functional microorganism groups
are inserted in this system which transforms matter and maintains the levels
of nutrients available on Earth. Due to their functional importance, they can
be used as biological indicators to determine any natural or artificial impact
which may occur in the soil. It is obvious that the complexity of the biological
interactions occurring on the soil–plant interface must be simplified to allow
quick and accurate assessment of these microorganism populations. Thus,
only those stages of the biogeochemical cycles which directly influence plant
growth should be chosen. However, different stages can be selected according
to the experimental objective.
Autotrophic organisms have the important function of matter de-mineralisation
and transform it into organic molecules. In this group are plants that
de-mineralise carbon, i.e. transform carbon dioxide (CO 2) into glucose, which
is then polymerised mainly into starch, cellulose, hemicellulose and lignin.
Plants are also responsible for transforming NO 3 – ,NH3 + ,and SO4 2– into amino
acids, PO 4 2– into nucleic acids while ATP, NADP, and SO4 2– can be transformed
into glutathione.
In a simplified way, plants can be considered as nutrients from the soil
solution plus solar energy accumulated in chemical form. Plants generally
release organic molecules into the soil in two ways: (1) by depositing dead
plant material to form the litter; and, (2) by exuding excretion and lysates into