plant surface microbiology.pdf

4 Microorganisms on the Rhizosphere Microcosm 67
the rhizosphere, a phenomenon known as rhizodeposition. These compounds,
which are continuously released into the soil, constitute the main
nutrient sources, maintain the microbiota, the fertility and participate in the
maintenance of the soil structure.
Microorganisms are classified into several categories according to the carbon
and energy source used, but only some groups will be considered in this
chapter. The heterotrophic microorganisms can use glucose or amino acids as
carbon sources. Glucose can be obtained from some macromolecules, such as
cellulose and starch, which undergo lytic action by enzymes produced by the
cellulose and starch-reducing microorganisms (Fig. 10).
Proteins are degraded to amino acids by proteolytic organisms, which can
use these compounds as carbon or nitrogen sources. On the other hand, sulphur
amino acids such as cystine and cysteine can also be used to obtain sulphur
which is used in the biosynthesis of other compounds necessary for cell
metabolism. The amino acids can also be used by the cell without lysis of the
molecule, as many microorganism species are not able to biosynthesise all the
amino acids required by the cell.
Protozoa, such as amoebas, ciliates and flagellates, are organisms which
have the function of immobilising and mineralising the nitrogen in the rhizosphere
system. Bacteria are their main nutrient source, and they obtain
nitrogen and other nutrients for their metabolism from them. Some of these
nitrogen compounds are released into the soil as inorganic NH 3 + and can be
absorbed by the root or by other microorganism groups such as nitrifiers, sulphate
reducers or oxidisers or phosphate solubilisers. Biological nitrogen fixation
is very important in the introduction of NH 3 + molecules into the rhizosphere
(free-living N fixers) or in the plant (symbiotic N fixers). These fixed
molecules can be transformed in NO 3 – or used in the biosynthesis of amino
acids that will form the cell proteins when polymerised. Sulphur amino acids
may be synthesised from SO 4 2– obtained by the oxidation of S by sulphur cycle
bacteria. NO 3 – and NH3 + can be used in amino acid biosynthesis and also as
final receptors of electrons for some groups of facultative anaerobic bacteria.
Phosphate exists in the soil mainly in the soluble inorganic form. Several
solubilisation mechanisms have been described and many microorganisms
produce compounds which can solubilise phosphates. The nitrogen cycle
functional group, the nitrifiers, produces NO 3 – that can form nitric acid. The
sulphur cycle functional group can produce SO 4 2– that can form H2SO 4 or
reduce it to H 2S, which will also solubilise insoluble inorganic phosphate. In
the degradation of sulphur amino acids, proteolytic microorganisms release
H 2S or CO 2, which can form carbonic acid. Both molecules can also solubilise
inorganic phosphate. The carbon cycle microorganisms form CO 2 and
organic acids as end products of their catabolism, and both compounds are
responsible for pH reduction and inorganic phosphate solubilisation.
Soluble inorganic phosphate is absorbed mainly by the mycorrhizal fungi
that transport these molecules to the plant, which in turn transform them into