BIOFERTILIZERS AS COMPLEMENTS TO SYNTHETIC AND ORGANIC FERTILIZATION - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 162

affinity and transport it inside the cells. Siderophores produced by different bacteria and fungi
include ferrichrome ( Ustilago sphaerogena ), mycobactin ( Mycobacterium sp), enterobactin
and bacillibactin ( Bacillus subtilis ), ferrioxamine B ( Streptomyces pilosus ), azotobactin
( Azotobacter vinelandii ), pseudobactin ( Pseudomonas B10), ornibactin ( Burkholderia
cepacia ) and coprogen, ferricrocin and palmitoylcoprogen ( Trichoderma spp ). Fluorescent
pseudomonads produce a peptide siderophore with high affinity for iron called pyoverdin
(Madigan & Martinko, 2005).
Phosphate Solubilization
Phosphorus (P) is, after nitrogen, the most important macronutrient for plant nutrition and
a critical element in agricultural and natural ecosystems worldwide. Phosphorus in an
essential component of key molecules for organisms, including RNA, DNA, AMP, ADP,
ATP and phospholipids.
The productivity of arid regions is particularly low due to scarce and erratic precipitation,
but also to the low availability of phosphorus in the soil. The soil phosphorus is readily
converted into insoluble complexes such as hydrous oxides (oxides, hydroxides and
oxyhydroxides) of iron and aluminum, amorphous and crystalline aluminum silicate and
calcium carbonate (Sample et al. , 1980).
Although arid soils contain a high concentration of phosphorus, i.e. 557-729 kg/ha,
depending on soil use only 2.4 to 3.9% is found in available forms for plants. Generally, 15 to
20% (97 to 110 kg/ha) of the total phosphorus is present in organic forms such as phytin,
lecithin, phospholipids and other compounds, while the remaining 77-82% is available in
inorganic forms such as tricalcium phosphate, and a smaller quantity as iron and aluminum
phosphates (Rao & Tarafdar, 2002). Insoluble forms of phosphorus include aluminum
phosphates on acid soils, and calcium phosphates on alkaline soils. Fixing reactions in the soil
cause that only a small portion (10 to 15%) of the phosphorus applied to the crops as
chemical fertilizer or manure can be used by plants during the same year of its application.
Phosphorus deficiency causes a reduction of plant growth, alteration of the leaves to a
bluish-green color and formation of acid tasting and small fruits. Strategies to address the low
availability of phosphorus include the use of organic sources of this element (phosphoric rock
or fish phosphate fertilizers) and the use of phosphate solubilizing bacteria (PSB), which
increase the availability of this element for plants. PSB constitute a beneficial bacterial group
capable of hydrolyzing both organic and inorganic phosphorus from insoluble sources
(Goldstein et al. , 2003). PSB secrete both organic acids and phosphatases to convert insoluble
phosphates into soluble ions of monobasic (H 2 PO 4 - ) and dibasic (HPO 4 - ) phosphate, through
the process known as mineral phosphate solubilization.
It is generally accepted that the main mechanism of mineral phosphate solubilization is
associated with the production of low molecular weight organic acids, which chelate the
cations bound to the phosphate through its hydroxyl and carboxyl groups, favoring its
conversion to a soluble form. In addition, some PSB produce phosphatases, such as phytase,
which efficiently hydrolyze the organic forms of phosphate. PSB possess the ability to
solubilize compounds such as tricalcium phosphate, dicalcium phosphate, hydroxyapatite and
phosphoric rock; the gluconic acid and 2-ketogluconic are the compounds most commonly
referred as phosphate solubilizers. Various enzymes, such as nonspecific phosphatases,