article · Microorganisms
Phosphorus is an essential plant nutrient, but much of it remains locked in insoluble forms within soil. In this study, twenty-eight bacterial isolates were recovered from soils across five regions in Tunisia to identify strains capable of releasing usable phosphorus. Genetic sequencing identified five distinct species: Pseudomonas fluorescens, Pseudomonas putida, Pseudomonas taiwanensis, Stenotrophomonas maltophilia, and Pantoea agglomerans. Laboratory testing showed that all five species successfully solubilised insoluble tricalcium phosphate in both solid and liquid growth media, accompanied by a drop in pH indicating organic acid production. Among these, Pseudomonas fluorescens displayed the highest phosphate-solubilising performance. Furthermore, all five bacterial species produced the plant growth-promoting hormone indole acetic acid, with the Pseudomonas fluorescens isolate producing the largest concentration. Strong correlations were also identified between bacterial phosphate solubilisation capacity and the native soil pH and total phosphorus levels.
Crops require phosphorus to grow, but chemical fertilisers can be inefficient and environmentally damaging when minerals become locked in the soil. Identifying native bacteria that can naturally convert insoluble soil phosphates into forms plants can absorb provides a pathway towards sustainable crop nutrition. Strains that also produce growth-promoting hormones can further assist plant development naturally.
The findings could inform the development of microbial biofertilisers for agricultural producers seeking alternatives or supplements to chemical fertilisers. Because the work remains at the stage of laboratory isolation, characterisation, and in vitro testing in synthetic growth media, it represents early-stage research. Extensive formulation, safety validation, greenhouse trials, and field testing would be necessary before these strains could be incorporated into commercial biofertiliser products.
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Soil microorganisms play an important role in maintaining natural ecological balance through active participation in carbon, nitrogen, sulfur, and phosphorous cycles. Phosphate-solubilizing bacteria (PSB) are of high importance in the rhizosphere, enhancing the solubilization of inorganic phosphorus complexes into soluble forms available for plant nutrition. The investigation of this species of bacteria is of major interest in agriculture, as they can be used as biofertilizers for crops. In the present study, 28 isolates of PSB were obtained after the phosphate enrichment of soil samples from five Tunisian regions. Five PSB species were identified by 16S rRNA gene sequencing including Pseudomonas fluorescens, P. putida, and P. taiwanensis, Stenotrophomonas maltophilia, and Pantoea agglomerans. Solid and liquid Pikovskaya’s (PVK) and National Botanical Research Institute’s (NBRIP) media containing insoluble tricalcium phosphate were used for the evaluation of the phosphate solubilization ability of the bacterial isolates by two methods: visual evaluation of the solubilization zone around colonies (halo) and determination of solubilized phosphates in liquid medium by the colorimetric method of the vanado-molybdate yellow. Based on the results of the halo method, the isolate of each species that showed the higher phosphate solubilization index was selected for evaluation of phosphate solubilization by the colorimetric method. In the liquid media, the bacterial isolates showed phosphate solubilization ranging from 535.70 to 618.57 µg mL−1 in the NBRIP medium, and 374.20 to 544.28 µg mL−1 in the PVK medium, with the highest values produced by P. fluorescens. The best phosphate solubilization ability and higher reduction in broth pH, which indicates higher organic acid production, were achieved in NBRIP broth for most of the PSB. Strong correlations were observed between the average capability of PSB to solubilize phosphates and both the pH and total phosphorous content in the soil. The production of the hormone indole acetic acid (IAA), which can promote plant growth, was observed for all five PSB species. Among them, P. fluorescens obtained from the forest soil of northern Tunisia showed the highest production of IAA (50.4 ± 0.9 µg mL−1).
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DOI: 10.3390/microorganisms11030783
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