Nikolay Vassilev

Biotechnological solubilization of rock phosphate on media containing agro-industrial wastes

Rock phosphate (RP) is an important natural material traditionally used for the production of phosphorus (P) fertilizers. Compared with chemical treatment, microbial solubilization of RP is an alternative environmentally mild approach. An overview of biotechnological techniques, mainly based on solubilization processes involving agro-industrial residues, is presented. Potential advantages of composting, solid-state fermentation, and liquid submerged fermentation employing free and immobilized microorganisms that produce organic acids and simultaneously solubilize RP are discussed. Subsequent introduction of the final fermented products into soil-plant systems promotes plant growth and P acquisition.

Immobilized cell technology applied in solubilization of insoluble inorganic (rock) phosphates and P plant acquisition

This paper reviews current knowledge of the production of organic acids by immobilized microorganisms with a simultaneous solubilization of rock phosphate in fermentation and soil conditions. The most widely applied methods are based on the passive immobilization in preformed porous carriers and entrapment of the microbial cells in natural gels. In general, immobilized systems show higher acid producing and rock phosphate solubilizing activity than freely suspended cells. The potential of gel-entrapped P-solubilizers and mycorrhizal fungi as microbial soil inoculants is also pointed out. Some advantages and constraints of using immobilized cells are discussed and a special emphasis on further research is given.

Rock phosphate solubilization by Aspergillus niger grown on sugar-beet waste medium

Solubilization of rock phosphate by Aspergillus niger was studied in solid-state fermentation on sugar-beet waste. This combination was selected after testing three agroindustrial waste materials, namely rice hulls, sugar-beet waste and alperujo. Sugar-beet waste was the best substrate for fungal growth with 69% mineralization, followed by rice hulls and alperujo. The fungus was successfully cultivated on sugar-beet waste supplemented with 3.0 g/l rock phosphate, acidifying the medium and thus decreasing the pH to 3–3.5. Solubilization of insoluble phosphate increased during the first half of the process, reaching a maximum of 292 μg phosphate/ml, although a part of it was probably consumed by the mycelium.

Reuse of microbially treated olive mill wastewater as fertiliser for wheat (Triticum durum Desf.).

Free cells of Aspergillus niger were grown on olive mill wastewater (OMW) supplemented with rock phosphate (RP) in an air-lift bioreactor in batch and repeated-batch processes. The fungus grew well and reduced the chemical oxygen demand of the waste by 35% and 64% in the batch and repeated-batch (fourth batch) processes, respectively. Total sugar content was consistently reduced (ca. 60%) in both processes while reduction of total phenols was minimal. RP was solubilised and maximum soluble P was 0.63 and 0.75 gl(-1) in the batch and repeated-batch (third batch), respectively. Several types of OMW+/-RP, microbially-treated or not, were tested in a greenhouse for their fertilising ability on a soil-wheat (Triticum durum Desf.) model system. Beneficial effects were highest using OMW treated by the repeated-batch process. The treated plants showed an increase in seed biomass, spike number, and kernel weight. Harvest index was highest (0.49+/-0.04) after treatment with OMW from the repeated-batch process.

Rock phosphate solubilization with gluconic acid produced by immobilized Penicillium variabile P16

Penicillium variabile P16 immobilized on polyurethane sponge produced gluconic acid in presence of rock phosphate, the latter being simultaneously solubilized during five repeated batches. A total production of 42, 60, and 90 g gluconic acid/l was obtained for 3, 7, and 14 g rock phosphate/l, respectively. Accordingly, soluble phosphorus concentration increased with gluconic acid production, reaching a maximum of 350 mg/l at the 3d batch in medium supplemented with 14 g rock phosphate/l.

Application of encapsulated Penicillium variabile P16 in solubilization of rock phosphate

Abstract Penicicllium variabile P16, encapsulated in agar or calcium alginate, produced gluconic acid in repeated batch shake-flask cultures. Agar-encapsulated mycelium produced 21% more gluconic acid than Ca-alginate-encapsulated fungus, and was utilized in experiments for rock phosphate solubilization. The latter was closely related to gluconic acid production, which was affected by the presence of rock phosphate. Maximum amounts of dissolved P (0.226 gl−1 batch−1) were found at 14.0 gl−1 rock phosphate, but the solubilization efficiency was higher at 3.5 gl−1 rock phosphate. Gluconic acid productivity pattern and the degree of phosphate solubilization were not influenced by decreasing the glucose concentration in the cultivation medium from 80 to 25 gl−1. Encapsulated fungal cells systems could substitute for chemical solubilization of rock phosphate and could be applied as soil microbial inoculants.

Olive mill waster water treatment by immobilized cells of Aspergillus niger and its enrichment with soluble phosphate

Abstract Olive mill waste water (OMW), supplemented or not with ammonium sulphate and rock phosphate (RP), was applied as a medium in a shake-flask repeated-batch fermentation with Aspergillus niger immobilized on polyurethane sponge. Compared to other treatments, the results showed higher growth of the immobilized mycelium and significant reduction of the total phenols when the waste material was enriched with RP and ammonium sulphate (N). The immobilized fungus solubilized the RP with a maximum level of soluble P of 0·58 g/litre reached during the fourth batch cycle of the OMW+RP treatment. Depending on the medium composition, three types of treated OMW were produced which could be further used for various purposes.

Rock phosphate solubilization by immobilized cells of Enterobacter sp. in fermentation and soil conditions

An Enterobacter sp., a phosphate-solubilizing bacterium, was immobilized in agar. Various amounts of the immobilized bioparticles were applied in a repeated-batch fermentation process in order to solubilize Venezuelan rock phosphate. A total production of 388 mg and 403 mg soluble P/l was obtained for 2.0 g rock phosphate/l in flasks containing 10 ml and 20 ml agar beads, respectively, that was significantly higher compared to a free-cell single-batch control. Whole and destroyed immobilized bioparticles were introduced into a soil enriched with rock phosphate to improve the growth of onion plants. Compared to the control supplemented with a free-cell bacterial suspension, the results showed at least equal plant growth of 80 mg/pot and shoot P concentration of 82 μg/plant when 10 destroyed beads were applied per pot.

SOLUBILIZATION OF ROCK PHOSPHATE BY IMMOBILIZED ASPERGILLUS NIGER

Aspergillus nigel; an acid-producing filamentous fungus, was immobilized on polyurethane foam. Various amounts of foam cubes and spore suspension were tested in order to obtain an efficient immobilization process. The best combination selected for further experiments was 0.2 g polyurethane foam and 3 ml spore suspension. Immobilized cells were reused, with higher levels of acid formation being maintained for longer periods (at least 240 h) than for free cells. The highest titratable acidity, of about 315 mmole/l, was reached with 0.5 cm foam cubes after 3 × 48 h batch cultures in a laboratory shake-flask experiment. Rock phosphate (2.5g/l and 5.0g/l) solubilization was carried out in repeated 48 h batch fermentation using A. niger immobilized on polyurethane foam cubes. The greatest accumulation of soluble P,, approximately 360 #g/ml, was obtained after the second batch in flasks with 5 g/l rock phosphate, but process efficiency was higher at lower concentration of the rock phosphate. After 10 days of culture a total production of 1.2-1.6 mg/ml soluble P was obtained depending on the concentration of rock phosphate in the medium.

Production and properties of inulinase from Aspergillus niger

SummaryA thermostable inulinase was identified in a strain of A. niger. The highest activity was observed at 50 °C (50 Lml−1) and 77% and 34% of this was retained at 60° and 65°C, respectively. pH stability, the effect of thermal stabilizers such as Propylene glycol (10%) and Sorbitol (10%) and effects of different cations were investigated. It was found that the activity was completely inhibited by Ag+ and Hg2+, while Na+ had an activator effect.