Phosphate solubilization potential and modeling of stress tolerance of rhizobacteria from rice paddy soil in northern Iran

The purposes of this study were to evaluate the phosphate solubilization activity of bacteria isolated from the rhizosphere of rice paddy soil in northern Iran, and to study the effect of temperature, NaCl and pH on the growth of these isolates by modeling. Three of the most effective strains from a total of 300 isolates were identified and a phylogenetic analysis was carried out by 16S rDNA sequencing. The isolates were identified as Pantoea ananatis (M36), Rahnella aquatilis (M100) and Enterobacter sp. (M183). These isolates showed multiple plant growth-promoting attributes such as phosphate solubilization activity and indole-3-acetic acid (IAA) production. The M36, M100 and M183 isolates were able to solubilize 172, 263 and 254 µg ml^?1 of Ca₃(PO₄)₂ after 5 days of growth at 28 °C and pH 7.5, and to produce 8.0, 2.0 and 3.0 μg ml^?1 of IAA when supplemented with l-tryptophan (1 mg ml^?1) for 72 h, at 28 °C and pH 7.0, respectively. The solubilization of insoluble phosphate was associated with a drop in the pH of the culture medium and there was an inverse relationship between pH and solubilized P (r = ?0.98, P < 0.0952). There were no significant differences among isolates in terms of acidity tolerance based on their confidence limits as assessed by segmented model analysis and all isolates were able to grow at pH 4.3–11 (with optimum at 7.0–7.5). Based on a sigmoidal trend of a three-parameter logistic model, the salt concentration required for 50 % inhibition was 8.15, 6.30 and 8.23 % NaCl for M36, M100 and M183 isolates, respectively. Moreover, the minimum and maximum growth temperatures estimated by the segmented model were 5.0 and 42.75 °C for M36, 12.76 and 40.32 °C for M100, and 10.63 and 43.66 °C for M183. The three selected isolates could be deployed as inoculants to promote plant growth in an agricultural environment.     

Potassium solubilising bacteria (KSB) isolated from rice paddy soil: from isolation,identification to K use efficiency

The present study was intended to isolate potassium-solubilizing bacteria (KSB) from paddy rhizosphere soil. The isolates were obtained from 40 rice paddy fields across Mazandaran province in northern Iran and screened for their K-solubilizing ability on modified Aleksandrov agar medium. The three selected isolates which showed the best solubilisation of potassium were identified using molecular marker 16S rDNA sequencing. The isolates were identified as Pantoea agglomerans, Rahnella aquatilis and Pseudomonas orientalis. From the flame photometry results, the amounts of potassium released by the isolates from mica at 21st day of incubation were 35.36, 76.04 and 56.58 μg ml^?1, respectively. The estimated optimal growth temperature (Ts) were 26.38, 29.17 and 26.80 °C based on segmented model analysis. The pH values of the culture medium with ranges from 6.75–7.26, had a more positive effect on the solubilization of potassium-bearing minerals. The pot experiment results showed that the inoculums of all three KSB enhanced the amount of grain yield and K uptake compared to the control treatment (without K fertilizer). Values were higher when KSB inoculums were used with ½ K chemical fertilizer (47.5 Kg/ha). Bacterial inoculums also increased K use efficiency (AE, PE, APE, ARE and UE) in plants. In conclusion, these findings have successfully demonstrated the effectiveness of locally isolated PGPR with multiple beneficial characteristics such as K solubilizing, IAA production and tolerance to different environment stresses. Therefore, they can be used as biofertilizers to enhance the availability of potassium in the soils and to improve the growth and yield of rice.     

Marine Actinobacteria Showing Phosphate-Solubilizing Efficiency in Chorao Island, Goa, India

The occurrence and distribution of an actinobacteria group of bacteria capable of dissolving insoluble phosphates were investigated in this study in marine environments, especially in sediments of Chorao Island, Goa Province, India. A total of 200 bacterial isolates of actinobacteria was isolated. All isolates were screened for phosphate-solubilizing activity on Pikovskaya’s agar. Thirteen different isolates exhibiting maximum formation of halos (zone of solubilization) around the bacterial colonies were selected for quantitative estimations of P-solubilization. Quantitative estimations for P-solubilization were analyzed for up to 10 days at intervals of 24 h. Maximum solubilization from 89.3 ± 3.1 to 164.1 ± 4.1 μg ml^?1 was observed after 6 days of incubation in six of all isolates, while the isolate NII-1020 showed maximum P-solubilization. The increase in solubilization coincided with the drop in pH. Many of these species showed wide range of tolerance to temperature, pH, and salt concentrations. Further, 16S rRNA gene sequence analyses were carried to identify the bacterial groups which are actively solubilized phosphate in vitro. Gene sequencing results reveal that all isolates were clustered into six different actinobacterial genera: Streptomyces, Microbacterium, Angustibacter, Kocuria, Isoptericola, and Agromyces. The presence of phosphate-solubilizing microorganisms and their ability to solubilize phosphate were indicative of the important role played by bacteria in the biogeochemical cycle of phosphorus and the plant growth in coastal ecosystems.     

The novel and efficient method for isolating potassium solubilizing bacteria from rhizosphere soil

Potassium (K) is the third major essential macronutrient for plant growth and more than 90% of potassium in the soil exists in the form of insoluble rocks and silicate minerals. 150 potassium solubilizing bacterial (KSB) strains were isolated from rhizosphere soil using Aleksandrov medium containing insoluble mica powder. Ten efficient KSB strains were selected and quantification studies showed that higher K solubilization (50.6?mg L^?1) was observed in the strain HMP27 followed by strain WHP47 (46.4?mg L^?1) in liquid medium. Potassium solubilization by the bacterial strains is determined by measuring zone of clearance around the bacterial colony. This procedure requires 10–15?days incubation. Therefore, a simple, rapid, and user-friendly method has been developed for screening of potassium solubilizing bacteria using the bromothymol blue dye in modified Aleksandrov medium. Microorganisms possessing potassium solubilization property developed a clear zone around bacterial colony and changed the colour of dye from greenish blue to yellow after two days incubation. High-performance liquid chromatography analysis of the filtrates showed the presence of oxalic, tartaric, citric, and succinic acid, which could be responsible for solubilization of potassium. This method will allow researchers to readily isolate new potassium solubilizing strains adapted to specific environments.     

Efficiency of Some Bacterial Strains in Potassium Release from Mica and Phosphate Solubilization under In Vitro Conditions

Phosphorus and potassium (K) are major essential macronutrients for biological growth and development. Application of beneficial microorganisms to soil is one approach to enhance crop growth. In this study, the ability of five bacterial strains, including four strains of Pseudomonas sp. (S10-3, S14-3, S19-1, and S21-1) and one strain of Azotobacter sp. SP16, to release K from muscovite and biotite was investigated. Furthermore, phosphate solubilization by these strains was measured when an insoluble source of P [Ca₃(PO₄)₂] was added to the medium. Among the bacterial strains, the highest average K release (73% higher than control) was observed with Pseudomonas sp. S14-3. The average amount of K released from biotite was 37% higher than that from muscovite in inoculated treatments. The enhanced release of mineral K might be attributed to the release of organic acids from the bacteria, a mechanism which plays a pivotal role in solubilizing phosphate from inorganic sources. The results confirmed the enhanced phosphate solubilization by the bacterial strains in the presence of muscovite. The highest P solubilizing activity (67% higher than control) was found in S21-1 and S14-3 strains. Concentrations of both K and P in the liquid phase were increased by increasing the time of experiment. X-ray diffraction analysis of muscovite specimens inoculated with S14-3 strain revealed a partial transformation of these minerals through the presence of 19.5 Å peak on the diffractogram of the magnesium-saturated sample. This may be due to the release of K from the interlayer space and subsequent filling with a number of bacterial metabolites. The findings of this research suggest K depletion from mica in the presence of bacteria, but further investigations are needed to clarify the mechanisms involved.     

Identification of Yeast Strains Isolated from Agricultural Soils for Releasing Potassium-bearing Minerals

Thirty-five yeast strains were isolated from soil samples that were collected from different locations in Upper Egypt. The purified isolates were screened for the release potassium from mica on Aleksandrov agar medium. Two yeast isolates (KSY-29 and KSY-33) showed an ability to solubilize potassium by inducing clear zones around their colonies. They were identified as Pichia anomala and Rhodotorula glutinis, respectively, based on PCR analysis of the ITSI-26S region that was amplified by NL1/NL4 species-specific primers. The amount of K released from muscovite mica in the broth culture of the yeast isolates was measured after 5, 10, 15 and 20 days of the incubation at 25°C. Both yeast isolates were very effective in releasing K of muscovite in broth culture, recording 8.11–13.21 μg/ml that were released from muscovite mica after 20 days of incubation. The inoculation of maize (Zea maize) plants with these yeast isolates under different K levels (25, 50 and 100% of recommended dose of potassium, RDK) as potassium sulfate was tested on growth and K uptake by these plants in the greenhouse. Significant increases (p < 0.05) in plant height, root and shoot dry weights as well as K uptakes by shoots and roots maize plants occurred through the inoculation with KSY-29 or KSY-33 isolates.     

Rock phosphate solubilization by four yeast strains

The solubilization of rock phosphate (RP) by four yeast strains, Rhodotorula sp., Candida rugosa, Saccharomyces cerevisiae and Saccharomyces rouxii, which were isolated from wheat rhizospheric soils, was investigated in this study. The yeast isolates demonstrated diverse levels of soluble phosphate releasing abilities in modified Pikovskaya liquid medium containing RP as sole phosphate source. C. rugosa was the most effective solubilizer under different conditions, followed by Rhodotorula sp., S. rouxii and S. cerevisiae. Acidification of the broth seemed to be the major mechanism for RP solubilization by the yeast isolates, and the increase in soluble phosphate released was correlated significantly with an increase in titratable acidity and a drop in pH. The optimal composition for the solubilization of RP by the yeast isolates in the broth was 20 g L^?1 glucose, 1 g L^?1 yeast extract, 0.5 g L^?1 (NH₄)₂SO₄, and 5 g L^?1 RP, respectively. The yeast isolates were able to solubilize RP at wide range of temperature and initial pH, with the maximum percentage of soluble phosphate released being recorded at 30–35 °C and pH 5–6, respectively.     

Phosphate solubilization potential and stress tolerance of rhizobacteria from rice soil in Northern Thailand

Plant growth-promoting rhizobacteria (PGPR) are known to influence plant growth by various direct or indirect mechanisms. A total of 216 phosphate-solubilizing bacterial isolates were isolated from different rice rhizospheric soil in Northern Thailand. These isolate were screened in vitro for their plant growth-promoting activities such as solubilization of inorganic phosphate, ammonia (NH₃), catalase and cell wall-degrading enzyme activity. It was found that 100% solubilized inorganic phosphate, 77.77% produced NH₃ and most of the isolates were positive for catalase. In addition, some strains also produced cell wall-degrading enzymes such as protease (7%), chitinase (1%), cellulase (3%) and β-glucanase (3%), as evidenced by phenotypic biochemical test and quantitative assay using spectrophotometry. The isolates could exhibit more than two or three plant growth-promoting (PGP) traits, which may promote plant growth directly or indirectly or synergistically. Part of this study focused on the effect of NaCl, temperature, and pH on a specific the bacterial isolate Acinetobacter CR 1.8. Strain CR 1.8 was able to grow on up to 25% NaCl, between 25 and 55°C, and at pH 5–9. Maximum solubilization of tricalcium phosphate and aluminium phosphate was obtained at neutral pH, and 37°C. Strain CR 1.8 had protease activity but no cellulase, β-glucanase and cellulase activities.     

Isolation of Phosphate Solubilizing Bacteria from Maize Rhizosphere and Their Potential for Rock Phosphate Solubilization–Mineralization and Plant Growth Promotion

Phosphate solubilizing bacteria (PSB) play a significant role in plant P nutrition by their effect on soil P dynamics and their subsequent ability to make P available to plants via solubilization and mineralization processes. This study aimed to evaluate the effect of separate and combined use of indigenous PSB, poultry manure (PM) and compost on solubilization and mineralization of rock phosphate (RP) and their subsequent effect on growth and P accumulation of maize (Zea mays L.). A group of fifty seven bacteria were isolated from the rhizosphere/rhizoplane of maize that had been grown in soils collected from varying altitudes (655–2,576 m) of the mountain region of Rawalakot, Azad Jammu and Kashmir, Pakistan. After screening, the capacity of eleven isolates to solubilize mineral phosphate was quantitatively evaluated using insoluble Ca₃(PO₄)₂ in culture medium as a time course study through spectrometer. The growth hormone producing (IAA) capacity of the isolates was also determined. Furthermore, five potential isolates were tested for their ability to increase P release capacity (mineralization) of insoluble RP in an incubation study. The effect of PSB inoculation on maize was determined in a completely randomized greenhouse experiment where root and shoot biomass and P accumulation in plants were assessed. The P solubilization index of selected isolates varied from 1.94 to 3.69, while the P solubilization efficiency ranged between 94.1% and 269.0%. The isolates MRS18 and MRS27 displayed the highest values. The P solubilization in the liquid medium was maximum at 6 and 9 days of incubation ranging between 9.91 and 44.04 µgmL^?1 and the isolates MRS27 and MRS34 exhibited the highest solubilization. Six isolates showed additional capability of producing IAA ranging between 2.66 and 28.41 µgmL^?1. Results of the incubation study indicated that P release capacity (P mineralization) of RP-amended soil varied between 6.0 and 11.8 µgPg^?1 that had been significantly increased to 30.6–36.3 µgPg^?1 (maximum value) when PSB were combined with RP. The combined application of PSB and organic amendments (PM, compost) with RP further increased P mineralization by releasing a maximum of 37.7 µgPg^?1 compared with separate application of RP (11.8 µgPg^?1) and organic amendments (21.5 and 16.5 µgPg^?1). The overall effect of PSB (as a group) with RP over RP alone on maize growth showing a relative increase in shoot length 21%, shoot fresh weight 42%, shoot dry weight 24%, root length 11%, root fresh weight 59%, root dry weight 35% and chlorophyll content 32%. This study clearly indicates that use of PSB, and organic amendments with insoluble RP could be a promising management strategy to enhance P availability in soil pool and improve plant growth in intensive cropping systems.     

Stabilization of dextransucrase from <Emphasis Type="Italic">Leuconostoc mesenteroides</Emphasis> NRRL B-640

Phosphate solubilizing yeast (PSY) were isolated from rhizosphere, non-rhizosphere and fruits from Bhavnagar district. The potential of 25 yeasts were analyzed on the basis of phosphate solubilizing zone to growth on solid medium denoted as solubilization index (SI) which ranged from 1.10 to 1.50. Among 25 yeast isolates, 6 yeast belonging to genus Saccharomyces (2), Hansenula, Klockera, Rhodotorula and Debaryomyces exhibited highest SI (1.33–1.50) were further examined for in vitro tricalcium phosphate (TCP) and low grade rock phosphate (RP) solubilization. TCP proved superior to RP with all the yeasts. Within low grade RPs tested, except isolate Y5, all isolates showed maximum solubilization with Hirapur RP (HRP) ranging from 7.24 to 19.30 mg% P₂O₅. Among six PSY screened, Debaryomyces hansenii showing maximal HRP solubilization was chosen for further physiological studies. Maximum HRP solubilization was expressed in following condition: pH optima 7.0, temperature optima 28°C and optimal period of incubation were 15 days. Acidic pH of the spent media was a constant feature in all the cases. No correlation could be established between final acidity produced by yeasts and the quantity of phosphate liberated.     

Phosphate solubilization by stress-tolerant soil fungus <Emphasis Type="Italic">Talaromyces funiculosus</Emphasis> SLS8 isolated from the Neem rhizosphere

A promising biotechnological strategy in the management of phosphorus (P) fertilization is the use of phosphate-solubilizing fungi to solubilize rock phosphates and allow the recovery of unavailable P fixed to soil particles. Phosphate-solubilizing rhizosphere fungus, Talaromyces funiculosus SLS8, isolated from Neem (Azadirachta indica) on saline soil, was tolerant to environmental stressors, salinity and agricultural systemic fungicides. Phosphate solubilization under different nutritional conditions was investigated by culturing T. funiculosus SLS8 in Pikovskaya liquid medium containing different nitrogen sources (ammonium sulfate, casein, urea, potassium nitrate or sodium nitrate) and carbon sources (glucose, fructose, galactose or sucrose), NaCl, and three systemic fungicides. The highest concentration of solubilised phosphate (187 mg P L^?1) was achieved after 5 days of incubation in the medium with glucose and ammonium sulphate. The culture pH decreased from 6.5 to 4.2 and HPLC demonstrated organic acid production. Phosphate solubilized was highly negatively correlated with pH (r?=??0.96). Increasing salinity had no effect on phosphate solubilization. The maximum tolerance limits to systemic fungicides carbendazim, mancozeb, and hexaconazole were 12.5 μg mL^?1, 2,000 μg mL^?1 and 250 μl mL^?1 respectively. At these concentrations carbendazim, mancozeb and hexaconazole were found to decrease phosphate solubilization by 55 %, 37 %, and 30 %, respectively. Our results indicate that T. funiculosus SLS8 may be a potential candidate for the development of a biofertilizer for maintaining available phosphate levels in environmentally stressed soils such as saline agricultural soils impacted by systemic fungicide application or seed treatment.     

Isolation and characterization of chromium(VI)-reducing bacteria from tannery effluents and solid wastes

In the present investigation, five novel Cr(VI) reducing bacteria were isolated from tannery effluents and solid wastes and identified as Kosakonia cowanii MKPF2, Klebsiella pneumonia MKPF5, Acinetobacter gerneri MKPF7, Klebsiella variicola MKPF8 and Serratia marcescens MKPF12 by 16S rDNA gene sequence analysis. The maximum tolerance concentration of Cr(VI) as K₂Cr₂O₇ of the bacterial isolates was varying up to 2000 mg/L. Among the investigated bacterial isolates, A. gerneri MKPF7 was best in terms of reduction rate. The optimum temperatures for growth and Cr(VI) reduction by the bacterial isolates were 35 and 40 °C, respectively except A. gerneri MKPF7 which grew and reduced Cr(VI) optimally at 40 °C. The optimum pH for growth and Cr(VI) reduction by K. cowanii MKPF2, A. gerneri MKPF7 and S. marcescens MKPF12 was 7.0 whereas the optimum pH for growth and Cr(VI) reduction by K. pneumoniae MKPF5 and K. variicola MKPF8 were 7.0, 8.0 and 6.0, 7.0, respectively. All the bacterial isolates showed maximum tolerance against Ni²⁺ and Zn²⁺ whereas minimum tolerance was observed against Hg²⁺ and Cd²⁺. The bacteria isolated in the present study thus can be used as eco-friendly biological expedients for the remediation and detoxification of Cr(VI) from the contaminated environments.     

Isolation and Characterization of Coal Solubilizing Aerobic Microorganisms from Salt Range Coal Mines,Pakistan

Microbial solubilization of coal has been considered as a promising technology to convert raw coal into valuable products. In the present study, initially a total of 50 different aerobic bacterial and fungal isolates have been isolated from soil, coal and water samples of Dulmial Coal Mines, Chakwal, Pakistan, but on the basis of solubilization potential, only four isolates were selected for further study. The intensity of biosolubilization was measured by determining the weight loss of the coal pieces, which was observed to be about 25.93% by Pseudomonas sp. AY2, 36.36% by Bacillus sp. AY3 and 50% by Trichoderma sp. AY6, while Phanerochaete sp. AY5 showed maximum coal solubilization potential i.e. 66.67% in 30 days. UV/Vis spectrum revealed an increase in the pattern of absorbance of all treated samples compared to control referring to solubilization. Fourier transform infrared spectroscopy indicated alterations in the structure of treated coal in comparison to control coal suggesting breakdown in the complex structure of coal. The major absorbance bands in infrared spectroscopy for solubilization product were attributed to carbonyl (1,600 cm^?1), hydroxyl (3,450 cm^?1), cyclane (2,925 cm^?1), ether linkage (1,000–1,300 cm^?1), carboxyl (3,300–2,500 cm^?1) and side chains of aromatic ring (1,000–500 cm^?1). The presence of microorganisms and surface erosion of coal residues compared to control samples were observed by scanning electron microscopy, which suggested that isolated microorganisms were able to survive in coal for a longer period of time. Therefore, the present study concluded that microorganisms isolated from coal mines have excellent potential for coal solubilization which is considered as a crucial step in coal methanogenesis allowing them to be used successfully for in situ methane production to meet future energy demands.     

Biotransformation of Heavy Metals from Soil in Synthetic Medium Enriched with Glucose and Shewanella sp. HN-41 at Various pH

The biotransformation of heavy metals from contaminated soil was examined using a facultative anaerobic bacterium Shewanella sp. HN-41. The experiments were carried out to assess the influence of glucose at various pH on the transformation of heavy metals from soil thorough solubilization. A preliminary study on the transformation of heavy metals from soil was first performed using a defined medium supplemented with glucose at 10, 20, and 30 mM to select the effective concentration. Among the three concentrations examined, glucose at 30 mM leached a highest level of metal ions. Therefore, 30 mM glucose was used as the representative carbon source for the subsequent experiments in a defined medium at various pH (5, 6, 7, 8, and 9). The organism HN-41 was not influenced by pH ranging from acidic to neutral and was able to metabolize all the metal elements from contaminated soil. The level of Fe, Cr, As, Mn, Pb, and Al solubilization ranged from 3 to 7664 mg kg^?1 at various initial pH. The rate of metal solubilization was found to be low at neutral pH compared with acidic and alkaline. These results are expected to assist in the development of heavy metal transformation processes for the decontamination of heavy metal-contaminated soil.     

Growth and uptake of caesium,rubidium, and potassium by ectomycorrhizal and saprotrophic fungi grown on either ammonium or nitrate as the N source

After the accident at the Fukushima Dai-ichi Nuclear Power Plant in 2011, high activities of radiocaesium have been reported in wild mushrooms in Japan. Fungi play an important role in the dynamics of radiocaesium in forest ecosystems. We examined the contents of caesium (Cs), rubidium (Rb), and potassium (K) in the mycelium of 15 isolates of ectomycorrhizal (EM) fungi and nine isolates of saprotrophic (SA) fungi in a synthetic medium with either ammonium chloride (NH₄Cl) or sodium nitrate (NaNO₃), supplemented with 1 ppm caesium chloride and rubidium chloride. The mycelia were harvested after 8 weeks of incubation, and the contents of Cs, Rb, and K were measured by inductively coupled plasma mass spectrometry. The dry weight of the mycelium in the medium with NH₄ was significantly higher than that with NO₃, although some EM species, Hebeloma, Astraeus, Scleroderma, and Pisolithus, grew well in the medium with NO₃. Among SA species, Crucibulum and Cyathus grew in the medium with NO₃. The uptakes of Cs, Rb, and K by Suillus, Pisolithus, and Rhizopogon were higher than that in other EM and SA species when they grew on the medium with NH₄, while the uptakes of these elements by Astraeus and Scleroderma were higher than those by other species grown on the medium with NO₃. The content of Rb was positively correlated with Cs (r = 0.85, p < 0.001) and K (r = 0.51, p < 0.001). The accumulation of Cs, Rb, and K was differently affected by the N source and fungal species.     

Bioleaching of Uranium From Egyptian Rocks Using Native Actinomycete Strains

Bioleaching is an economic, novel practice for extraction of metals from their sources by microorganisms. The current study aimed to extract uranium from Egyptian ores using native strains of actinomycetes. Two types of rocks and one ore sample were collected from west-central Sinai, Egypt. Major oxides of the samples and fourteen heavy metals, including uranium, were determined. X-ray diffraction analysis proved that uranium was present in the samples in various structures. Uranium was present in different concentrations, 220, 770, and 550 mg/kg in sandstone, granite, and manganese ore, respectively. Thirty-four actinomycete isolates were recovered from the studied samples using four different isolation media. Acid production capabilities were employed to select isolates for further leaching experiments. Bioleaching experiments were carried out using sterile and non-sterile ore samples. Using sterile ore samples, the highest solubilization percentages of U₃O₈ were 44.5, 38.55, and 16.76% from sandstone, manganese ore, and granite sample, achieved by isolates UA12, UA5, and U7, respectively. Lower solubilization percentages of U₃O₈ were recorded by using non-sterile ore samples. Investigating the factors affecting the bioleaching abilities of the tested organisms revealed that 10 days of incubation with 4% pulp density were the best conditions for U₃O₈ solubilization. The most efficient isolates were identified using 16S rRNA gene sequence analysis. UA12 identified to be Streptomyces bacillaris, while UA5 could not be identified, and U7 was assigned as uncultured bacterium clone. Scanning electron microscope examination of the bioleaching experiment showed different growth intensity within the active isolates. For larger-scale extraction purposes, a kilogram of sandstone, containing 220 mg of U₃O₈, was used in the form of a truncated cone in a heap leaching experiment. After 20 cycles, 14.72 mg/l (6.7%) of U₃O₈ was leached by S. bacillaris, while 19.36 mg/l (8.8%) of U₃O₈ was leached by chemical leaching using sulfuric acid. The results of this study prove that the extraction of uranium using actinomycetes could be exploited as less polluting, more economical, and more effective than traditional chemical extraction especially from low-grade ores or mining wastes.     

Biological nitrogen fixation and phosphate solubilization by bacteria isolated from tropical soils

Introduction

In addition to fixing atmospheric nitrogen, some bacterial isolates can also solubilize insoluble phosphates, further contributing to plant growth.

Aims

The objectives of this study were the following: isolate, select, and identify nodulating bacteria in the cowpea that are efficient not only in biological nitrogen fixation (BNF) but also in the solubilization of insoluble inorganic phosphates; identify and quantify the organic acids produced; and establish the relationship between those acids and the solubilizing capacity.

Methods

The bacteria were captured from two soils containing high concentrations of insoluble phosphorus from the cities of Lavras and Patos de Minas, using the cowpea [Vigna unguiculata (L.) Walp.] as bait. We obtained 78 strains, which were characterized according to their cultural attributes in culture medium 79 with the strains UFLA 03-84, INPA 03-11B, and BR3267 (approved by the Ministry of Livestock and Supply Agriculture—MAPA, as inoculants for the cowpea) and Burkholderia cepacia (LMG1222^T), which was used as a positive control for phosphate solubilization. Strains that were selected for their efficiency in both processes were identified by 16S rDNA sequence analysis. We evaluated the symbiotic efficiency (BNF) in a greenhouse and the solubilization efficiency of CaHPO₄, Al(H₂PO₄)₃, and FePO₄.2H₂O in solid and liquid GELP media. Strains that excelled at the solubilization of these phosphate sources were also evaluated for the production of the following organic acids: oxalic, citric, gluconic, lactic, succinic, and propionic.

Results

The presence of Acinetobacter, Bacillus, Firmicutes, Microbacterium, Paenibacillus, and Rhizobium was detected by 16S rDNA sequencing and analysis. Bacterial strains obtained from cowpea nodules varied greatly in the efficiency of their BNF and phosphate solubilization processes, especially in the strains UFLA 03-09, UFLA 03-10, UFLA 03-12, and UFLA 03-13, which were more efficient in both processes. More strains were able to solubilize insoluble inorganic calcium and iron phosphates in liquid medium than in solid medium. The production of organic acids was related to the solubilization of CaHPO₄ and FePO₄.2H₂O for some strains, and the type and concentration of the acid influenced this process.

Conclusions

These are the first results obtained with bacterial isolates from tropical soils in which the production of organic acids was detected and quantified to examine the solubilization of insoluble inorganic phosphates.     

Selection of phosphate-solubilizing diazotrophic Herbaspirillum and Burkholderia strains and their effect on rice crop yield and nutrient uptake

Background and Aims

Plant growth-promoting bacteria, mainly diazotrophs and phosphate solubilizers, can reduce the use of chemical fertilizers for rice crops. Here, diazotrophic bacteria isolated from rice were screened for their ability to solubilize inorganic P (P_i) in vitro and in association with rice plants cultivated in pots.

Methods

Forty-nine isolates were tested for the ability to solubilize P_i on NBRIP and GL agar plate media and seven selected strains were further evaluated in NBRIP liquid medium. Three of these strains were inoculated in rice plants grown in soil pots containing ¹⁵N-labeled fertilizer and two sources of P: tricalcium phosphate (TCP) or simple superphosphate (SSP). The dry matter, yield, N, P, and the ¹⁵N content accumulated in plant tissues were measured at 135 days after planting.

Results

Seven strains belonging to the genera Herbaspirillum and Burkholderia formed a halo of solubilized P_i on agar plates. The Burkholderia strains showed peak soluble P (around 200 mg P L^?1) on the fifth day when grown in NBRIP liquid medium for 14 days. Inoculation of Herbaspirillum strains (H18, ZA15) and a Burkholderia vietaminensis strain (AR114) increased rice grain yield from 33 to 47 % with TCP and 18 to 44 % with TSS, respectively. The bacterial inoculation led to enhanced N-use efficiency of the ¹⁵N-labeled fertilizer.

Conclusion

These results suggest that the selection and use of P-solubilizing diazotrophic bacteria are a good strategy to promote P solubilization and/or N use efficiency in rice plants.     

Colonization and Biodegradation of Photo-Oxidized Low-Density Polyethylene (LDPE) by New Strains of Aspergillus sp. and Lysinibacillus sp.

The primary objective of this study was the isolation of low-density polyethylene (LDPE)-degrading microorganisms. Soil samples were obtained from an aged municipal landfill in Tehran, Iran, and enrichment culture procedures were performed using LDPE films and powder. Screening steps were conducted using linear paraffin, liquid ethylene oligomer, and LDPE powder as the sole source of carbon. Two landfill-source isolates, identified as Lysinibacillus xylanilyticus XDB9 (T) strain S7-10F and Aspergillus niger strain F1-16S, were selected as super strains. Photo-oxidation (25 days under ultraviolet [UV] irradiation) was used as a pretreatment of the LDPE samples without pro-oxidant additives. The PE biodegradation process was performed for 56 days in a liquid mineral medium using UV-irradiated pure LDPE films without pro-oxidant additives in the presence of the bacterial isolate, the fungal isolate, and the mixture of the two isolates. The process was monitored by measuring the fungal biomass, the bacterial growth, and the pH of the medium. During the process, the fungal biomass and the bacterial growth increased, and the pH of the medium decreased, which suggests the utilization of the preoxidized PE by the selected isolates as the sole source of carbon. Carbonyl and double bond indices exhibited the highest amount of decrement and increment, respectively, in the presence of the fungal isolate, and the lowest indices were obtained from the treatment of a mixture of both fungal and bacterial isolates. Fourier transform infrared (FT-IR), x-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses showed that the selected isolates modified and colonized preoxidized pure LDPE films without pro-oxidant additives.     

Molecular identification and in vitro screening of antagonistic bacteria from agricultural byproduct compost: Effect of compost on development and photosynthetic efficiency of tomato plant

The dynamics of mesophilic and thermophilic bacterial population of compost was studied. The bacteria population in the compost ranged from 10⁹ to 10⁵ CFU g^?1 and was found to be maximum during mesophilic phase, and then decreased during the thermophilic, the cooling and maturation phases. Assessment of culturable bacteria by 16S rDNA revealed phylogenetic lineage of different polymorphic class bacilli, γ, β-proteobacteria and actinobacteria. Bacterial isolates produced extracellular enzymes: proteases, cellulase, xylanase, pectinase, tannase and amylase. Among them, mesophilic bacteria exhibited xylanolytic (81.25 %) and cellulolytic (63 %) activity. Thermophilic bacteria showed cellulolytic (75 %) and xylanolytic (66.6 %) activity, but a few isolates also produced tannase and pectinase. All bacterial isolates were observed to cause inhibition of three isolates of Bacillus pumilus and one isolate each of Staphylococcus sciuri and Kocuria sp. The physiological effect of compost on shoot length, leaf size and fruit maturation of tomato have been evaluated; the compost (75 g/pot) improved these parameters as compared to known compost (SOM). The efficacy of compost and SOM on photochemistry of tomato leaves was studied, based on imaging-PAM of the chlorophyll fluorescence parameters. F_v/F_m and electron transport rate (ETR) were increased significantly in compost (75 g) amended pot within 30 days of growth. Likewise, highest Y (II) of photosystem II (PS II) yield was found in compost (75 g) pot in 15 days. The findings of this study proved that the compost comprising of various bacteria involved in degradation of substrates was found to be beneficial for enhancement of tomato growth and development.