Tunisian Soil Research Articles

Application of structural equation modeling for assessing relationships between organic carbon and soil properties in semiarid Mediterranean region

Restoration of atmospheric carbon in soils has principal many good effects. Arid lands cover more than 40 % of the global earth area, but only stock 16 % from the global carbon stock. It seems to be a suitable solution for this environmental issue, but still all variables controlling organic carbon in such kinds of soil, was ignored. This study aims to develop two models of organic carbon under clayey and sandy soils in semi-arid Mediterranean zones basing on physical and chemical soil properties. For establishing both models, structural equation modeling was used. For modeling organic carbon, two Tunisian soil databases composed from clayey and sandy soils made respectively, of 450 and 602 soil horizons were used. Using the two databases for all properties, the principal component analysis shows two components for clayey soil; (i) chemical properties and bulk density and (ii) physical properties. For the sandy soil it reveals two components; (i) chemical properties and (ii) physical properties. According to the derived components for each soil category, two models have been built. Structural equation modeling results show that clayey model has proved that organic carbon was controlled by chemical properties and bulk density more than physical properties and sandy model has proved that organic carbon was controlled by chemical properties more than physical properties. The root mean square errors of approximation were 0.079 and 0.050 for the clayey and sandy models, respectively. Then these two models were validated with two other databases from Tunisian dryland soils.

A mineralogical identification of a Tunisian clayey soil and fabric changes during wetting

AbstractA clayey soil from southern Tunisia has been studied in order to assess its possible use as an anti-pollution barrier. The soil has been characterized from both geotechnical and physicochemical points of view. The physicochemical characterization was carried out using X-ray diffraction, thermogravimetric and differential thermal analyses, specific surface area and cation exchange capacity measurements. Hydration tests were carried out using oedometers to determine the swelling behaviour of compacted specimens under different stress conditions. The large swelling pressure (420 kPa) is in agreement with the mainly smectitic character of the soil highlighted by the physicochemical tests. The changes in the fabric of the soil and in its porosity after the hydration tests were investigated using the same physicochemical methods, and related to the macroscopic hydro-mechanical properties. X-ray diffraction and mercury intrusion measurements show that there are changes in the interlayer spacing, particle size and inter-aggregate pore distribution. Comparing the properties of the soil with the Belgian guidelines for the construction of anti-pollution barriers showed that the material was suitable for that use.

Early stages of copper corrosion behaviour in a Tunisian soil

AbstractThe early stages of copper corrosion in a Tunisian soil were studied using mass loss, surface analysis (optical microscopy, visible spectroscopy, IRTF and atomic force microscopy) and electrochemical characterizations (polarization curves and cyclic voltammetry). The corrosion rate dependence with immersion time was Δm = atb. Two behaviours for the material surface were evidenced when varying the soil concentration and temperature. Then, the apparent kinetic constant, the soil reaction order and the apparent activation energy were calculated. Analytical and electrochemical characterizations showed that a rough patina layer was build up as a result of the interaction between soil and copper.

Chemical Changes in Soil and Plants After Successive Amendments of Tunisian MSW Compost

A field study was carried out to evaluate long-term heavy metal (HM) accumulation in the top 20 cm of a Tunisian clayey loam soil amended for (four years) with municipal solid waste compost (MSWC) at three levels (0, 40 and 80 t ha−1 year−1). HM uptake and translocation within wheat plants grown on these soils were also investigated. Compared to untreated soils, MSWC-amended soils showed significant increases in the content of all measured HM (Cd, Cr, Cu, Ni, Pb and Zn) in the last three years, especially for the 80 t ha−1 year−1 MSWC-amended plots. Wheat plants grown on MSWC-amended soils showed a general increase in metal uptake and translocation, especially for Cr and Ni. This HM uptake was about three fold greater for treatment 80 t h−1 as compared to plots amended at a rate of 40 t h−1. At times, the diluting effect resulting from enhanced growth rates of the plants with compost application resulted in lower concentrations in the plants grown (grain part) on treated plots. On the other hand, Cr and Ni were less mobile in the aerial part of wheat plants and were accumulated essentially in root tissues. Plant/soil transfer coefficients for MSWC-amended treatments were higher than threshold range reported in the literature, indicating that there was an important load/transfer of HM ions from soils to wheat plants.

Improvement of Highly Thermostable Xylanases Production by Talaromyces thermophilus for the Agro-industrials Residue Hydrolysis

A newly isolated thermophilic fungal strain from Tunisian soil samples was identified as Talaromyces thermophilus and was selected for its ability to produce extracellular hemicellulases when grown on various lignocellulosic substrates. Following the optimization of carbon source, nitrogen source, and initial pH of the growth medium in submerged liquid cultures, yields as high as 10.00 +/- 0.15 and 0.21 +/- 0.02 U/ml were obtained for xylanase and beta-xylosidase, respectively. In fact, wheat bran was found to be a good inducer of hemicellulase enzymes, mainly beta-xylosidase. The optimal temperature and pH of the xylanase activity were 75 degrees C and 8.0, respectively. This enzyme exhibited a remarkable stability and retained 100% of its original activity at 50 degrees C for 7 days at pH 7.0-8.0. The half-lives of the enzyme were 4 h at 80 degrees C, 2 h at 90 degrees C, and 1 h at 100 degrees C. T. thermophilus could therefore be considered as a satisfactory and promising producer of thermostable xylanases. Crude enzyme of T. thermophilus rich in xylanase and beta-xylosidase was established for the hydrolysis of lignocellulosic materials as wheat bran.

Enhancement of oxytetracycline production after gamma irradiation-induced mutagenesis of Streptomyces rimosus CN08 strain

Streptomyces rimosus CN08 isolated from Tunisian soil produced 8.6mgl(-1) of oxytetracycline (OTC) under submerged fermentation (SmF). Attempts were made for enhancing OTC production after irradiation-induced mutagenesis of Streptomyces rimosus CN08 with Co(60)-γ rays. 125 OTC-producing colonies were obtained after screening on kanamycin containing medium. One mutant called Streptomyces rimosus γ-45 whose OTC production increased 19-fold (165mgl(-1)) versus wild-type strain was selected. γ-45 mutant was used for OTC production under solid-state fermentation (SSF). Wheat bran (WB) was used as solid substrate and process parameters influencing OTC production were optimized. Solid-state fermentation increased the yield of antibiotic production (257mgg(-1)) when compared with submerged fermentation. Ammonium sulphate as additional nitrogen source enhanced OTC level to 298mgg(-1). Interestingly, OTC production by γ-45 mutant was insensitive to phosphate which opens the way to high OTC production even in medium containing phosphate necessary for optimal mycelia growth.

Isolation, identification and characterization of a new lipolytic Pseudomonas sp., strain AHD‐1, from Tunisian soil

A novel, lipid‐degrading bacterium (strain AHD‐1) was isolated from soil regularly contaminated with washing‐machine wastewater in Sfax, Tunisia. When this strain was grown in a medium containing 2% triacylglycerol, the hydrolysis products were found to be diacylglycerols, monoacylglycerols and free fatty acids. This strain was an aerobic, mesophilic, Gram‐negative, motile, non‐sporulating bacterium, capable of growing optimally at pH 7 and 27 °C. The predominant fatty acids were found to be C16:1ω7c (31%), C16:0 (28.1%), C18:1ω7c (16.3%) and C17:0 (5.8%). Phylogenetic analysis of the 16S rRNA gene showed that this isolate is a new strain belonging to the genus Pseudomonas. Strain AHD‐1 was found to be closely related to Pseudomonas azotoformans IAM 1603T, Pseudomonas gessardii CIP 105469T and Pseudomonas libanensis CIP 105460T with 99.7%, 99.56% and 99.54% of similarity, respectively.

Improvement of Bacillus thuringiensis Bacteriocin Production Through Culture Conditions Optimization

BUPM4 is a Bacillus thuringiensis subsp. kurstaki strain, isolated from Tunisian soil, producing an original bacteriocin named Bacthuricin F4. The optimization of the latter production conditions was carried out under several physicochemical conditions. It was found that the highest bacteriocin activity was reached at low aeration while bacteriocin synthesis yields were strongly reduced at higher ones. A balance between growth and bacteriocin synthesis, both highly dependent on aeration, was taken into account for the overproduction of bacteriocin. Both glucose and glycerol were shown to be necessary for Bacthuricin F4 maximal synthesis. In addition, the optimal carbon/nitrogen ratio for bacteriocin production is 9. In such optimal conditions, more than 4-fold greater bacteriocin production was obtained than when using TSB medium.

A new Tunisian strain of Bacillus thuringiensis kurstaki having high insecticidal activity and δ-endotoxin yield

BLB1 is a new Bacillus thuringiensis kurstaki strain, isolated from a Tunisian soil sample. Assay of toxicity of BLB1 crystal proteins resulted in an LC50 of 70.32 ng of toxin per mg of flour against third instar Ephestia kuehniella with confidence limits of (31.6-109.04 ng). This LC50 is less than that of the commercial strains HD1 used as a reference. The characterization of this strain by scanning transmission electron microscopy, analysis of its cry genes content by PCR-sequencing, and analysis of its delta-endotoxin patterns demonstrate that it belongs to the same subgroup than HD1, but ruled out the involvement of cry gene content or protoxin activation in the hypertoxicity of this strain. Taking into account the delta-endotoxin/spore ratio for each strain, and by allowing the estimation of the production level per spore, it might be concluded that BLB1 production is the highest, when compared with that of HD1. On the basis of its toxicity, BLB1 could be considered as a strain of great interest and would allow the production of quantities of bioinsecticides at low cost.

Effects of 5-year application of municipal solid waste compost on the distribution and mobility of heavy metals in a Tunisian calcareous soil

It is obvious that the application of solid waste compost improves the soil fertility. These wastes, however, may also have some negative effects on the agricultural environment due to their metal content. This research aimed at evaluating the influence of Tunisian municipal solid waste compost and farmyard manure on some chemical properties and the distribution of heavy metals in a calcareous Tunisian soil (clayey–loamy soil). A field plot experiment, without vegetation, was installed since 1999 at the experimental farm of the Agronomic National Institute of Tunis (INAT) in the region of Mornag (20 km south of Tunis, Tunisia). During 5 years, the field received yearly the following treatments: 0, 40, 80 and 120 t/ha of municipal solid waste compost and 0, 40 and 120 t/ha of manure. The fractionation of heavy metals in the soil was evaluated after 5 years using a sequential extraction procedure. The application of the two amendments was found to increase the content of organic matter, the total nitrogen content and the electrical conductivity, whereas it slightly decreased the soil pH. The addition of manure did not have a significant effect on the accumulation of heavy metals in the soil, whereas compost application increased the total concentration of heavy metals in the soil. The distribution of heavy metals between the different fractions in untreated and treated soils showed the residual fraction to be dominant, followed by the fraction bound to Fe and Mn oxides. The amount of Cu bound to the organic fraction increased with the application rate, which is probably caused by the formation of organic complexes. For the other metals, the increase of the association with organic matter is very limited. The application of compost moreover increases the amount of Zn associated with Fe and Mn oxides. The “Mobility Factor (MF)” was quite low and did not change after the 5-year application of the two organic amendments. It always remained lower than 10%, although for Cd it amounted to 17%.

Genetic characterisation and symbiotic proprieties of native sinorhizobia trapped by Medicago sativa on Tunisian soils

Medicago sativa, a perennial species of the genus Medicago, is particularly cultivated in the Tunisian oases. Three M. sativa cultivated varieties (Magali A, Gabès2355, ABT805) and 1 genotype from a local provenance (Widhref) were cultivated on 4 Tunisian soil samples exhibiting contrasting pedo-climatic characteristics. Results showed that nodulation and aerial growth were significantly dependent on both variety and soil factors. Magali A showed the highest number of nodules, and varieties grown in Amra and Mateur soils produced the largest nodulation and aerial growth. A collection of 250 rhizobial isolates was obtained from the 4 soils by trapping them on M. sativa. Molecular characterisation by PCR/RFLP of 16S genes showed that almost all isolates (158 of 160 isolates) belong to Sinorhizobium meliloti. The subsequent analysis of isolates from 2 contrasting soils (Mateur, in the north of Tunisia, and Rgim Maatoug, in the south) demonstrated that strains harbour polymorphism at symbiotic genes, with 4 nodC genotypes, 2 nifDK genotypes and 2 nodA genotypes. The 2 soils contained different symbiont genotypes. The distribution of various profiles types was different according to geographic sites. The soils of Mateur and Rgim Maatoug do not contain the same genotypes. Eleven representative strains from this collection were evaluated for their nodulation and nitrogen fixation capacities. Results showed a high degree of variability among strains for nodulation and aerial growth parameters, and only 2 strains, GII/M1 and LII/M1, seem to be effective with all 4 varieties of M. sativa tested. Strains isolated from the soil of northern Tunisia were more efficient and produced fewer nodules than those trapped on southern soil.

Production of Anti-Methicillin-Resistant Staphylococcus Activity from Bacillus subtilis sp. Strain B38 Newly Isolated from Soil

B38 bacterial strain, isolated from Tunisian soil showed a strong antimicrobial activity. Based on biochemical characterization and 16S rDNA sequence analysis, B38 strain was identified as Bacillus subtilis. Cell culture supernatant showed antibacterial activity against clinical isolates of methicillin-resistant Staphylococcus species and several Gram-positive and Gram-negative bacteria. Antifungal activity against phytopathogenic fungi was also observed. Antibacterial activity production started at early exponential growth phase, and maximum activity was reached at the stationary phase. This antibacterial activity was neither affected by proteases, lipase, and organic solvents, nor by surfactants. It was stable over a wide pH range and still active after autoclaving at 121 degrees C during 20 min. Thin layer chromatography followed by bioautography assay allowed the detection of four active spots with R(f) values of 0.30, 0.47, 0.70, and 0.82. The single spot with R (f) 0.30 showed antifungal activity, whereas the spots with R(f) values of 0.47, 0.70, and 0.82 exhibited antibacterial activity.

New Thermostable Amylase from Bacillus cohnii US147 with a Broad pH Applicability

A new thermophilic bacterial strain identified as Bacillus cohnii US147 was isolated from the southern Tunisian soil. The identification was based on physiological tests and molecular techniques related to the 16S ribosomal ribonucleic acid. The isolated strain produced amylase, which was purified. This amylase had an apparent molecular mass of 30 kDa as estimated by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis. Amylase US147 showed K (m) and V (max) values of 0.7 mg/ml and 2.2 U/ml, respectively, with starch as the substrate. The enzyme was active in acid and basic pH and had a maximal activity on starch at pH 9 and 70 degrees C. The enzyme was stable at pH 9 for 72 h and retained half of its activity after incubation at 70 degrees C for 150 min. A partially inhibition (15%, 25%, 23%, 20%, and 22%) was obtained with 1 mM SDS, 1 mM NaBO(3), 1 mM H(2)O(2,) 1 mM Zn(+2), and 5 mM ethylenediamine tetraacetic acid (EDTA), respectively. The amylase recovered its original activity by the addition of 10 mM Ca (2+) to the 5 mM EDTA. These properties indicated a possible use of this amylase in starch saccharification, in detergent, and in other industrial applications.

Interaction of aqueous acidic-fluoride waste with natural tunisian soil

AbstractClayey soils are essential materials used to reduce hydraulic conductivity and pollutant migration, common at sites of waste disposal. This study investigates the possible use of a Tunisian soil as a lining material for disposal sites for acidic-fluoride wastes. A permeability test on a waste-solution sample (pH = 2.7) obtained from a disposal site in southern Tunisia was conducted over a period of about 2 years. The test results show that the permeability decreased with time until stabilized at 8.33 × 10−11 m/s. After the permeability test, the samples retrieved from the permeameter show a degradation state which varied with the thickness of the specimen. These samples can be classified into three zones (Z1: unaffected, Z2: moderately affected; and Z3: extensively affected). Physicochemical characterization of the three samples (Z1, Z2, and Z3), and of the original argillaceous soil, was by X-ray diffraction, Fourier transform infrared spectroscopy, differential thermal and thermal gravimetric analysis, 29Si and 27Al nuclear magnetic resonance, and N2-adsorption techniques. The original sample consists essentially of palygorskite, kaolinite, and quartz. Sample Z3 underwent complete dissolution of kaolinite which supports the precipitation of fluoroaluminate and the appearance of an X-ray amorphous silica phase. In samples Z1 and Z2, the soil adsorbs fluoride at a rate of ∼68.5 mg/g and is highly resistant to acidic attack.

Phenotypic and genotypic diversity of Genista saharae microsymbionts from the infra-arid region of Tunisia

Genista saharae, indigenous of Sahara, is a spontaneous shrub that plays an important ecological role for the preservation and fertility of poor and eroded soils. This legume has not been examined for its root nodule bacteria. The taxonomic diversity of bacteria from root nodules of G. saharae growing in the infra-arid region of Tunisia was investigated. A total of 28 bacterial strains isolated from root nodules of G. saharae grown in Tunisian soil were characterized using a polyphasic approach including phenotypic characteristics, PCR-RFLP of 16S rDNA and 16S rRNA gene sequencing. It was found that new isolates are diverse and affiliated to Ensifer (75%), Rhizobium (10%) and Phyllobacterium (15%). The Phyllobacterium isolates lacked the capacity for nodule formation on this plant. Genista saharae formed nodules with diverse rhizobia in Tunisian soils. Furthermore, our results support the presence of non-nodulating commensal strains (Phyllobacterium) in legumes nodule. This study is the first report on the characterization of G. saharae microsymbionts in Tunisia.

Occurrence and diversity of Frankia in Tunisian soil

There is a lack of studies on the occurrence and diversity of Frankia in African soils, including those in northern African regions. The present study on Tunisian soils is an attempt to address this issue using Alnus glutinosa, Elaeagnus angustifolia and Casuarina glauca in a plant capturing bioassay on 30 soil samples, followed by amplified 16S ribosomal DNA restriction pattern analysis (ARDRA). A total of seven ARDRA haplotypes of Frankia have been detected in root actinorhizas that have been affiliated to theoretical ARDRA haplotypes upon in silico digestion of selected 16S ribosomal RNA (rRNA) gene sequences retrieved from GeneBank and confirmed by their partial 16S rRNA gene sequencing. Elaeagnus‐compatible Frankia isolates were widespread and form four ARDRA haplotypes affiliated to Frankia, colonizing Elaeagnaceae and Rhamnaceae in two different phylogenetic subgroups. Alnus‐compatible strains occurring in northern subhumid area were closely related to Alnus–Morella‐compatible strains and clustered in two ARDRA haplotypes. Casuarina‐compatible strains lack variability in several northern arboreta. The relatively wide diversity of Tunisian Frankia strains opens the perspective that African soil could be an interesting reservoir for the isolation of new actinorhizal strains that could be used as potential biofertilizers to counteract the progressive soil desertification which indeed is a crucial environmental problem in Northern Africa.

Comparison of rhizobia that nodulate Medicago laciniata and Medicago truncatula present in a single Tunisian arid soil

The rhizobia present in a single arid region Tunisian soil that nodulate Medicago laciniata and Medicago truncatula were compared. All isolates, 40 from each host, were Sinorhizobium meliloti based on 16S rRNA polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) patterns and subsequent confirmation by sequence analysis of the 16S rRNA genes in four representatives from each host species. There was no apparent relationship between Medicago host species of isolation and the nodulating rhizobial genome as determined by repetitive extragenic palandromic PCR. The isolates of M. laciniata were distinguished from those of M. truncatula present in the same soil by variation in PCR-RFLP of nifDK, indicating that this dissimilarity is originally genetic and not geographic. While forming effective symbioses with their own respective isolates, both M. laciniata and M. truncatula formed ineffective true nodules, nodule-like structures, or no nodules at all in cross-inoculation tests, as confirmed by the histological observations.

Targeted Gene Disruption of the Cyclo (L-Phe, L-Pro) Biosynthetic Pathway inStreptomycessp. US24 Strain

We have previously isolated a new actinomycete strain from Tunisian soil called Streptomyces sp. US24, and have shown that it produces two bioactive molecules including a Cyclo (L-Phe, L-Pro) diketopiperazine (DKP). To identify the structural genes responsible for the synthesis of this DKP derivative, a PCR amplification (696 bp) was carried out using the Streptomyces sp. US24 genomic DNA as template and two degenerate oligonucleotides designed by analogy with genes encoding peptide synthetases (NRPS). The detection of DKP derivative biosynthetic pathway of the Streptomyces sp. US24 strain was then achieved by gene disruption via homologous recombination using a suicide vector derived from the conjugative plasmid pSET152 and containing the PCR product. Chromatography analysis, biological tests and spectroscopic studies of supernatant cultures of the wild-type Streptomyces sp. US24 strain and three mutants obtained by this gene targeting disruption approach showed that the amplified DNA fragment is required for Cyclo (L-Phe, L-Pro) biosynthesis in Streptomyces sp. US24 strain. This DKP derivative seems to be produced either directly via a nonribosomal pathway or as a side product in the course of nonribosomal synthesis of a longer peptide.

A β-cyclodextrin glycosyltransferase from a newly isolated Paenibacillus pabuli US132 strain: Purification, properties and potential use in bread-making

A bacterial strain designed US132, isolated from a Tunisian soil was selected for its production of a potent cyclodextrin glycosyltransferase (CGTase) activity. This strain was identified as Paenibacillus pabuli by sequencing of the 16S rDNA and the 16S-23S internal transcribed spacer (ITS). The US132 CGTase, purified to homogeneity by hydrophobic interaction chromatography and starch adsorption, is a monomer of approximately 70 kDa. This enzyme exhibited a maximal activity at 65 °C, in presence of 10 mM calcium, and was most active at pH range 5.5–9 with an optimum at 6.5. Using 10% (w/v) of potato starch, this CGTase produced a high level of cyclodextrins reaching 42 g/l with a β-cyclodextrin ratio of 63%. Furthermore, this enzyme can be used in the bread-baking process since its addition in the dough mix improved significantly the loaf volume and decreased the firmness of bread during storage.

Genetic diversity among Elaeagnus compatible Frankia strains and sympatric-related nitrogen-fixing actinobacteria revealed by nifH sequence analysis

Elaeagnus compatible Frankia isolates from Tunisian soil have been previously clustered with Frankia, colonizing Elaeagnaceae and Rhamnaceae in two different phylogenetic subgroups, while strain BMG5.6 was described as a new lineage closely related to Frankia and Micromonospora genera. In this study we further assess the diversity of captured Frankia and the relationship with BMG5.6-like actinobacteria, by using nifH gene sequences. Using PCR-RFLP screening on DNA extracted from lobe nodules, additional microsymbionts sharing BMG5.6 features have been detected proving a widespread occurrence of these actinobacteria in Elaeagnus root nodules. Neighbour-Joining trees of Frankia nifH sequences were consistent with previously published 16S rRNA and GlnII phylogenetic trees. Although four main clades could be discerned, actinobacterial strain BMG5.6 was clustered with Frankia strains isolated from Elaeagnus. The present study underscored the emanation of new diazotrophic taxon isolated from actinorhizal nodules occupying intermediate taxonomic position between Frankia and Micromonospora. Moreover, its aberrant position in nifH phylogeny should open network investigations on the natural history of nitrogen-fixing gene among actinobacteria.