Bacteria immobilisation on hydroxyapatite surface for heavy metals removal.

Selected bacterial strains were immobilised on the surface of hydroxyapatite (Ca10(PO4)6(OH)2 - HAp) of natural origin (fish bones). The capacity of the material, alone and in combination with the bacterial strains to act as heavy metal removers from aqueous streams was assessed. Pseudomonas fluorescens (S3X), Microbacterium oxydans (EC29) and Cupriavidus sp. (1C2) were chosen based on their resistance to heavy metals and capacity of adsorbing the metals. These systems were tested using solutions of Zn(II), Cd(II) and in solutions containing both metals. A synergistic effect between the strains and HAp, which is effective in removing the target heavy metals on its own, was observed, as the combination of HAp with the bacterial strains led to higher adsorption capacity for both elements. For the solutions containing only one metal the synergistic effect was greater for higher metal concentrations; 1C2 and EC29 were the most effective strains for Zn(II) and Cd(II) respectively, while S3X was less effective. Overall, an almost four-fold increase was observed for the maximum adsorption capacity for Zn(II) when 1C2 was employed - 0.433 mmol/g in comparison of 0.121 mmol/g for the unmodified HAp. For Cd(II), on the other hand, an almost three-fold increase was registered with EC29 bacterial strain - 0.090 vs 0.036 mmol/g for the unmodified HAp. When the solutions containing both metals were tested, the effect was more marked for lower concentrations.

Synergistic effect of organo-mineral amendments and plant growth-promoting rhizobacteria (PGPR) on the establishment of vegetation cover and amelioration of mine tailings.

Mine tailings pose a huge hazard for environmental and human health, and the establishment of vegetation cover is crucial to reduce pollutant dispersion for the surroundings. However, their hostile physicochemical conditions hamper plant growth, compromising phytoremediation strategies. This study aims to investigate the role of organo-mineral amendments and plant growth-promoting rhizobacteria (PGPR) on the improvement of mine tailings properties and Lolium perenne L. (ryegrass) growth. Plants were grown in mine tailings mixed with an agricultural soil (1:1), 10% compost, and supplied with two different inorganic amendments - rock phosphate (6%) or lime (3%), and inoculated with the rhizobacterial strains Advenellakashmirensis BKM20 (B1) and Mesorhizobium tamadayense BKM04 (B2). The application of organo-mineral amendments ameliorated tailings characteristics, which fostered plant growth and further enhanced soil fertility and microbial activity. These findings were consistent with the increase of total organic carbon levels, with the higher numbers of heterotrophic and phosphate solubilizing bacteria, and higher dehydrogenase and urease activities, found in these substrates after plant establishment. Plant growth was further boosted by PGPR inoculation, most noticeable by co-inoculation of both strains. Moreover, inoculated plants showed increased activities for several antioxidant enzymes (catalase, peroxidase, polyphenoloxidase, and glutathione reductase) which indicate a reinforced antioxidant system. The application of agricultural soil, compost and lime associated with the inoculation of a mixture of PGPR proved to enhance the establishment of vegetation cover, thus promoting the stabilization of Kettara mine tailings. Nonetheless, further studies are needed in order to confirm its effectiveness under field conditions.

Plant growth-promoting rhizobacteria (PGPR) improve the growth and nutrient use efficiency in maize (Zea mays L.) under water deficit conditions.

Drought is one of the major abiotic stresses that affects crop yield worldwide. An eco-friendly tool that can broadly improve plants' tolerance to water stress is bioionocula comprising plant growth-promoting rhizobacteria (PGPR). In this study, the effect of two PGPR Cupriavidus necator 1C2 (B1) and Pseudomonas fluorescens S3X (B2), singly and/or co-inoculated at two inocula sizes (S1 - 3 × 103 cells g-1 dry weight (dw) soil and S2 - 3 × 106 cells g-1 dw soil), on growth, nutrient uptake, and use efficiency was assessed in maize (Zea mays L.) plants grown at three levels of irrigation (80% of water holding capacity (WHC) - well-watered, 60% of WHC - moderate water deficit stress, and 40% of WHC - severe water deficit stress) in a greenhouse experiment. The impact of water deficit and bioinoculants on soil microbial activity (fluorescein diacetate hydrolysis) was also evaluated. Moderate and severe water deficit negatively affected soil microbial activity, as well as, maize growth, by reducing plants' shoot biomass and increasing root/shoot ratio at 60 and 40% of WHC. Bioinoculants mitigated the negative effects on shoot biomass, especially when PGPR were co-inoculated, increasing up to 89% the aerial biomass of plants exposed to moderate water deficit. Bioinoculation also increased nitrogen (N) and phosphorous (P) use efficiency, which may have led to higher maize growth under water deficit conditions. The size of the inocula applied had marginal influence on biometric and nutrient parameters, although the higher concentration of the mixture of PGPR was the most effective in improving shoot biomass under moderate water deficit. This study shows that rhizobacterial strains are able to increase nutrient use efficiency and to alleviate water stress effects in crops with high water demands and have potential applications to keep up with productivity in water stress scenarios.

Assessment of plant growth promoting bacterial populations in the rhizosphere of metallophytes from the Kettara mine, Marrakech.

Soil heavy metal contamination resulting from mining activities constitutes a major environmental problem worldwide. The spread of heavy metals is often facilitated by scarce vegetation cover, so there is an urgent need to improve plant survival and establishment in these metalliferous areas. This study is aimed at the isolation and analysis of the phylogenetic relationship of culturable bacteria from the rhizosphere of metallophyte plants growing in the Kettara mine, in Marrakech, in order to select plant growth-promoting rhizobacteria (PGPR), which could be used in assisted-phytoremediation. Bacterial isolates were grouped by random amplified polymorphic DNA analysis and identified by 16S rRNA gene sequencing. Strains were further characterized for the production of plant growth-promoting (PGP) substances, such as NH3, siderophores, indol-3-acetic acid (IAA), hydrogen cyanide, and extracellular enzymes, for ACC-deaminase activity, their capacity to solubilize phosphate, and for their tolerance to heavy metals and acidic pH. Rhizosphere soils were highly contaminated with Cu and Zn and presented low fertility. Phylogenetic analysis showed that the rhizobacteria were affiliated to three major groups: γ-Proteobacteria (48 %), ß-Proteobacteria (17 %), and Bacilli (17 %). The most represented genera were Pseudomonas (38 %), Bacillus (10 %), Streptomyces (10 %), and Tetrathiobacter (10 %). Overall, rhizobacterial strains showed an ability to produce multiple, important PGP traits, which may be helpful when applied as plant growth promoter agents in contaminated soils. PGPR were also able to withstand high levels of metals (up to 2615.2 mg Zn l-1, 953.29 mg Cu l-1, and 1124.6 mg Cd l-1) and the order of metal toxicity was Cd > Cu > Zn. The rhizobacterial strains isolated in the present study have the potential to be used as efficient bioinoculants in phytoremediation strategies for the recovery of Kettara mine soils.

Diversity and characterization of culturable bacterial endophytes from Zea mays and their potential as plant growth-promoting agents in metal-degraded soils.

In this study, we evaluated the phylogenetic diversity of culturable bacterial endophytes of Zea mays plants growing in an agricultural soil contaminated with Zn and Cd. Endophytic bacterial counts were determined in roots and shoots, and isolates were grouped by random amplified polymorphic DNA and identified by 16S ribosomal RNA (rRNA) gene sequencing. Endophytes were further characterized for the production of plant growth-promoting (PGP) substances, such as NH3, siderophores, indol-3-acetic acid (IAA), hydrogen cyanide and extracellular enzymes, and for the capacity to solubilize phosphate. The endophytes producing higher amounts of IAA were screened for their tolerance to Zn and Cd and used as bioinoculants for maize seedlings grown in the Zn/Cd-contaminated soil. The counts of endophytes varied between plant tissues, being higher in roots (6.48 log10 g(-1) fresh weight) when compared to shoots (5.77 log10 g(-1) fresh weight). Phylogenetic analysis showed that endophytes belong to three major groups: α-Proteobacteria (31 %), γ-Proteobacteria (26 %) and Actinobacteria (26 %). Pseudomonas, Agrobacterium, Variovorax and Curtobacterium were among the most represented genera. Endophytes were well-adapted to high Zn/Cd concentrations (up to 300 mg Cd l(-1) and 1,000 mg Zn l(-1)) and showed ability to produce several PGP traits. Strains Ochrobactrum haematophilum ZR 3-5, Acidovorax oryzae ZS 1-7, Frigoribacterium faeni ZS 3-5 and Pantoea allii ZS 3-6 increased root elongation and biomass of maize seedlings grown in soil contaminated with Cd and Zn. The endophytes isolated in this study have potential to be used in bioremediation/phytoremediation strategies.