Response of Birch and Alder Root Endophytes as Well as Rhizosphere and Bulk Soil Microorganisms to Heavy Metal Pollution

Cadmium-affected synthesis of exopolysaccharides by rhizosphere bacteria

AIM

Study is focused on the influence of cadmium addition to growth media on production yield, their size and molecular mass of exopolysaccharides (EPS) synthesized by three rhizosphere bacteria strains. Inhibition of bacterial growth by increasing concentrations of Cd²⁺ was also analysed.

METHODS AND RESULTS

The highest impact of Cd²⁺ was noticed on the growth of Arthrobacter sp. and Rhizobium metallidurans. Chryseobacterium sp. and Arthrobacter sp. produced significantly lower when compared to R. metallidurans amounts of EPS under the influence of Cd²⁺ . In all bacterial strains both size and molecular mass decreased after addition of Cd²⁺ to growth media. It causes a change in EPS conformation to more planar, which minimizes the volume of liquid in the interglobular space next to the bacterial wall. Results confirmed strong effect of Cd²⁺ on the structure and synthesis of bacterial EPS what can be a key factor in the interactions between rhizosphere bacteria and host plants in heavy metal polluted soils.

CONCLUSION

This work proves that due to the presence of cadmium ions, the size and conformation of EPS produced by selected bacterial strains is changed to minimize their impact on cell. We suggest that shifting in EPS conformation from bigger globular particles to the smaller planar ones could be one of the probable mechanisms of Cd resistance in metallotolerant bacteria, and finally explain increased efficiency of heavy metal phytoextraction by EPS-producing plant growth-promoting micro-organisms.

SIGNIFICANCE AND IMPACT OF THE STUDY

One of the most promising remediation technique for Cd-contaminated areas is the phytoremediation in which rhizosphere bacteria play an important role by protecting plants' roots from toxic condition thus enhancing efficiency of intake. EPS secretion by bacteria is one of the most common mechanisms to protect the cell from impact of unpleasant environmental conditions, for example, toxicity of heavy metals like Cd.

Phytoremediation: Metal decontamination of soils after the sequential forestation of former opencast coal land

Phytoremediation through forestry may be an effective means for reducing the metal loading in lands reclaimed after surface-coal-mining in the UK. Planted with mixed woodland, the soil loading of 5 key metals (Zn, Cd, Mn, Pb and Cu) decreased, significantly and progressively, compared to soils left as grassland through a 14 year forestation chronosequence on land reclaimed from the former Varteg opencast coalmine, South Wales. Fourteen years after initial tree planting, soil metal loadings decreased by 52% for Cd (4.3 mg∙kg^-1 per year), 48% for Cu (2.1 mg∙kg^-1 per year), 47% for Zn (7.3 mg∙kg^-1 per year), 44% for Pb. (7.1 mg∙kg^-1 per year) and 35% for Mn (45 mg.kg-1 per year). Analysis of metal loadings in the leaves of Alnus glutinosa (L. Gaertn) (Common Alder) and Betula pendula (Roth) (Silver Birch) found both to be involved in metal uptake with birch taking up more Cd, Cu, Zn and Mn and Alder more Pb. Concentrations of Zn, Mn and Cd (Birch only) increased significantly in leaves from, but not in soils, under older plantings. Since different tree species take up metals at different rates, mixed plantings may be more effective in forest phytoremediation.

Modeling of phytoextraction efficiency of microbially stimulated Salix dasyclados L. in the soils with different speciation of heavy metals

Bioaugmentation of soils with selected microorganisms during phytoextraction can be the key solution for successful bioremediation and should be accurately calculated for different physicochemical soil properties and heavy metal availability to guarantee the universality of this method. Equally important is the development of an accurate prediction tool to manage phytoremediation process. The main objective of this study was to evaluate the role of three metallotolerant siderophore-producing Streptomyces sp. B1-B3 strains in the phytoremediation of heavy metals with the use of S. dasyclados L. growing in four metalliferrous soils as well as modeling the efficiency of this process based on physicochemical and microbiological properties of the soils using artificial neural network (ANN) analysis. The bacterial inoculation of plants significantly stimulated plant biomass and reduced oxidative stress. Moreover, the bacteria affected the speciation of heavy metals and finally their mobility, thereby enhancing the uptake and bioaccumulation of Zn, Cd, and Pb in the biomass. The best capacity for phytoextraction was noted for strain B1, which had the highest siderophore secretion ability. Finally, ANN model permitted to predict efficiency of phytoextraction based on both the physicochemical properties of the soils and the activity of the soil microbiota with high precision.

Synthesis of siderophores by plant-associated metallotolerant bacteria under exposure to Cd(2.)

Rhizosphere and endophytic bacteria are well known producers of siderophores, organic compounds that chelate ferric iron (Fe(3+)), and therefore play an important role in plant growth promotion in metalliferous areas, thereby improving bioremediation processes. However, in addition to their primary function in iron mobilization, siderophores also have the capacity to chelate other heavy metals, such as Al(3+), Zn(2+), Cu(2+), Pb(2+) and Cd(2+), that can affect homeostasis and the heavy metal tolerance of microorganisms. The main goal of our study was to select the most efficient siderophore-producing bacterial strains isolated from the roots (endophytes) and rhizosphere of Betula pendula L. and Alnus glutinosa L. growing at two heavy metal contaminated sites in southern Poland. Siderophore biosynthesis of these strains in the presence of increasing concentrations of Cd(2+) (0, 0.5, 1, 2 and 3 mM) under iron-deficiency conditions was analysed using spectrophotometric chemical tests for hydroxamates, catecholates and phenolates, as well as the separation of bacterial siderophores by HPLC and characterization of their structure by UHPLC-QTOF/MS. We proved that (i) siderophore-producing bacterial strains seems to be more abundant in the rhizosphere (47%) than in root endophytes (18%); (ii) the strains most effective at siderophore synthesis belonged to the genus Streptomyces and were able to secrete three types of siderophores under Cd(2+) stress: hydroxamates, catecholates and phenolates; (iii) in general, the addition of Cd(2+) enhanced siderophore synthesis, particularly ferrioxamine B synthesis, which may indicate that siderophores play a significant role in tolerance to Cd(2+) in Streptomyces sp.

Analysis of microbiologically stimulated biomass of Salix viminalis L. in the presence of Cd2+ under in vitro conditions – implications for phytoremediation

The efficiency of phytoremediation might be highly affected by plant-associated microorganisms, and understanding of the underlying mechanisms is still a great challenge. The primary aim of this study was to evaluate the efficiency parameters for Cd2+accumulation in the biomass of willow (Salix viminalis) as well as to define the biochemical response of the host plant when it is inoculated with selected bacterial strains (Massiliasp. andPseudomonassp.) or saprophytic fungus (Clitocybesp.) under controlledin vitroconditions. Inoculation of plants with bacterial strains affected the efficiency of phytoremediation process and was expressed as the quantity of accumulated Cd (Q), the bioaccumulation factor (BCF) and the translocation index (Ti); however, the effect was strain and plant organ specific. The level of hydrogen peroxide (H2O2), which is both an indicator of plant response to biological and/or abiotic environmental stress and a molecule involved in plant-microbial interactions, decreased under the influence of Cd2+in uninoculated plants (plant growth was inhibited by Cd2+) and increased in the inoculated variants of plants growing in the presence of Cd2+(microbiologically stimulated biomass). The saprophytic fungusClitocybesp. generally stimulated biomass and increased the level of H2O2synthesis in all the investigated plant organs and variants of the experiment. We suggest that determination of phytoremediation efficiency, and biochemical response (H2O2) of the host plant underin vitroconditions can help in predicting the final effect of plant-microbial systems in further field trials.