Phytoprotective influence of bacteria on growth and cadmium accumulation in the aquatic plant Lemna minor

The important role of EPS in mediated biosynthesis of CdS QDs: Comparative study of EPS-intact and EPS-free

In this study, we investigated the adsorption of Cd(II) and the biosynthesis of CdS quantum dots (QDs) mediated by cells of sulfate-reducing bacteria before and after the removal of EPS to determine whether EPS or the cell wall plays a major role. Potentiometric titration revealed that the concentration of proton-active binding sites on cells with EPS (EPS-intact) was notably higher than that on cells without EPS (EPS-free) and that the sites were predominantly carboxyl, phosphoryl, hydroxyl, and amine groups. The protein content in EPS-intact cells was higher, and thus the Cd(II) adsorption capacity was stronger. The CdS QDs biosynthesized using EPS-intact possessed better properties, including uniform size distribution, good crystallinity, small particle size, high fluorescence, and strong antimicrobial activity, and the yields were significantly higher than those of EPS-free by a factor of about 1.5-3.7. Further studies revealed that alkaline amino acids in EPS play a major role and serve as templates in the biosynthesis of QDs, whereas they were rarely detected in the cell wall. This study emphasizes the important role of EPS in the bacterial binding of metals and efficient recycling of hazardous waste in water.

Duckweeds for Phytoremediation of Polluted Water

Tiny aquatic plants from the Lemnaceae family, commonly known as duckweeds, are often regarded as detrimental to the environment because of their ability to quickly populate and cover the surfaces of bodies of water. Due to their rapid vegetative propagation, duckweeds have one of the fastest growth rates among flowering plants and can accumulate large amounts of biomass in relatively short time periods. Due to the high yield of valuable biomass and ease of harvest, duckweeds can be used as feedstock for biofuels, animal feed, and other applications. Thanks to their efficient absorption of nitrogen- and phosphate-containing pollutants, duckweeds play an important role in the restorative ecology of water reservoirs. Moreover, compared to other species, duckweed species and ecotypes demonstrate exceptionally high adaptivity to a variety of environmental factors; indeed, duckweeds remove and convert many contaminants, such as nitrogen, into plant biomass. The global distribution of duckweeds and their tolerance of ammonia, heavy metals, other pollutants, and stresses are the major factors highlighting their potential for use in purifying agricultural, municipal, and some industrial wastewater. In summary, duckweeds are a powerful tool for bioremediation that can reduce anthropogenic pollution in aquatic ecosystems and prevent water eutrophication in a simple, inexpensive ecologically friendly way. Here we review the potential for using duckweeds in phytoremediation of several major water pollutants: mineral nitrogen and phosphorus, various organic chemicals, and heavy metals.

Microbial Population Dynamics in Lemnaceae (Duckweed)-Based Wastewater Treatment System

The dynamic microflora associated within, and in the surrounding aquatic environment, has been found to be responsible for the functional properties of many aquatic plants. The aim of the current work was to evaluate the effectiveness of Lemnaceae-based wastewater treatment system under tropical conditions and investigate the changes in the aquatic microflora upon plant growth. A biological wastewater treatment system was designed and investigated using mixed Lemnaceae culture comprising Lemna minor and Spirodela polyrhiza in a batch mode. A significant reduction in total solids (31.8%), biochemical oxygen demand (93.5%), and chemical oxygen demand (73.2%) was observed after seven days of duckweed growth using a low inoculum. A preliminary study on the change in the microbial population diversity and functionality, in the wastewater before and after treatment, revealed an increase in the denitrifying microflora in wastewater post-Lemnaceae treatment. Dominance of 10 bacterial phyla, contributing for 98.3% of the total bacterial communities, was recorded, and ~ 50.6% loss of diversity post-treatment of wastewater was revealed by the Shannon Index. Among 16 bacterial families showing relative abundance of ≥ 1% in untreated wastewater, Methylobacteriaceae, Pseudomonadaceae, Brucellaceae, Rhodobacteraceae, and Acetobacteraceae prevailed in the water post-treatment by duckweeds. This is a novel work done on the dynamics of aquatic microflora associated with Lemnaceae under tropical Indian conditions. It confirms the application of Lemnaceae-based wastewater treatment system as effective biofilter and calls for further studies on the active involvement of the endophytic and aquatic microflora in the functions of these plant.

Isolation, Identification and Pollution Prevention of Bacteria and Fungi during the Tissue Culture of Dwarf Hygro (Hygrophila polysperma) Explants

Microbial contamination causes serious damage in plant tissue culture, and attention is always being paid regarding how to control and prevent the unwanted pollution. Dwarf hygro (Hygrophila polysperma) is a popular ornamental aquatic plant and its tissue culture has been reported, but the microbial pollution and the cure of microbial pollution was unknown. In this study, a number of bacteria and fungi were isolated from contaminants in MS culture media. Based on the 16S rDNA and ITS sequencing, it was identified that fifteen bacteria belong to Bacillus, Enterobacter, Pantoea, Kosakonia, Ensifer and Klebsiella, and three fungi belong to Plectosphaerella, Cladosporium and Peniophora, respectively. In addition, some drugs were further tested to be free of the bacteria and fungi pollution. The results revealed that 10 μg/mL of kanamycin, 5 μg/mL of chloramphenicol, and 0.015625% potassium sorbate could be applied jointly in MS media to prevent the microbial pollution, and the survival rate of H. polysperma explants was highly improved. This study reveals the bacteria and fungi species from the culture pollution of H. polysperma and provides a practical reference for optimizing the tissue culture media for other aquatic plants.

Changes in the prokaryotic diversity in response to hydrochemical variations during an acid mine drainage passive treatment

Acid mine drainage (AMD) causes major environmental problems and consequently, several treatments are proposed, favoring the passive systems because of their many advantages. The main goal of these procedures is the neutralization and removal of potentially toxic elements (PTE), yet little is known about the changes in the microbial assemblages in response to the hydrochemical variations during the treatments. Therefore, the main objective of this research was to determine the changes in the diversity and structure of the prokaryotic assemblages in a hybrid abiotic and biological (wetland) passive treatment system. The 16S rRNA gene survey showed that the AMD coming from the mine (pH 2.6) was mainly composed of acidophilic genera such as Acidithiobacillus, Leptospirillum, Ferritrophicum, and Cuniculiplasma (up to 76 % relative abundance). In the abiotic treatment, Acidiphilium was dominant in the sections with limestone filters (pH 2.2-4.8), followed by Limnobacter in the subsequent dolomite/limestone and phosphoric rock filters (pH 5.2-5.8). In these abiotic passive treatment sections, the microbial assemblage showed a limited diversity and richness. However, when the treated AMD reached the two final wetlands (pH ~6.8), the microbial diversity and richness increased, suggesting that further bioattenuation mechanisms might be occurring. Limnobacter and Novosphingobium were the main bacterial genera in the water samples of the wetland sections (Arundo donax). These changes in the composition of the microbial assemblages were highly correlated with the pH and Eh values during the treatment (p-value <0.001); however, the concentration of metal(loid)s such as Al, Cd, Fe, Mn, Ni, and Zn were also significantly related (p-value <0.05). In conclusion, the studied passive AMD treatment system enhanced the chemical quality of the treated AMD, showing high removal efficiencies for Al and Fe (> 99 %), and increasing the microbial diversity and richness in the effluent.

Accumulation, distribution and removal of triazine pesticides by Eichhornia crassipes in water-sediment microcosm

After application, pesticides remained in the field may contaminate water resources through surface runoff and leaching, posing a threat to aquatic ecosystem. In the current study, the accumulation, translocation, distribution and removal of four triazine pesticides (simazine, atrazine, terbuthylazine and metribuzin) by free floating aquatic plant Eichhornia crassipes (E. crassipes) in water-sediment microcosm were investigated and the removal mechanisms were explored. E. crassipes was exposed to an initial concentration of 50 μg·L^-1 and the pesticide levels in water, sediment, roots and shoots of E. crassipes were monitored during 30 days. The results demonstrated that E. crassipes was capable of accumulating triazine pesticides with the bio-concentration factor (BCF) ranging from 0.8 to 18.4. Triazine pesticides were mainly stored in roots, and root accumulation and translocation amount depend on the hydrophobicity of the pesticides. The removal of the pesticides in water were significantly accelerated by the presence of E. crassipes, with the removal efficiency ranging from 66% to 79% after 30 days of treatment. Though phytoaccumulation only constituted 2-18% of the total spiked pesticides in the microcosm, E. crassipes played a vital role in removing simazine, atrazine and metribuzin. However, microbial degradation in sediment was the main pathway for the removal of terbuthylazine in the microcosm. This study demonstrated the potential application of E. crassipes to accelerate removal of contaminants from aquatic environment.

Responsiveness change of biochemistry and micro-ecology in alkaline soil under PAHs contamination with or without heavy metal interaction

Co-presence of organic pollutants and heavy metals in soil is causing increasing concerns, but the lack of knowledge of relation between soil ecology and pollutant fate is limiting the developing of specific control strategy. This study investigated soil change under pyrene stress and its interaction with cadmium (Cd). Soil physicochemical properties were not seriously influenced. However, pollutants' presence easily varied soil microbial activity, quantity, and diversity. Under high-level pyrene, Cd presence contributed to soil indigenous microorganisms' adaption and soil microbial community structure stability. Soils with both pyrene and Cd presented 7.11-12.0% higher pyrene degradation compared with single pyrene treatment. High-throughput sequencing analysis indicated the proportion of Mycobacterium sp., a commonly known PAHs degrader, increased to 25.2-48.5% in treatments from 0.52% in control. This phenomenon was consistent with the increase of PAHs probable degraders (the ratio increased to 2.86-6.57% from 0.24% in control). Higher Cd bioavailability was also observed in soils with both pollutants than that with Cd alone. And Cd existence caused the elevation of Cd resistant bacterium Limnobacter sp. (increased to 12.2% in CdCK from 2.06% in control). Functional gene prediction also indicated that abundance of genes related to nutrient metabolism decreased dramatically with pollutants, while the abundances of energy metabolism, lipid metabolism, secondary metabolites biosynthesis-related genes increased (especially for aromatic compound degradation related genes). These results indicated the mutual effect and internal-interaction existed between pollutants and soils resulted in pollutants' fate and soil microbial changes, providing further information regarding pollutants dissipation and transformation under soil microbial response.

Mutualistic Outcomes Across Plant Populations, Microbes, and Environments in the Duckweed Lemna minor

The picture emerging from the rapidly growing literature on host-associated microbiota is that host traits and fitness often depend on interactive effects of host genotype, microbiota, and abiotic environment. However, testing interactive effects typically requires large, multi-factorial experiments and thus remains challenging in many systems. Furthermore, most studies of plant microbiomes focus on terrestrial hosts and microbes. Aquatic habitats may confer unique properties to microbiomes. We grew different populations of duckweed (Lemna minor), a floating aquatic plant, in three microbial treatments (adding no, "home", or "away" microbes) at two levels of zinc, a common water contaminant in urban areas, and measured both plant and microbial performance. Thus, we simultaneously manipulated plant source population, microbial community, and abiotic environment. We found strong effects of plant source, microbial treatment, and zinc on duckweed and microbial growth, with significant variation among duckweed genotypes and microbial communities. However, we found little evidence of interactive effects: zinc did not alter effects of host genotype or microbial community, and host genotype did not alter effects of microbial communities. Despite strong positive correlations between duckweed and microbe growth, zinc consistently decreased plant growth, but increased microbial growth. Furthermore, as in recent studies of terrestrial plants, microbial interactions altered a duckweed phenotype (frond aggregation). Our results suggest that duckweed source population, associated microbiome, and contaminant environment should all be considered for duckweed applications, such as phytoremediation. Lastly, we propose that duckweed microbes offer a robust experimental system for study of host-microbiota interactions under a range of environmental stresses.

Community dynamics of duckweed-associated bacteria upon inoculation of plant growth-promoting bacteria

Plant growth-promoting bacteria (PGPB) have recently been demonstrated as a promising agent to improve wastewater treatment and biomass production efficiency of duckweed hydrocultures. With a view to their reliable use in aqueous environments, this study analysed the plant colonization dynamics of PGPB and the ecological consequences for the entire duckweed-associated bacterial community. A PGPB strain, Aquitalea magnusonii H3, was inoculated to duckweed at different cell densities or timings in the presence of three environmental bacterial communities. The results showed that strain H3 improved duckweed growth by 11.7–32.1% in five out of nine experiments. Quantitative-PCR and amplicon sequencing analyses showed that strain H3 successfully colonized duckweed after 1 and 3 d of inoculation in all cultivation tests. However, it significantly decreased in number after 7 d, and similar bacterial communities were observed on duckweed regardless of H3 inoculation. Predicted metagenome analysis suggested that genes related to bacterial chemotactic motility and surface attachment systems are consistently enriched through community assembly on duckweed. Taken together, strain H3 dominantly colonized duckweed for a short period and improved duckweed growth. However, the inoculation of the PGPB did not have a lasting impact due to the strong resilience of the natural duckweed microbiome.

Phenol Removal Capacity of the Common Duckweed (Lemna minor L.) and Six Phenol-Resistant Bacterial Strains From Its Rhizosphere: In Vitro Evaluation at High Phenol Concentrations

The main topic of this study is the bioremediation potential of the common duckweed, Lemna minor L., and selected rhizospheric bacterial strains in removing phenol from aqueous environments at extremely high initial phenol concentrations. To that end, fluorescence microscopy, MIC tests, biofilm formation, the phenol removal test (4-AAP method), the Salkowski essay, and studies of multiplication rates of sterile and inoculated duckweed in MS medium with phenol (200, 500, 750, and 1000 mg L⁻¹) were conducted. Out of seven bacterial strains, six were identified as epiphytes or endophytes that efficiently removed phenol. The phenol removal experiment showed that the bacteria/duckweed system was more efficient during the first 24 h compared to the sterile duckweed control group. At the end of this experiment, almost 90% of the initial phenol concentration was removed by both groups, respectively. The bacteria stimulated the duckweed multiplication even at a high bacterial population density (>10⁵ CFU mL⁻¹) over a prolonged period of time (14 days). All bacterial strains were sensitive to all the applied antibiotics and formed biofilms in vitro. The dual bacteria/duckweed system, especially the one containing strain 43-Hafnia paralvei C32-106/3, Accession No. MF526939, had a number of characteristics that are advantageous in bioremediation, such as high phenol removal efficiency, biofilm formation, safety (antibiotic sensitivity), and stimulation of duckweed multiplication.

Toxicological effects of AgNPs on duckweed (Landoltia punctata)

Silver nanoparticles (AgNPs) are widely applied in several types of products since they act as a biocide. However, their high level of release into the environment can bring risks to ecosystems. Thus, the toxicity of AgNPs toward duckweed (Landoltia punctata) was investigated by monitoring the growth rate inhibition and the effect on the photosynthetic metabolism through morphological and ultrastructural analysis. The AgNPs were characterized by transmission electron microscopy and the effective diameter (dynamic light scattering) and zeta potential were determined. Plants were grown according to the environmental conditions recommended in ISO/DIS 20079 and then exposed to different concentrations of AgNPs. Inhibition of the growth rate was measured based on the EC₅₀ and changes in the morphology, cellular structures and photosynthetic pigments were evaluated along with the silver accumulation. Although the results showed low growth inhibition when compared to other studies, significant damage to the ultrastructure, decreases in the photosynthetic pigments and starch grains, an increase in the phenolic compounds and physiological changes, such as a loss of color, were observed. Moreover, the accumulation of silver ions was noted and this could lead to bioamplification in consumer organisms, since duckweed belongs to the first level of the food chain.

Resilience to multiple stressors in an aquatic plant and its microbiome

PREMISE

Outcomes of species interactions, especially mutualisms, are notoriously dependent on environmental context, and environments are changing rapidly. Studies have investigated how mutualisms respond to or ameliorate anthropogenic environmental changes, but most have focused on nutrient pollution or climate change and tested stressors one at a time. Relatively little is known about how mutualisms may be altered by or buffer the effects of multiple chemical contaminants, which differ fundamentally from nutrient or climate stressors and are especially widespread in aquatic habitats.

METHODS

We investigated the impacts of two contaminants on interactions between the duckweed Lemna minor and its microbiome. Sodium chloride (salt) and benzotriazole (a corrosion inhibitor) often co-occur in runoff to water bodies where duckweeds reside. We tested three L. minor genotypes with and without the culturable portion of their microbiome across field-realistic gradients of salt (3 levels) and benzotriazole (4 levels) in a fully factorial experiment (24 treatments, tested on each genotype) and measured plant and microbial growth.

RESULTS

Stressors had conditional effects. Salt decreased both plant and microbial growth and decreased plant survival more as benzotriazole concentrations increased. In contrast, benzotriazole did not affect microbial abundance and even benefited plants when salt and microbes were absent, perhaps due to biotransformation into growth-promoting compounds. Microbes did not ameliorate duckweed stressors; microbial inoculation increased plant growth, but not at high salt concentrations.

CONCLUSIONS

Our results suggest that multiple stressors matter when predicting responses of mutualisms to global change and that beneficial microbes may not always buffer hosts against stress.

Efficient genetic transformation and CRISPR/Cas9-mediated genome editing in Lemna aequinoctialis

The fast growth, ease of metabolic labelling and potential for feedstock and biofuels production make duckweeds not only an attractive model system for understanding plant biology, but also a potential future crop. However, current duckweed research is constrained by the lack of efficient genetic manipulation tools. Here, we report a case study on genome editing in a duckweed species, Lemna aequinoctialis, using a fast and efficient transformation and CRISPR/Cas9 tool. By optimizing currently available transformation protocols, we reduced the duration time of Agrobacterium-mediated transformation to 5-6 weeks with a success rate of over 94%. Based on the optimized transformation protocol, we generated 15 (14.3% success rate) biallelic LaPDS mutants that showed albino phenotype using a CRISPR/Cas9 system. Investigations on CRISPR/Cas9-mediated mutation spectrum among mutated L. aequinoctialis showed that most of mutations were short insertions and deletions. This study presents the first example of CRISPR/Cas9-mediated genome editing in duckweeds, which will open new research avenues in using duckweeds for both basic and applied research.

Phytotoxicity and degradation of antibiotic ofloxacin in duckweed (Spirodela polyrhiza) system

The phytotoxicity and degradation of ofloxacin (OFX) in duckweed Spirodela polyrhiza based system was estimated in this study. For that, OFX was added in an environmentally relevant range (0.01-1.0 mg L^-1) in medium (Hoagland nutrient) and toxicity biomarkers, i.e. changes in plant biomass, relative growth rate (RGR), photopigment (Chl-a, Chl-b and carotenoids), protein content, antioxidative enzymes (catalase, CAT; superoxide dismutase, SOD; and ascorbate peroxidases, APX) in fronds were estimated. The batch-scale setups (250 ml) was prepared in triplicate for each concentration of OFX and reared in growth chambers (Algae Tron AG 230) for 7 d. Results suggested that the high concentrations of OFX caused a reduction in biomass (4.8-41.3%), relative root growth (RGR), protein (4.16-11.28%) and photopigment contents. The fronds in OFX spiked setups showed an increased level of antioxidative enzymes: CAT (0.230-0.338 mmol_H2O2 mg^-1 protein), APX (0.043-0.074 mmol_ascorbate mg^-1 protein), and SOD (0.267-0.317 U mg^-1 protein) than control. At the end (7 d), the residual OFX content in the medium was also estimated, and results suggested a significant (p < 0.05) reduction (93.73-98.36%) in OFX content than control setup (54.76-75.53%) at the end of the experimentation. The trend of residual OFX suggested phytodegradation as a significant mechanism of antibiotic degradation other than hydrolysis and photodegradation processes. This study indicates that duckweed can be an effective bio-tool for the removal of environmental relevant concentration of the antibiotics from the wastewater.

Application of common duckweed (Lemna minor) in phytoremediation of chemicals in the environment: State and future perspective

Over the past 50 years, different strategies have been developed for the remediation of polluted air, land and water. Driven by public opinion and regulatory bottlenecks, ecological based strategies are preferable than conventional methods in the treatments of chemical effluents. Ecological systems with the application of microbes, fungi, earthworms, plants, enzymes, electrode and nanoparticles have been applied to varying degrees in different media for the remediation of various categories of pollutants. Aquatic macrophytes have been used extensively for the remediation of pollutants in wastewater effluents and aquatic environment over the past 30 years with the common duckweed (L. minor) as one of the most effective macrophytes that have been applied for remediation studies. Duckweed has shown strong potentials for the phytoremediation of organic pollutants, heavy metals, agrochemicals, pharmaceuticals and personal care products, radioactive waste, nanomaterials, petroleum hydrocarbons, dyes, toxins, and related pollutants. This review covers the state of duckweed application for the remediation of diverse aquatic pollutants and identifies gaps that are necessary for further studies as we find pragmatic and sound ecological solutions for the remediation of polluted environment for sustainable development.

Phytoremediation applications in natural condition and in mesocosm: The uptake of cadmium by Lemna minuta Kunth, a non-native species in Italian watercourses

Metal pollution in water and soil is an environmental and public health issue. Cadmium (Cd) is included in the list of priority hazardous substances in the European Water Framework Directive. Phytoremediation system is a cost-effective, plant-based approach that takes advantage of the ability of plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues. We studied the presence and the importance of an invasive species, such as Lemna minuta, in the environment and the effects of Cd pollution on this species. Growth, removal, and tolerance were evaluated for different Cd concentrations and different times of plant exposure. Overall, the results show that L. minuta has a good capacity of growth, metal bioconcentration, and tolerance up to 3 days of exposure at 0.5 and 1.5 mg L^-1 of Cd. In particular, L. minuta was able to accumulate Cd up to 3771 mg kg^-1 on dry mass basis. We can conclude that L. minuta possesses a great capability of Cd absorption and accumulation, thus supporting a potential use of this species in designing a metal bioremediation system in phytoremediation field.

Ibuprofen exposure in Lemna gibba L.: Evaluation of growth and phytotoxic indicators, detection of ibuprofen and identification of its metabolites in plant and in the medium

Ibuprofen (IBU) is detected worldwide in water bodies due to the incomplete removal by wastewater treatments. Contrasting results have been reported on the toxicity of IBU on aquatic biomonitor plants such as duckweed, and no data about IBU detection and metabolism in plants has been reported. In this work, the effects of 1 mg L(-1) IBU on Lemna gibba L. were monitored in an 8-day laboratory test. In particular, an increase in frond number (+12%) and multiplication rate (+10%) while no variations in photosynthetic pigment content were observed. Moreover, UPLC-HRMS analysis of the presence of IBU and its metabolites in plants and in the growth medium was performed. The results showed that, besides IBU, 11 IBU metabolites were detected in plants. Among the IBU metabolites, hydroxyl- and dihydroxyl-IBU were found, whereas carboxyl-IBU was undetectable. Interestingly, some IBU metabolites were detected in the plant growth solution at the end of the IBU treatment, while no IBU products were found in the IBU solution without plants, suggesting a role for L. gibba in IBU metabolism. The findings of this work represent an important step for a better evaluation of the effects of IBU and its metabolites in duckweed, with notable implications for the eco-toxicological assessment of IBU in the aquatic ecosystem.

Survey of duckweed diversity in Lake Chao and total fatty acid, triacylglycerol, profiles of representative strains

Lemnaceae (duckweeds) are widely distributed aquatic flowering plants. Their high growth rate, starch content and suitability for bioremediation make them potential feedstock for biofuels. However, few natural duckweed resources have been investigated in China, and there is no information about total fatty acid (TFA) and triacylglycerol (TAG) composition of duckweeds from China. Here, the genetic diversity of a natural duckweed population collected from Lake Chao, China, was investigated using multilocus sequence typing (MLST). The 54 strains were categorised into four species in four genera, representing 12 distinct sequence types. Strains representing Lemna aequinoctialis and Spirodela polyrhiza were predominant. Interestingly, a surprisingly high degree of genetic diversification within L. aequinoctialis was observed. The four duckweed species revealed a uniform fatty acid composition, with three fatty acids, palmitic acid, linoleic acid and linolenic acid, accounting for more than 80% of the TFA. The TFA in biomass varied among species, ranging from 1.05% (of dry weight, DW) for L. punctata and S. polyrhiza to 1.62% for Wolffia globosa. The four duckweed species contained similar TAG contents, 0.02% mg · DW(-1). The fatty acid profiles of TAG were different from those of TFA, and also varied among the four species. The survey investigated the genetic diversity of duckweeds from Lake Chao, and provides an initial insight into TFA and TAG of four duckweed species, indicating that intraspecific and interspecific variations exist in the content and composition of both TFA and TAG in comparison with other studies.

Effects of a rhizobacterium on the growth of and chromium remediation by Lemna minor

Duckweed has shown great potential for both energy and environmental applications, particularly in wastewater treatment and fuel ethanol production. A rhizobacterium, Exiguobacterium sp. MH3, has been reported to associate with the duckweed Lemna minor for symbiotic growth. The aim of this work is to study the effects of rhizobacterium MH3 on L. minor growth and chromium (Cr) remediation. It appeared to have a synergism between the rhizobacterium MH3 and duckweed; the presence of strain MH3 promoted the growth of duckweeds by increasing both the frond number and dry weight of duckweed by more than 30%, while duckweed in turn provided essential carbon source and energy for the growth of rhizobacterium MH3. Under Cr(VI) exposure, particularly at higher Cr(VI) concentrations, Exiguobacterium sp. MH3 significantly alleviated the harmful effects of the stress on the duckweed by promoting duckweed growth and preventing duckweed from excessive uptake of Cr. Potential mechanisms were also discussed in light of the genome sequence of strain MH3, and it was speculated that siderophores and indole-3-acetic acid (IAA) secreted by strain MH3 might contribute to promoting duckweed growth.

Assessment of physico-chemical qualities and heavy metal concentrations of Umgeni and Umdloti Rivers in Durban, South Africa

We assessed the effects of seasonal dynamics on the physico-chemical qualities and heavy metals concentrations of the Umgeni and Umdloti Rivers in Durban, South Africa. Water samples were taken from nine different sampling points and analysed for the following parameters; temperature, pH, turbidity, electrical conductivity (EC), biological oxygen demand (BOD5), chemical oxygen demand (COD), phosphate (PO4(2-)), nitrate (NO3(2-)), ammonium (NH4(+)), sulphate (SO4(2-)), lead (Pb(2+)), mercury (Hg(2+)), cadmium (Cd(2+)), aluminium (Al(3+)), and copper (Cu(2+)) using standard methods. The data showed variations it terms of the seasonal fluctuations and sampling regime as follows: temperature 12-26.5 °C; pH 5.96-8.45; turbidity 0.53-18.8 NTU; EC 15.8-5180 mS m(-1); BOD5 0.60-7.32 mg L(-1); COD 10.5-72.9 mg L(-1); PO4 (2-) < 500-2,460 μg L(-1); NO3 (2-) <0.05-4.21 mg L(-1); NH4 (+) < 0.5-1.22 mg L(-1); SO4 (2-) 3.90-2,762 mg L(-1); Pb(2+) 0.023-0.135 mg L(-1); Hg(2+) 0.0122-0.1231 mg L(-1) Cd(2+) 0.068-0.416 mg L(-1); Al(3+) 0.037-1.875 mg L(-1), and Cu(2+)0.006-0.144 mg L(-1). The concentrations of most of the investigated parameters exceeded the recommended limit of the South African Guidelines and World Health Organization tolerance limits for freshwater quality. We conclude that these water bodies are potentially hazardous to public health and this highlights the need for implementation of improved management strategies of these river catchments for continued sustainability.

Characterization of Cu-tolerant bacteria and definition of their role in promotion of growth, Cu accumulation and reduction of Cu toxicity in Triticum aestivum L

The effects of Cu-tolerant bacteria strain USTB-O on Cu accumulation, plant growth and reduction of Cu toxicity in wheat seedlings Triticum aestivum L. were investigated. The strain was identified as belonging to Bacillus species and showed a specific tolerance to Cu through binding the Cu ions to the cell walls to reduce their entry into the cells. The bacteria not only increased Cu accumulation in wheat seedlings, but also secreted indole-3-acetic acid (IAA) and therefore promoted plant growth. Moreover, the bacteria effectively improved the antioxidant defence system to alleviate the oxidative damage induced by Cu. The bacteria promoted superoxide dismutase (SOD) in both shoots and roots to reduce superoxide radicals. The bacteria stimulated all enzymes activities under Cu exposure conditions, peroxidase (POD) and catalase (CAT) in shoots and ascorbate peroxidase (APX) and dehydroascorbate reductase (DHAR) in roots were major enzymes to eliminate H2O2 in wheat seedlings.

Cadmium-accumulating plants

Plants are categorized in three groups concerning their uptake of heavy metals: indicator, excluder, and hyperaccumulator plants, which we explain in this chapter, the former two groups briefly and the hyperaccumulators in detail. The ecological role of hyperaccumulation, for example, the prevention of herbivore attacks and a possible substitution of Zn by Cd in an essential enzyme, is discussed. As the mechanisms of cadmium hyperaccumulation are a very interesting and challenging topic and many aspects are studied worldwide, we provide a broad overview over compartmentation strategies, expression and function of metal transporting proteins and the role of ligands for uptake, transport, and storage of cadmium. Hyperaccumulators are not without reason a topic of great interest, they can be used biotechnologically for two main purposes which we discuss here for Cd: phytoremediation, dealing with the cleaning of anthropogenically contaminated soils as well as phytomining, i.e., the use of plants for commercial metal extraction. Finally, the outlook deals with topics for future research in the fields of biochemistry/biophysics, molecular biology, and biotechnology. We discuss which knowledge is still missing to fully understand Cd hyperaccumulation by plants and to use that phenomenon even more successfully for both environmental and economical purposes.

Monitoring heavy metal pollution by aquatic plants: a systematic study of copper uptake

INTRODUCTION

The copper bioaccumulation by the floating Lemna minor and by the completely submerged Ranunculus tricophyllus as a function of exposure time and copper concentration was studied, with the aim of proposing these species as environmental biosensors of the water pollution.

RESULTS

The results show that both these aquatic angiosperms are good indicators of copper pollution because the copper uptake is the only function of metal concentration (water pollution).

CONCLUSION

Uptake behavior is reported as a function of the time and concentration, based on the results of a 3-year study. Kinetic evaluations are proposed.