Metal-resistant rhizobacteria isolates improve Mucuna deeringiana phytoextraction capacity in multi-metal contaminated soils from a gold mining area

Assessing ecological risks and spatial distribution of potentially toxic elements in soils from anthropized environments in a watershed at the caatinga-Atlantic forest ecotone in Brazil

Understanding the processes of mobility and availability of potentially toxic elements in soil is crucial for informed decision-making in the development of public policies aimed at minimizing environmental impacts. Monitoring, in combination with the determination of natural concentrations, can provide effective tools for controlling pollution sources. In this study, enrichment, pollution, and ecological risk indices were used for some potentially toxic elements in an anthropogenically influenced watershed in southwestern Bahia, Brazil. The study involved 63 composite surface soil samples collected from areas with natural forest, crops, pastures, and urbanization. The samples were analyzed for fertility and particle size. Metal extraction followed the EPA 3051A method, and element determination was carried out via ICP-OES. The soils in the Verruga River watershed exhibit a high variability in fertility and granulometric attributes. The Kruskal-Wallis test at a 5% significance level was employed to assess the impact of land management on the availability of elements (As, Co and Pb), while Spearman's correlation, along with hierarchical clustering analysis, was used to comprehend element dynamics. Geostatistics were applied to identify pollution hotspots. Consequently, it became evident that potentially toxic elements can accumulate in the soil depending on land use and management practices (As, Co, and Pb), as well as the weathering process linked to the type of source material, such as diamictite deposits (Ni and Co). Soils in the Verruga River watershed qualify as having minimal enrichment, low pollution levels, and individual ecological risk concerning Cd. The percentage of samples enriched with Cu, As, Zn, and Cd exceeded 67%, with agricultural activities being the primary source of pollution. Meanwhile, in pasture and urban areas, Co and Pb were notably prominent, respectively.

Soil bacteria around a derelict tailings pile with different metal pollution gradients: community composition, metal tolerance and influencing factors

Bacteria play a vital role in ecological processes of soil contaminated by heavy metals. Here, soil sampling was carried out around a tailings pile contaminated to different degrees by cadmium (Cd), lead (Pb) and arsenic (As). The bacteria in the soil were cultured, separated and purified on Luria-Bertani medium, and the changes in bacterial communities in soils with different pollution levels were analysed with 16S rRNA sequencing. Bacillus pacificus strain MZ520364 was found to be highly tolerant to Cd, Pb and As, and single-metal and multimetal tolerance experiments were further conducted with this strain. The results obtained from alpha diversity and operational taxonomic unit (OTU) statistical analyses showed a significant difference in bacterial composition among soils with different metal pollution levels, and the highest bacterial diversity was found at the most severely polluted site. Evidence from variance partitioning analysis (VPA) and the Spearman correlation heatmap analysis showed that the leading factors affecting bacterial community composition were cation exchange content (CEC), pH, total Zn, total As, and available As concentrations in soil. Additionally, in the single-metal treatments, B. pacificus MZ520364 could tolerate 600 mg/L Cd²⁺, 1000 mg/L Pb²⁺ or 700 mg/L As³⁺. When Cd, Pb and As coexisted, the best growth of B. pacificus MZ520364 was present at 120 mg/L Cd²⁺, 200 mg/L Pb²⁺ and 150 mg/L As³⁺. The effect of Cd, Pb and As on the growth of the strain followed the order of Cd > As > Pb, and the heavy metal combination showed more toxicity than single metals. In summary, our results revealed the ecological impact of soil physicochemical properties on the diversity and richness of soil bacterial communities and suggested that B. pacificus MZ520364 may be used for the remediation of Cd-Pb-As co-contaminated soil.

Improving cadmium accumulation by Solanum nigrum L. via regulating rhizobacterial community and metabolic function with phosphate-solubilizing bacteria colonization

Soil cadmium (Cd) mobilized with phosphate-solubilizing bacteria (PSB), especially for strains effectively colonized in rhizosphere, is an important pathway for promoting its accumulation by Cd-hyperaccumulators. In this study, screened PSB strains, Acinetobacter pittii (AP) and Escherichia coli (EC), were used to evaluate their effects on Cd mobilization in rhizosphere, Cd accumulation by Solanum nigrum L., and rhizobacterial community and metabolic function under different colonization condition. Results indicated that AP or EC inoculated in soils significantly promoted plant growth, and simultaneously motivated Cd accumulation in S. nigrum L. by 119% and 88%, respectively, when compared with that of uninoculated treatment. Higher efficiency colonization of AP contributed to more organic acids (malic, l-proline, l-alanine, and γ-aminobutanoic) production in the rhizosphere soil and Cd accumulation by S. nigrum L., when compared with that of EC treatment. Taxonomic distribution and co-occurrence network analyses demonstrated that inoculation of AP or EC enriched dominant microbial taxa with plant growth promotion function and keystone taxa related to Cd mobilization in the rhizosphere soil, respectively. Inoculated strains up-regulated the expression of genes related to bacterial mobility, amino acid metabolism, and carbon metabolism among rhizobacterial community. Overall, this study provided a feasible method for soil Cd phytoremediation by promoting Cd mobilization with the enhancement of keystone taxa and organic acid secretion based on the high-efficiency colonization of PSB.

Tolerance and phytoremediation potential of grass species native to South American grasslands to copper-contaminated soils

Grass species native to South American can have mechanisms to tolerate copper (Cu) excess, which improves their use to phytoremediate Cu-contaminated soils . The aims of the present study are to assess the tolerance of grass species native to South American grasslands to copper-contaminated soils, as well as their adaptive responses under high Cu-stressed condition and to identify native grass species presenting the highest potential to be used for phytoremediation purposes. Soil samples were air-dried and their acidity, phosphorus and potassium levels were corrected, and the samples were incubated. Three Cu levels were used in the experiment: natural (Dose 0), with added of 40 mg kg^-1 of Cu and with added of 80 mg kg^-1 of Cu. Three Axonopus affinis, Paspalum notatum and Paspalum plicatulum seedlings were transferred to 5-L pots filled with soil in August and grown for 121 days. Soil solution was collected during cultivation with the aid of Rhizon lysimeters. Main concentrations of cations and anions, dissolved organic carbon and pH in the soil solution were analyzed and the ionic speciation was carried out. Cu toxicity impaired the growth of grass species native to South America, since Cu excess led to both changes in root morphology and nutritional unbalance. Among all assessed native species, Paspalum plicatulum was the one presenting the greatest potential to phytostabilize in Cu-contaminated soils, since it mainly accumulates Cu absorbed in the roots; therefore, its intercropping with grapevines is can be beneficial in Cu-contaminated soils.

Enhanced uptake of Cd, Cr, and Cu in Catharanthus roseus (L.) G.Don by Bacillus cereus: application of moss and compost to reduce metal availability

Heavy metals (HMs) being the notorious and toxic are being introduced into the environment credited to natural and anthropogenic activities. The use of ornamental plants is being ignored as potential candidates for HMs phytoremediation. In this study, pot experiments were conducted on Catharanthus roseus (L.) G.Don to evaluate selected heavy metals tolerance and accumulation potential with reference to the bacterial endophyte (Bacillus cereus) and organic amendments (moss and compost at 5% v/v). Results indicated improvement in uptake of Cd (230 mg kg^-1), Cu (229 mg kg^-1), and Cr (458 mg kg^-1) by C. roseus with B. cereus. The concentration of Ni and Pb was found highest in controls (without strain) that were 420 and 904 mg kg^-1, respectively. Conversely, the addition of organic amendments enhanced biomass production, as compared to controls, 441, 471, and 763% by peat moss (T3), compost (T4), and peat moss + compost + inoculum treatments (T6), respectively, while reduction of plant HMs content was observed. Microbial-aided phytoremediation/phytoextraction could be a potential method for removal of Cd, Cr, and Cu, while organic amendments can significantly improve plant growth in the presence of heavy metals.

Influence of Pseudomonas japonica and organic amendments on the growth and metal tolerance of Celosia argentea L

In this study, a pot experiment was piloted in a greenhouse to evaluate the potential of Celosia argentea var. cristata L. for tolerating/accumulating heavy metals in synthetic wastewater in the presence of Pseudomonas japonica and organic amendment, i.e., moss and compost. Two-week-old seedlings were transferred to pots, and after 4 weeks, the bacterial strain was inoculated, then watered with synthetic wastewater for 5 weeks and harvested after 9 weeks. After harvesting, physiological and biochemical parameters, as well as metal contents of plants, were quantified. The results indicated highest growth and biomass production in moss- and compost-associated plants while highest metal uptake has been found in the presence of P. japonica and synthetic wastewater-irrigated plants. Synthetic wastewater-irrigated plants have shown highest Pb uptake of 2899 mg kg^-1 DW, while with P. japonica in soil those plants have shown highest Cd, Cu, Ni, and Cr uptake of 962, 1479, 1042, and 956 mg kg^-1 DW, respectively. The production of antioxidant enzymes, i.e., catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), and glutathione-s-transferase (GST), was high in P. japonica-amended plants because of increased uptake of metals. It is concluded that moss and compost have improved growth while P. japonica improved metal accumulation and translocation to aerial parts with little involvement in plant growth.

Prospect of phytoremediation combined with other approaches for remediation of heavy metal-polluted soils

Accumulation of heavy metals in agricultural soils due to human production activities-mining, fossil fuel combustion, and application of chemical fertilizers/pesticides-results in severe environmental pollution. As the transmission of heavy metals through the food chain and their accumulation pose a serious risk to human health and safety, there has been increasing attention in the investigation of heavy metal pollution and search for effective soil remediation technologies. Here, we summarized and discussed the basic principles, strengths and weaknesses, and limitations of common standalone approaches such as those based on physics, chemistry, and biology, emphasizing their incompatibility with large-scale applications. Moreover, we explained the effects, advantages, and disadvantages of the combinations of common single repair approaches. We highlighted the latest research advances and prospects in phytoremediation-chemical, phytoremediation-microbe, and phytoremediation-genetic engineering combined with remediation approaches by changing metal availability, improving plant tolerance, promoting plant growth, improving phytoextraction and phytostabilization, etc. We then explained the improved safety and applicability of phytoremediation combined with other repair approaches compared to common standalone approaches. Finally, we established a prospective research direction of phytoremediation combined with multi-technology repair strategy.

Extended antibiotic treatment in salmon farms select multiresistant gut bacteria with a high prevalence of antibiotic resistance genes

The high use of antibiotics for the treatment of bacterial diseases is one of the main problems in the mass production of animal protein. Salmon farming in Chile is a clear example of the above statement, where more than 5,500 tonnes of antibiotics have been used over the last 10 years. This has caused a great impact both at the production level and on the environment; however, there are still few works in relation to it. In order to demonstrate the impact of the high use of antibiotics on fish gut microbiota, we have selected four salmon farms presenting a similar amount of fish of the Atlantic salmon species (Salmo salar), ranging from 4,500 to 6,000 tonnes. All of these farms used treatments with high doses of antibiotics. Thus, 15 healthy fish were selected and euthanised in order to isolate the bacteria resistant to the antibiotics oxytetracycline and florfenicol from the gut microbiota. In total, 47 bacterial isolates resistant to florfenicol and 44 resistant to oxytetracycline were isolated, among which isolates with Minimum Inhibitory Concentrations (MIC) exceeding 2048 μg/mL for florfenicol and 1024 μg/mL for oxytetracycline were found. In addition, another six different antibiotics were tested in order to demonstrate the multiresistance phenomenon. In this regard, six isolates of 91 showed elevated resistance values for the eight tested antibiotics, including florfenicol and oxytetracycline, were found. These bacteria were called “super-resistant” bacteria. This phenotypic resistance was verified at a genotypic level since most isolates showed antibiotic resistance genes (ARGs) to florfenicol and oxytetracycline. Specifically, 77% of antibiotic resistant bacteria showed at least one gene resistant to florfenicol and 89% showed at least one gene resistant to oxytetracycline. In the present study, it was demonstrated that the high use of the antibiotics florfenicol and oxytetracycline has, as a consequence, the selection of multiresistant bacteria in the gut microbiota of farmed fish of the Salmo salar species at the seawater stage. Also, the phenotypic resistance of these bacteria can be correlated with the presence of antibiotic resistance genes.