Deciphering the bacterial microbiome in response to long-term mercury contaminated soil.

Hg contaminated soils are of concern due to the toxic effects on soil microbes. Currently, the adaptation of bacterial community to long-term Hg contamination remains largely unknown. Here, we assessed the effects of Hg contaminated soils on the bacterial communities under controlled conditions using 16S rRNA gene amplicon sequencing. The results showed that the bacterial α-diversity and richness were significant positively correlated with total Hg (p < 0.05). Land-use type, pH, EC, TK, and nitrate-N played important roles in shaping the bacterial communities. Long-term Hg-contaminated soils can be divided into three types based on land use types: slag type, farmland type, and mining area type. The dominant phyla include Proteobacteria, Actinobacteriota, Acidobacteriota, Chloroflexi, and Firmicutes. The dominant genera identified were Pseudomonas, Gaiella, Sphingomonas, Bacillus, Arthrobacter, Nocardioides. Network analysis showed that dominant taxa had non-random co-occurrence patterns and module 1 had an important role in responding Hg stress. Keystone genera identified were Bauldia, Phycicoccus, Sphingomonas, Gaiella, Nitrospira. The above results further our understanding of the adaptation of the bacterial community in long-term Hg-contaminated soil. This study has important guiding significance for the use of bacterial consortia to remediate Hg-contaminated soil.

Effects of Pseudomonas alkylphenolica KL28 on immobilization of Hg in soil and accumulation of Hg in cultivated plant.

OBJECTIVE: The available content of mercury (Hg) in farmland soil is directly related to the safety of agricultural products. Meanwhile, humans may accumulate high concentrations of Hg through the food chain, resulting in health damage. Regarding the remediation technologies of Hg-contaminated soil, research and development is mainly concentrated on the immobilisation of Hg in soil and efficient extraction by accumulators. Therefore, in this work, the highly Hg-tolerant strain Pseudomonas alkylphenolica KL28 was used to study the removal effect of Hg in a solution, immobilization effect of Hg in soil, and its effect on growth, Hg accumulation and photosynthetic characteristics of Brassica campestris L. RESULTS: KL28 could effectively remove Hg2+ in the solution, with the removal ratio of 96.0% at 24 h. This strain could reduce decreases in shoots' and roots' dry weights by 31% and 16%, respectively, at a Hg concentration of 20 mg/L. The available Hg in the soil decreased to 4.7-9.4% in 8 days treated with KL28 bacterial solution at a dosage of 100 L/hm2. Meanwhile, with increases in Hg concentrations, Fv/Fm, Y(II), Y(I) and Y(NPQ) in the leaves of B. campestris showed a downward trend while Y(ND) and Y(NO) displayed an upward trend. Under the stress of 20 mg/L Hg2+, KL28 could reduce the Fv/Fm from 11.2 to 6.1%. CONCLUSIONS: KL28 could effectively remove Hg in the solution, immobilize Hg in soil, promote growth, decrease Hg accumulation and affect photosynthetic characteristics of B. campestris, indicating its potential use in Hg contaminated soils.

Characterization of cadmium-resistant rhizobacteria and their promotion effects on Brassica napus growth and cadmium uptake.

Excessive cadmium (Cd) accumulation in soil can adversely affect plants, animals, microbes, and humans; therefore, novel and uncharacterized Cd-resistant plant-growth-promoting rhizobacteria (PGPR) are required to address this issue. In the paper, 13 bacteria were screened, their partial 16S rRNA sequences determined, and the isolates, respectively, clustered into Curtobacterium (7), Chryseobacterium (4), Cupriavidus (1), and Sphingomonas (1). Evaluation of PGP traits, including indole-3-acetic acid (IAA) production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, siderophore secretion, and cyanhydric acid production, identified Cupriavidus necator GX_5, Sphingomonas sp. GX_15, and Curtobacterium sp. GX_31 as promising candidates for PGPR based on high IAA or ACC deaminase production. Additionally, root-elongation assays indicated that inoculating GX_5, _15, or _31 increased Brassica napus root length both in the presence and absence of Cd by 19.75-29.96% and 19.15-31.69%, respectively. Pot experiments indicated that inoculating B. napus with GX_5, _15, and _31 significantly increased the dry weight of above-ground tissues and root biomass by 40.97-85.55% and 18.99-103.13%, respectively. Moreover, these isolates significantly increased Cd uptake in the aerial parts and root tissue of B. napus by 7.38-11.98% and 48.09-79.73%, respectively. These results identified GX_5, _15, or _31 as excellent promoters of metal remediation by using microorganism-associated phytoremediation.

Effect of organic substrates on available elemental contents in nutrient solution.

In this paper, the changes of available elemental contents in the nutrient solution extracts of organic substrates (peat moss, charred rice husk, chicken manure, sawdust, turfgrass clipping and weathered coal) were studied and compared with that in the water extracts. Results showed that available elemental contents in the nutrient solution extracts are significantly different between organic substrates, whereas ionic concentrations are basically under steady condition after treatment for 36-108 h. Ionic contents in the nutrient solution extracts are not equal to the value of adding ionic concentrations in the supplied nutrient solution to that in the water extract. Thus, a mathematical model was proposed for adjusting the composition of supplied nutrient solution to match plant requirements in the organic soilless culture system.

Adsorption of cadmium by live and dead biomass of plant growth-promoting rhizobacteria.

Plant growth-promoting rhizobacteria (PGPR) have been extensively investigated in combination remediation with plants in heavy metal contaminated soil. However, being biosorbent, few studies of live and dead cells of PGPR have been undertaken. Meanwhile, the application of live or dead biomass for the removal of heavy metals continues to be debated. Therefore, this study uses living and non-living biosorbents of Cupriavidus necator GX_5, Sphingomonas sp. GX_15, and Curtobacterium sp. GX_31 to compare their Cd(ii) adsorption capacities by SEM-EDX, FTIR, and adsorption experiments. In the present study, whether the cells were living or dead and whatever the initial Cd(ii) concentration was, removal efficiency and adsorption capacity can be arranged as GX_31 > GX_15 > GX_5 (p < 0.05). However, removal efficiency in live and dead biosorbents was quite different and it greatly affected by the initial Cd(ii) concentrations. The dead cells exhibited a higher adsorption capacity than the live cells of GX_31. Nevertheless, for GX_5 and GX_15, the loading capacity of the non-living biomass was stronger than that of the living biomass at 20 mg L-1 of Cd(ii), but the capacity was similar at 100 mg L-1 of Cd(ii). Minor changes of spectra were found after autoclaving and it seemed that more functional groups of the dead biosorbent were involved in Cd(ii) binding by FTIR analysis, which also illustrated that the hydroxyl, amino, amide, and carboxyl groups played an important role in complexation with Cd(ii). Based on these findings, we concluded that the dead cells were more potent for Cd(ii) remediation, especially for GX_31.

Biosorption and bioaccumulation characteristics of cadmium by plant growth-promoting rhizobacteria.

Plant growth-promoting rhizobacteria (PGPR) not only promote growth and heavy metal uptake by plants but are promising biosorbents for heavy metals remediation. However, there exist arguments over whether extracellular adsorption (biosorption) or intracellular accumulation (bioaccumulation) play dominant roles in Cd(ii) adsorption. Therefore, three cadmium-resistant PGPR, Cupriavidus necator GX_5, Sphingomonas sp. GX_15, and Curtobacterium sp. GX_31 were used to study bioaccumulation and biosorption mechanisms under different initial Cd(ii) concentrations, using batch adsorption experiments, desorption experiments, scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX) spectroscopy, transmission electron microscopy (TEM), and Fourier-transform infrared (FTIR) spectroscopy. In this study, with the increase of the initial Cd(ii) concentrations, the removal efficiency of strains decreased and the adsorption capacity improved. The highest Cd(ii) removal efficiency values were 25.05%, 53.88%, and 86.06% for GX_5, GX_15, and GX_31 with 20 mg l-1 of Cd(ii), while the maximum adsorption capacity values were 7.97, 17.13, and 26.43 mg g-1 of GX_5, GX_15, and GX_31 with 100 mg l-1 of Cd(ii). Meanwhile, the removal efficiency and adsorption capacity could be ordered as GX_31 > GX_15 > GX_5. The dominant adsorption mechanism for GX_5 was bioaccumulation (50.66-60.38%), while the dominant mechanisms for GX_15 and GX_31 were biosorptions (60.29-64.89% and 75.93-79.45%, respectively). The bioaccumulation and biosorption mechanisms were verified by SEM-EDX, TEM and FTIR spectroscopy. These investigations could provide a more comprehensive understanding of metal-bacteria sorption reactions as well as practical application in remediation of heavy metals.

Effect of chitosan on the available contents and vertical distribution of Cu2+ and Cd2+ in different textural soils.

Chitosan, an environment-friendly biopolymer, has been adopted to remedy contaminated soils by heavy metals of Cu(2+) and Cd(2+). Experimental results demonstrated that, within the first 7d, available Cu(2+) and Cd(2+) contents in three textural soils (clay, loam, and sandy soil) decreased significantly after chitosan application. Moreover, the available Cu(2+) and Cd(2+) contents in soil layers of 14-16 cm and 24-26 cm were significantly reduced than that in 4-6 cm after 7d of chitosan application. Our investigation suggested that application of 0.9 g chitosan kg(-1) DW soil for 7d could be perfect for the remediation of the soil contaminated by Cu(2+) and Cd(2+).