Rhizobacterial communities associated with spontaneous plant species in long-term arsenic contaminated soils

Straw removal reduces Cd availability and rice Cd accumulation in Cd-contaminated paddy soil: Cd fraction, soil microorganism structure and porewater DOC and Cd

The impacts of straw removal on rice Cd absorption, behaviour of Cd and microbial community in rhizosphere soil were investigated in paddy fields over two consecutive seasons. The results of the experiments in two fields revealed that straw removal promoted the transformation of soil Cd from acid-extractable and oxidisable fraction to residual fraction and reduced soil DTPA-Cd content with the reduction in DOC and Cd ions in soil porewater, thereby decreasing Cd content in rice. Specifically, the Cd content in brown rice of early rice was below 0.2 mg·kg-1 when all rice straw and roots were removed in the slightly Cd-contaminated soils. The α-diversity of soil microbial communities was less influenced by continuous straw removal, β-diversity was altered and the relative abundances of Anaeromyxobacter, Methylocystis and Mycobacterium microbes were increased. Redundancy analysis and network analysis exhibited that soil pH predominantly influenced the microbial community. Path analysis revealed that the Cd content in brown rice could be directly influenced by the soil Total-Cd and DTPA-Cd, as well as soil pH and OM. Straw removal, including roots removal, is an economical and effective technique to reduce Cd accumulation in rice plants.

Effect of cadmium stress on the bacterial community in the rhizosphere of mulberry (Morus alba L.)

Mulberry has a good tolerance to cadmium (Cd) and is considered a candidate plant for phytoremediation. The rhizosphere microbial community plays an important role in phytoremediation. Nevertheless, little information on the rhizosphere microbial community mechanisms in mulberry during the phytoremediation of Cd-contaminated soil is available. In this study, the remediation efficiency of mulberry in pots subjected to three simulated Cd pollution levels and their rhizosphere bacterial communities during the remediation process were analyzed. "Yuesang 11" was used as the test mulberry variety, and three simulated Cd pollution levels were set by adding three concentrations of Cd (Cd5, 5 mg kg-1; Cd3, 3 mg kg-1; Cd2, 2 mg kg-1). The results showed that the elimination rates of Cd in the rhizosphere soil were 81.7%, 85.3%, and 57.9% under the stress of the Cd2, Cd3, and Cd5 conditions, respectively. Meanwhile, 3,082,583 high-quality sequence reads and 976 operational taxonomic units were successfully obtained from the mulberry rhizosphere soil by high-throughput absolute quantification sequencing and further assigned to 11 bacterial phyla and 26 families. Of these, decreased abundances of 19 bacteria at the family level and increased abundances of seven bacteria under Cd stress were revealed by comparative analysis. Based on the alpha diversity indices (Chaol, Shannon and Simpson) and principal component analysis, the rhizosphere bacterial diversity of the Cd5 condition was significantly decreased, but that of the Cd2 and Cd3 conditions was not different from that of soil without Cd (CK). Likewise, redundancy analysis showed that the abundances of Acidobacteria Gp2, Acidobacteria Gp13, and Sphingobacteria were significantly positively associated with the elimination rates of Cd. This study suggested that the mulberry rhizosphere contains a relatively stable bacterial community consisting of diverse Cd-resistant bacteria, providing a scientific basis for remediating heavy-metal polluted soils using mulberry.

Spontaneous Plant Diversity in Urban Contexts: A Review of Its Impact and Importance

To promote sustainability in urban green spaces, it is necessary to know the diversity of spontaneous species in these spaces. Based on the investigation and analysis of the relevant scientific literature, the diversity of spontaneous species and their importance was contextually discussed, along with the assessment of local biodiversity impact in green spaces. Studies on green spaces, spontaneous flora, biodiversity or ecosystem services, and studies on exotic species and adaptability were summarized. Finally, the existing issues regarding biodiversity and urbanization, and the role of spontaneous plants in restoring industrial areas were discussed. Based on the research carried out, it is considered that green spaces contain unique and useful biodiversity resulting from their management. Spontaneous flora can be a generator of plants with aesthetic character, which can be grown in an ecologically sound way in private gardens and natural spaces in town and village zones, with certain remarkable farming-biological characteristics (ecological plasticity, high hardiness, etc.). Biodiversity is a relevant feature of urban landscapes, offering multiple gains, and the conservation of this biodiversity in urban green spaces is fundamental and requires an integrated approach. However, urbanisation usually has a detrimental influence on local species’ diversity.

Nanoparticles as a potential protective agent for arsenic toxicity alleviation in plants

Aggrandized technological and industrial progression in past decades have occasioned immense depreciation in the quality of environment and ecosystem, majorly due to augmentation in the number of obnoxious pollutants incessantly being released in soil, water or air. Arsenic (As) is one such hazardous metalloid contaminating the environment which has the potential to detrimentally affect the life on earth. Even in minute quantity, As is known to cause various critical diseases in humans and toxicity in plants. Recent studies on nanoparticles (NPs) approve of their ability to qualify the criterion of becoming a potent tool for mitigating As-induced phytotoxicity. Nanoparticles are reported to promote plant growth under As-stress by stimulating various alterations at physiological, biochemical, and molecular levels. In this review, we provide an up-to-date compilation of research that has been carried out in comprehending the mechanisms utilized by nanoparticles including controlled As uptake and distribution in plants, maintenance of ROS homeostasis during stress and chelation and vacuolar sequestration of As so as to reduce the severity of toxicity induced by As, and potential areas of research in this field will also be indicated for future perspectives.

Research and application of arsenic-contaminated groundwater remediation by manganese ore permeable reactive barrier

Arsenic pollution in the water environment is one of the important environmental problems at present. High arsenic groundwater and its resulting local arsenic poisoning have caused a great threat to human life and health. The permeable reactive barrier (PRB) is an underground in-situ remediation technology, which has the advantages of high efficiency, low energy consumption, long aging, low operating and maintenance costs. By studying the arsenic removal effects of different materials, this paper selected natural manganese ore, manganese ore granulation, loaded manganese ore and mixed manganese ore as fillers for PRB. And it conducted a simulated experiment to study the feasibility of actual PRB engineering to repair arsenic-containing groundwater. The experiment proves that the removal rate of arsenic by four manganese ore materials exceeds 90%. After examining the geographical location and hydrogeological conditions of the PRB project, the Dengjiatang area of Chenzhou City, Hunan Province was selected as the construction area. Studies show that after the completion of PRB, the arsenic content of the effluent at each monitoring point is below 10 μg/L. It indicates that all four fillers achieve the purpose of removing arsenic, and can be applied to the project according to actual needs. Finally, the safety evaluation of the PRB project was carried out. And FeCl₃·6H₂O was selected as the base curing material and cement was as the process auxiliary stabilizer to solidify the arsenic-containing waste residue. The arsenic concentration in the leaching solution of the arsenic slag after curing is only 1 μg/L.

Abatement of arsenic-induced phytotoxic effects in rice seedlings by an arsenic-resistant Pantoea dispersa strain

Population detonation and rapid industrialization are the major factors behind the reduction in cultivable land that affects crop production seriously. This situation is further being deteriorated due to the negative effects of abiotic stresses. Under such conditions, plant growth-promoting rhizobacteria (PGPR) are found to improve crop production which is essential for sustainable agriculture. This study is focused on the isolation of potent arsenic (As)-resistant PGPR from the agricultural land of West Bengal, India, and its application to reduce As translocation in rice seedlings. After screening, an As-resistant PGPR strain AS18 was identified by phenotypic characters and 16S rDNA sequence-based homology as Pantoea dispersa. This strain displayed nitrogen fixation, phosphate solubilization, 1-aminocyclopropane-1-carboxylic acid deaminase (ACCD) activity, indole-3-acetic acid (IAA) production, in addition to As (III) resistance up to 3750 μg/mL. The As removal efficiency of this strain was up to 93.12% from the culture medium as evidenced by AAS. The bioaccumulation property of AS18 strain was further validated by TEM-EDAX-XRD-XRF-FTIR studies. This strain showed significant morpho-biochemical improvements including antioxidant enzymatic activities and As-minimization in plant (rice) cells. Thus, being an As-resistant potent PGPR, AS18 strain is expected to be applied in As-spiked agricultural fields for bioremediation and phytostimulation.

Niche Differentiation in the Composition, Predicted Function, and Co-occurrence Networks in Bacterial Communities Associated With Antarctic Vascular Plants

Climate change directly affecting the Antarctic Peninsula has been reported to induce the successful colonization of ice-free lands by two Antarctic vascular plants (Deschampsia antarctica and Colobanthus quitensis). While studies have revealed the importance of microbiota for plant growth and stress tolerance in temperate climates, the role that plant-associated microbes play in the colonization of ice-free lands remains unknown. Consequently, we used high-throughput DNA sequence analyses to explore the composition, predicted functions, and interactive networks of plant-associated microbial communities among the rhizosphere, endosphere, and phyllosphere niches of D. antarctica and C. quitensis. Here we report a greater number of operational taxonomic units (OTUs), diversity, and richness in the microbial communities from the rhizosphere, relative to endosphere and phyllosphere. While taxonomic assignments showed greater relative abundances of Proteobacteria, Bacteroidetes, and Actinobacteria in plant niches, principal coordinate analysis revealed differences among the bacterial communities from the other compartments examined. More importantly, however, our results showed that most of OTUs were exclusively found in each plant niche. Major predicted functional groups of these microbiota were attributed to heterotrophy, aerobic heterotrophy, fermentation, and nitrate reduction, independent of plant niches or plant species. Co-occurrences network analyses identified 5 (e.g., Microbacteriaceae, Pseudomonaceae, Lactobacillaceae, and Corynebacteriaceae), 23 (e.g., Chitinophagaceae and Sphingomonadaceae) and 7 (e.g., Rhodospirillaceae) putative keystone taxa present in endosphere, phyllosphere, and rhizosphere, respectively. Our results revealed niche differentiation in Antarctic vascular plants, highlighting some putative microbial indicators and keystone taxa in each niche. However, more studies are required to determine the pivotal role that these microbes play in the successful colonization of ice-free lands by Antarctic plants.

The role of arsenic resistant Bacillus aryabhattai MCC3374 in promotion of rice seedlings growth and alleviation of arsenic phytotoxicity

The biological agents have been utilized as an affordable alternative to conventional costly metal remediation technologies for last few years. The present investigation introduces arsenic (As) resistant plant growth promoting rhizobacteria (PGPR) isolated from the As-contaminated agricultural field of West Bengal, India that alleviates arsenic-induced toxicity and exhibited many plant growth promoting traits (PGP). The isolated strain designated as AS6 has identified as Bacillus aryabhattai based on phenotypic characteristics, physio-biochemical tests, MALDI-TOFMS bio-typing, FAME analysis and 16S rDNA sequence homology. The strain found to exhibit five times more resistance to arsenate than arsenite with minimum inhibitory concentrations (MIC) being 100 mM and 20 mM respectively. The result showed that accumulation of As was evidenced by SEM- EDAX, TEM-EDAX studies. The intracellular accumulation of arsenic was also confirmed as in bacterial biomass by AAS, FTIR, XRD and XRF analyses. The increased rate of As (V) reduction by this strain found to be exploited for the remediation of arsenic in the contaminated agricultural field. The strain also found to exhibit important PGP traits viz., ACC deaminase activity (2022 nmol α-ketobutyrate/mg protein/h), IAA production (166 μg/ml), N₂ fixation (0.32 μgN fixed/h/mg proteins) and siderophore production (72%) etc. Positive influenced of AS6 strain on rice seedlings growth promotion under As stress was observed considering the several morphological, biochemical parameters including antioxidants activities as compared with an uninoculated set. Thus this strain might be exploited for stress amelioration and plant growth enhancement of rice cultivar under arsenic spiked agricultural soil.

Do apes smell like humans? The role of skin bacteria and volatiles of primates in mosquito host selection

Anthropophilic mosquitoes are effective vectors of human diseases because of their biting preference. To find their host, these mosquitoes are guided by human odours, primarily produced by human skin bacteria. By analysing the skin bacterial and skin volatile profiles of humans, bonobos, chimpanzees, gorillas, lemurs and cows, we investigated whether primates that are more closely related to humans have a skin bacterial community and odour profile that is similar to humans. We then investigated whether this affected discrimination between humans and closely related primates by anthropophilic and zoophilic mosquitoes that search for hosts. Humans had a lower skin bacterial diversity than the other animals and their skin bacterial composition was more similar to the other primates than to the skin bacterial composition of cows. Like the skin bacterial profiles, the volatile profiles of the animal groups were clearly different from each other. The cow and lemur volatile profiles were more closely related to the human profiles than expected. Human volatiles were indeed preferred above cow volatiles by anthropophilic mosquitoes and no preference was observed when tested against non-human primate odour, except for bonobo volatiles that were preferred over human volatiles. Unravelling the differences between mosquito hosts and their effect on host selection is important for a better understanding of cross-species transmission of vector-borne diseases.

Novel arsenic-transforming bacteria and the diversity of their arsenic-related genes and enzymes arising from arsenic-polluted freshwater sediment

Bacteria are essential in arsenic cycling. However, few studies have addressed 16S rRNA and arsenic-related functional gene diversity in long-term arsenic-contaminated tropical sediment. Here, using culture-based, metagenomic and computational approaches, we describe the diversity of bacteria, genes and enzymes involved in AsIII and AsV transformation in freshwater sediment and in anaerobic AsIII- and AsV-enrichment cultures (ECs). The taxonomic profile reveals significant differences among the communities. Arcobacter, Dechloromonas, Sedimentibacter and Clostridium thermopalmarium were exclusively found in ECs, whereas Anaerobacillus was restricted to AsV-EC. Novel taxa that are both AsV-reducers and AsIII-oxidizers were identified: Dechloromonas, Acidovorax facilis, A. delafieldii, Aquabacterium, Shewanella, C. thermopalmarium and Macellibacteroides fermentans. Phylogenic discrepancies were revealed among the aioA, arsC and arrA genes and those of other species, indicating horizontal gene transfer. ArsC and AioA have sets of amino acids that can be used to assess their functional and structural integrity and familial subgroups. The positions required for AsV reduction are conserved, suggesting strong selective pressure for maintaining the functionality of ArsC. Altogether, these findings highlight the role of freshwater sediment bacteria in arsenic mobility, and the untapped diversity of dissimilatory arsenate-reducing and arsenate-resistant bacteria, which might contribute to arsenic toxicity in aquatic environments.

Arsenic-contaminated freshwater: assessing arsenate and arsenite toxicity and low-dose genotoxicity in Gammarus elvirae (Crustacea; Amphipoda)

Arsenic (As) contamination of freshwater is largely due to geogenic processes, but As is also released into the environment because of improper anthropic activities. The European regulatory limits in drinking water are of 10 μg L^-1 As. However, knowledge of the genotoxic effects induced by low doses of As in freshwater environments is still scanty. This study was designed to investigate arsenate (As(V)) and arsenite (As(III)) toxicity and low-dose genotoxicity in Gammarus elvirae, which has proved to be a useful organism for genotoxicity assays in freshwater. As(V) and As(III) toxicity was assessed on the basis of the median lethal concentration, LC(50), while estimates of DNA damage were based on the Comet assay. The G. elvirae LC (50-240 h) value we calculated was 1.55 mg L^-1 for As(V) and 1.72 mg L^-1 for As(III). Arsenic exposure (240 h) at 5, 10, and 50 µg L^-1 of As in assays with either arsenate or arsenite-induced DNA damage in hemocytes of G. elvirae in a concentration-dependent manner. Our study provides a basis for future genotoxic research on exposure to freshwater that contains low levels of arsenic.