Stimulated phytoextraction of metals from fly ash by microbial interventions

Insights into heavy metal tolerance mechanisms of Brassica species: physiological, biochemical, and molecular interventions

Heavy metal (HM) contamination of soil due to anthropogenic activities has led to bioaccumulation and biomagnification, posing toxic effects on plants by interacting with vital cellular biomolecules such as DNA and proteins. Brassica species have developed complex physiological, biochemical, and molecular mechanisms for adaptability, tolerance, and survival under these conditions. This review summarizes the HM tolerance strategies of Brassica species, covering the role of root exudates, microorganisms, cell walls, cell membranes, and organelle-specific proteins. The first line of defence against HM stress in Brassica species is the avoidance strategy, which involves metal ion precipitation, root sorption, and metal exclusion. The use of plant growth-promoting microbes, Pseudomonas, Psychrobacter, and Rhizobium species effectively immobilizes HMs and reduces their uptake by Brassica roots. The roots of Brassica species efficiently detoxify metals, particularly by flavonoid glycoside exudation. The composition of the cell wall and callose deposition also plays a crucial role in enhancing HMs resistance in Brassica species. Furthermore, plasma membrane-associated transporters, BjCET, BjPCR, BjYSL, and BnMTP, reduce HM concentration by stimulating the efflux mechanism. Brassica species also respond to stress by up-regulating existing protein pools or synthesizing novel proteins associated with HM stress tolerance. This review provides new insights into the HM tolerance mechanisms of Brassica species, which are necessary for future development of HM-resistant crops.

Beneficial Microorganisms Improve Agricultural Sustainability under Climatic Extremes

The challenging alterations in climate in the last decades have had direct and indirect influences on biotic and abiotic stresses that have led to devastating implications on agricultural crop production and food security. Extreme environmental conditions, such as abiotic stresses, offer great opportunities to study the influence of different microorganisms in plant development and agricultural productivity. The focus of this review is to highlight the mechanisms of plant growth-promoting microorganisms (especially bacteria and fungi) adapted to environmental induced stresses such as drought, salinity, heavy metals, flooding, extreme temperatures, and intense light. The present state of knowledge focuses on the potential, prospective, and biotechnological approaches of plant growth-promoting bacteria and fungi to improve plant nutrition, physio-biochemical attributes, and the fitness of plants under environmental stresses. The current review focuses on the importance of the microbial community in improving sustainable crop production under changing climatic scenarios.

Phytoremediation of Heavy Metal-Contaminated Sites: Eco-environmental Concerns, Field Studies, Sustainability Issues, and Future Prospects

Environmental contamination due to heavy metals (HMs) is of serious ecotoxicological concern worldwide because of their increasing use at industries. Due to non-biodegradable and persistent nature, HMs cause serious soil/water pollution and severe health hazards in living beings upon exposure. HMs can be genotoxic, carcinogenic, mutagenic, and teratogenic in nature even at low concentration. They may also act as endocrine disruptors and induce developmental as well as neurological disorders, and thus, their removal from our natural environment is crucial for the rehabilitation of contaminated sites. To cope with HM pollution, phytoremediation has emerged as a low-cost and eco-sustainable solution to conventional physicochemical cleanup methods that require high capital investment and labor alter soil properties and disturb soil microflora. Phytoremediation is a green technology wherein plants and associated microbes are used to remediate HM-contaminated sites to safeguard the environment and protect public health. Hence, in view of the above, the present paper aims to examine the feasibility of phytoremediation as a sustainable remediation technology for the management of metal-contaminated sites. Therefore, this paper provides an in-depth review on both the conventional and novel phytoremediation approaches; evaluates their efficacy to remove toxic metals from our natural environment; explores current scientific progresses, field experiences, and sustainability issues; and revises world over trends in phytoremediation research for its wider recognition and public acceptance as a sustainable remediation technology for the management of contaminated sites in the twenty-first century.

Bacterial community shaped by heavy metals and contributing to health risks in cornfields

Scientists are increasingly aware that heavy metal contamination in soils, especially in farmland ecosystems, can negatively affect human health and alter the bacterial community that plays a critical role in plant growth and heavy metal accumulation. The goal of the present paper was to uncover how various heavy metals and non-metallic elements affect human health and bacterial diversity in cornfields and to explore the contribution of soil bacteria to heavy metal accumulation in crops. Soil samples were collected from five counties in Shandong Province, China, where abnormally high levels of heavy metals and metalloids were caused by mining and heavy industry. We calculated a hazard quotient (HQ) to evaluate the health risk these heavy metals cause and analyzed the soil bacterial community using 16S rRNA gene sequencing. The HQ results showed that As posed the greatest threat to human health followed by Pb although concentrations of all metals did not reach the health risk threshold. Meanwhile, principal component analysis (PCA) and redundancy analysis (RDA) revealed soil bacterial richness was significantly influenced by As, Ni, and Cr as well as pH and phosphorus, but not by the species diversity of aboveground weeds. The most abundant bacteria in our study region were heavy metal tolerant groups, specifically Actinobacteria and Proteobacteria. Moreover, correlation analysis suggested that Actinobacteria might reduce the phytoaccumulation of Cr, Cu, Zn, and Hg in corn, while Proteobacteria might weaken phytoaccumulation of Pb, Ni, As, and Cd. Our results verified that heavy metals play an important role in shaping the soil bacterial community. Using native bacteria in farmland provides a potential biological strategy for reducing the health risk posed by heavy metals related to food consumption.

Heavy Metal Stress, Signaling, and Tolerance Due to Plant-Associated Microbes: An Overview

Several anthropogenic activities including mining, modern agricultural practices, and industrialization have long-term detrimental effect on our environment. All these factors lead to increase in heavy metal concentration in soil, water, and air. Soil contamination with heavy metals cause several environmental problems and imparts toxic effect on plant as well as animals. In response to these adverse conditions, plants evolve complex molecular and physiological mechanisms for better adaptability, tolerance, and survival. Nowadays conventional breeding and transgenic technology are being used for development of metal stress resistant varieties which, however, are time consuming and labor intensive. Interestingly the use of microbes as an alternate technology for improving metal tolerance of plants is gaining momentum recently. The use of these beneficial microorganisms is considered as one of the most promising methods for safe crop-management practices. Interaction of plants with soil microorganisms can play a vital role in acclimatizing plants to metalliferous environments, and can thus be explored to improve microbe-assisted metal tolerance. Plant-associated microbes decrease metal accumulation in plant tissues and also help to reduce metal bioavailability in soil through various mechanisms. Nowadays, a novel phytobacterial strategy, i.e., genetically transformed bacteria has been used to increase remediation of heavy metals and stress tolerance in plants. This review takes into account our current state of knowledge of the harmful effects of heavy metal stress, the signaling responses to metal stress, and the role of plant-associated microbes in metal stress tolerance. The review also highlights the challenges and opportunities in this continued area of research on plant-microbe-metal interaction.

A study of subsurface wastewater infiltration systems for distributed rural sewage treatment

Three types of subsurface wastewater infiltration systems (SWIS) were developed to study the efficiency of organic pollutant removal from distributed rural sewage under various conditions. Of the three different layered substrate systems, the one with the greatest amount of decomposed cow dung (5%) and soil (DCDS) showed the highest removal efficiency with respect to total nitrogen (TN), where the others showed no significant difference. The TN removal efficiency was increased with an increasing filling height of DCDS. Compared with the TN removal efficiency of 25% in the system without DCDS, the removal efficiency of the systems in which DCDS filled half and one fourth of the height was increased by 72% and 31%, respectively. Based on seasonal variations in the discharge of the typical rural family, the SWIS were run at three different hydraulic loads of 6.5, 13 and 20 cm/d. These results illustrated that SWIS could perform well at any of the given hydraulic loads. The results of trials using different inlet configurations showed that the effluent concentration of the contaminants in the system operating a multiple-inlet mode was much lower compared with the system operated under single-inlet conditions. The effluent concentration ofa pilot-scale plant achieved the level III criteria specified by the Surface Water Quality Standard at the initial stage.