Biofilm mediated strategies to mitigate heavy metal pollution: A critical review in metal bioremediation

Emerging trends and future outlook on chromium removal in the lab, pilot scale, and industrial wastewater system: an updated review exploring 10 years of research

Chromium (Cr) is widely recognized as a carcinogenic metal, and numerous technologies have been studied on a lab scale to manage the pollution caused by Cr contamination in wastewater. However, the removal of Cr presents several challenges and limitations in industrial wastewater management. These issues highlight the ongoing need for research to discover more efficient methods for remediating Cr from wastewater. The proposed review summarizes the current limitations, gaps, and state-of-the-art technologies on Cr removal in industrial wastewater systems over the past 10 years. It aims to lay the groundwork for future research and innovation in Cr remediation for industrial applications. The review emphasizes that conventional physicochemical techniques are often insufficient and highlights the necessity of implementing advanced integrated systems. The limitations related to industrial scaling up are also deeply investigated. Special attention is given to differentiating research conducted at laboratory, pilot, and industrial levels. The findings reveal that limited research has been conducted on an industrial scale, with most investigations focusing on treating tannery and electroplating wastewater. A few studies have also been reported on wastewater from textile, mining, steel mills, pigments, and wood processing. Despite the existence of high-performance systems demonstrated in lab-scale studies, only a handful of treatment techniques have effectively removed Cr at an industrial scale. Nevertheless, innovative breakthroughs in advanced integrated systems show promise for improved performance in the future.

Bacterial biomineralization of heavy metals and its influencing factors for metal bioremediation

Increasing industrial pollution and certain hazardous agricultural practices have led to the discharge of heavy toxic metals into the environment. Among different bioremediation techniques, biomineralization is the synthesis of biomineral crystals extracellularly or intracellularly. Several bacteria, such as Bacillus cereus, Pseudomonas stutzeri, Bacillus subtilis, and Lactobacillus sphaericus have been found to induce heavy metal precipitation and mineralization for bioremediation. This article summarizes the different biomineralization mechanisms of bacterial-induced heavy metal biomineralization, mainly microbial-induced carbonate precipitation (MICP), microbial-induced phosphate precipitation (MIPP), and microbial-induced sulphide precipitation (MISP). Moreover, bacterial structures such as cell wall, biofilm, and extracellular polymeric substances (EPS) influence mineralization and control bacterial compartmentalization of heavy metal precipitation. Several genes control the efficiency of biomineralization in bacteria, such as ureA, ureB, ureC, phoA, dsrA, dsrB, dsrC, dsrD, dsrE, luxS, and ompR. This biomineralization mechanism provides new and broad prospects for its application in soil improvement, industrial applications, and wastewater treatments. In addition, bacterial genetic modification holds immense potential for advancing the biomineralization process to meet diverse environmental and industrial needs.

Characterization and Molecular Insights of a Chromium-Reducing Bacterium Bacillus tropicus

Environmental pollution from metal toxicity is a widespread concern. Certain bacteria hold promise for bioremediation via the conversion of toxic chromium compounds into less harmful forms, promoting environmental cleanup. In this study, we report the isolation and detailed characterization of a highly chromium-tolerant bacterium, Bacillus tropicus CRB14. The isolate is capable of growing on 5000 mg/L Cr (VI) in an LB (Luria Bertani) agar plate while on 900 mg/L Cr (VI) in LB broth. It shows an 86.57% reduction ability in 96 h of culture. It can also tolerate high levels of As, Cd, Co, Fe, Zn, and Pb. The isolate also shows plant growth-promoting potential as demonstrated by a significant activity of nitrogen fixation, phosphate solubilization, IAA (indole acetic acid), and siderophore production. Whole-genome sequencing revealed that the isolate lacks Cr resistance genes in their plasmids and are located on its chromosome. The presence of the chrA gene points towards Cr(VI) transport, while the absence of ycnD suggests alternative reduction pathways. The genome harbors features like genomic islands and CRISPR-Cas systems, potentially aiding adaptation and defense. Analysis suggests robust metabolic pathways, potentially involved in Cr detoxification. Notably, genes for siderophore and NRP-metallophore production were identified. Whole-genome sequencing data also provides the basis for molecular validation of various genes. Findings from this study highlight the potential application of Bacillus tropicus CRB14 for bioremediation while plant growth promotion can be utilized as an added benefit.

"Strategies for microbes-mediated arsenic bioremediation: Impact of quorum sensing in the rhizosphere"

Plant growth-promoting rhizobacteria (PGPR) are gaining recognition as pivotal agents in bioremediation, particularly in arsenic-contaminated environments. These bacteria leverage quorum sensing, an advanced communication system, to synchronize their activities within the rhizosphere and refine their arsenic detoxification strategies. Quorum Sensing enables PGPR to regulate critical processes such as biofilm formation, motility, and the activation of arsenic-resistance genes. This collective coordination enhances their capacity to immobilize, transform, and detoxify arsenic, decreasing its bioavailability and harmful effects on plants. Furthermore, quorum sensing strengthens the symbiotic relationship between growth-promoting rhizobacteria and plant roots, facilitating better nutrient exchange and boosting plant tolerance to stress. The current review highlights the significant role of quorum sensing in improving the efficacy of PGPR in arsenic remediation. Understanding and harnessing the PGPR-mediated quorum sensing mechanism to decipher the complex signaling pathways and communication systems could significantly advance remediation strategy, promoting sustainable soil health and boosting agricultural productivity.

Synergetic effects of chlorinated paraffins and microplastics on microbial communities and nitrogen cycling in deep-sea cold seep sediments

Chlorinated paraffins (CPs) and microplastics (MPs) are commonly found in deep-sea cold seep sediments, where nitrogen cycling processes frequently occur. However, little is known about their combined effects on sedimentary microbial communities and nitrogen cycling in these environments. This study aimed to investigate the synergistic impacts of CPs and MPs on microbial communities and nitrogen cycling in deep-sea cold seep sediments through microcosm experiments. Our results demonstrated that the presence of CPs and MPs induced significant alterations in microbial community composition, promoting the growth of Halomonas. Furthermore, CPs and MPs were found to enhance nitrification, denitrification and anammox processes, which was evidenced by the higher abundance of genes associated with nitrification and denitrification, as well as increased activity of denitrification and anammox in the CPs and MPs-treatment groups compared to the control group. Additionally, the enhanced influence of CPs and MPs on denitrification was expected to promote nitrate-dependent and sulfate-dependent anaerobic oxidation of methane, thereby resulting in less methane released into the environment. These findings shed light on the potential consequences of simultaneous exposure to CPs and MPs on biogeochemical nitrogen cycling in deep-sea cold seep sediments.

Microbial Primer: The catalytic biofilm matrix

The extracellular matrix of microbial biofilms has traditionally been viewed as a structural scaffold that retains the resident bacteria in the biofilm. Moreover, a role of the matrix in the tolerance of biofilms to antimicrobials and environmental stressors was recognized early in biofilm research. However, as research progressed it became apparent that the biofilm matrix can also be involved in processes such as bacterial migration, genetic exchange, ion capture and signalling. More recently, evidence has accumulated that the biofilm matrix can also have catalytic functions. Here we review foundational research on this fascinating catalytic role of the biofilm matrix.

An Overview of Bacteria-Mediated Heavy Metal Bioremediation Strategies

Contamination-free groundwater is considered a good source of potable water. Even in the twenty-first century, over 90 percent of the population is reliant on groundwater resources for their lives. Groundwater influences the economical state, industrial development, ecological system, and agricultural and global health conditions worldwide. However, different natural and artificial processes are gradually polluting groundwater and drinking water systems throughout the world. Toxic metalloids are one of the major sources that pollute the water system. In this review work, we have collected and analyzed information on metal-resistant bacteria along with their genetic information and remediation mechanisms of twenty different metal ions [arsenic (As), mercury (Hg), lead (Pb), chromium (Cr), iron (Fe), copper (Cu), cadmium (Cd), palladium (Pd), zinc (Zn), cobalt (Co), antimony (Sb), gold (Au), silver (Ag), platinum (Pt), selenium (Se), manganese (Mn), molybdenum (Mo), nickel (Ni), tungsten (W), and uranium (U)]. We have surveyed the scientific information available on bacteria-mediated bioremediation of various metals and presented the data with responsible genes and proteins that contribute to bioremediation, bioaccumulation, and biosorption mechanisms. Knowledge of the genes responsible and self-defense mechanisms of diverse metal-resistance bacteria would help us to engineer processes involving multi-metal-resistant bacteria that may reduce metal toxicity in the environment.

A mini-review on indigenous microbial biofilm from various wastewater for heavy-metal removal - new trends

Biofilm, as a form of the microbial community in nature, represents an evolutionary adaptation to the influence of various environmental conditions. In nature, the largest number of microorganisms occur in the form of multispecies biofilms. The ability of microorganisms to form a biofilm is one of the reasons for antibiotic resistance. The creation of biofilms resistant to various contaminants, on the other hand, improves the biological treatment process in wastewater treatment plants. Heavy metals cannot be degraded, but they can be transformed into non-reactive and less toxic forms. In this process, microorganisms are irreplaceable as they interact with the metals in a variety of ways. The environment polluted by heavy metals, such as wastewater, is also a source of undiscovered microbial diversity and specific microbial strains. Numerous studies show that biofilm is an irreplaceable strategy for heavy metal removal. In this review, we systematize recent findings regarding the bioremediation potential of biofilm-forming microbial species isolated from diverse wastewaters for heavy metal removal. In addition, we include some mechanisms of action, application possibilities, practical issues, and future prospects.

Microbial Biofilms for Environmental Bioremediation of Heavy Metals: a Review

Heavy metal pollution caused due to various industrial and mining activities poses a serious threat to all forms of life in the environment because of the persistence and toxicity of metal ions. Microbial-mediated bioremediation including microbial biofilms has received significant attention as a sustainable tool for heavy metal removal as it is considered safe, effective, and feasible. The biofilm matrix is dynamic, having microbial cells as major components with constantly changing and evolving microenvironments. This review summarizes the bioremediation potential of bacterial biofilms for different metal ions. The composition and mechanism of biofilm formation along with interspecies communication among biofilm-forming bacteria have been discussed. The interaction of biofilm-associated microbes with heavy metals takes place through a variety of mechanisms. These include biosorption and bioaccumulation in which the microbes interact with the metal ions leading to their conversion from a highly toxic form to a less toxic form. Such interactions are facilitated via the negative charge of the extracellular polymeric substances on the surface of the biofilm with the positive charge of the metal ions and the high cell densities and high concentrations of cell-cell signaling molecules within the biofilm matrix. Furthermore, the impact of the anodic and cathodic redox potentials in a bioelectrochemical system (BES) for the reduction, removal, and recovery of numerous heavy metal species provides an interesting insight into the bacterial biofilm-mediated bioelectroremediation process. The review concludes that biofilm-linked bioremediation is a viable option for the mitigation of heavy metal pollution in water and ecosystem recovery.

Advances in bioleaching of waste lithium batteries under metal ion stress

In modern societies, the accumulation of vast amounts of waste Li-ion batteries (WLIBs) is a grave concern. Bioleaching has great potential for the economic recovery of valuable metals from various electronic wastes. It has been successfully applied in mining on commercial scales. Bioleaching of WLIBs can not only recover valuable metals but also prevent environmental pollution. Many acidophilic microorganisms (APM) have been used in bioleaching of natural ores and urban mines. However, the activities of the growth and metabolism of APM are seriously inhibited by the high concentrations of heavy metal ions released by the bio-solubilization process, which slows down bioleaching over time. Only when the response mechanism of APM to harsh conditions is well understood, effective strategies to address this critical operational hurdle can be obtained. In this review, a multi-scale approach is used to summarize studies on the characteristics of bioleaching processes under metal ion stress. The response mechanisms of bacteria, including the mRNA expression levels of intracellular genes related to heavy metal ion resistance, are also reviewed. Alleviation of metal ion stress via addition of chemicals, such as spermine and glutathione is discussed. Monitoring using electrochemical characteristics of APM biofilms under metal ion stress is explored. In conclusion, effective engineering strategies can be proposed based on a deep understanding of the response mechanisms of APM to metal ion stress, which have been used to improve bioleaching efficiency effectively in lab tests. It is very important to engineer new bioleaching strains with high resistance to metal ions using gene editing and synthetic biotechnology in the near future.

Bioremediation of heavy metals polluted environment and decolourization of black liquor using microbial biofilms

BACKGROUND

With increased urbanization and industrialization, modern life has led to an anthropogenic impact on the biosphere. Heavy metals pollution and pollutants from black liquor (BL) have caused severe effects on environment and living organisms. Bacterial biofilm has potential to remediate heavy metals and remove BL from the environment. Hence, this study was planned to investigate the potential of microbial biofilms for the bioremediation of heavy metals and BL polluted environments.

METHODS AND RESULTS

Eleven biofilm forming bacterial strains (SB1, SB2, SC1, AF1, 5A, BC-1, BC-2, BC-3, BC-4, BC-5 and BC-6) were isolated and identified upto species level via 16S rRNA gene sequencing. Biofilm strains belonging to Bacillus and Lysinibacillus sphaericus were used to remediate heavy metals (Pb, Ni, Mn, Zn, Cu, and Co). Atomic absorption spectroscopy showed significantly high (P ≤ 0.05) bioremediation potential by L. sphaericus biofilm (1462.0 ± 0.67 µgmL^-1) against zinc (Zn). Similarly, Pseudomonas putida biofilm significantly (P ≤ 0.05) decolourized (65.1%) BL. Fourier transform infrared (FTIR) analysis of treated heavy metals showed the shifting of major peaks (1637 & 1629-1647, 1633 & 1635-1643, and 1638-1633 cm^-1) corresponding to specific amide groups due to C = O stretching.

CONCLUSION

The study suggested that biofilm of the microbial flora from tanneries and pulp paper effluents possesses a strong potential for heavy metals bioremediation and BL decolourization. To our knowledge, this is the first report showing promising biofilm remediation potential of bacterial flora of tanneries and pulp-paper effluent from Kasur and Sheikhupura, Punjab, Pakistan, against heavy metals and BL.

Applying fulvic acid for sediment metals remediation: Mechanism, factors, and prospect

Fulvic acid (FA) has been shown to play a decisive role in controlling the environmental geochemical behavior of metals. As a green and natural microbial metabolite, FA is widely used in environmental remediation because of its good adsorption complexation and redox ability. This paper introduces the reaction mechanism and properties of FA with metals, and reviews the progress of research on the remediation of metal pollutant by FA through physicochemical remediation and bioremediation. FA can control the biotoxicity and migration ability of some metals, such as Pb, Cr, Hg, Cd, and As, through adsorption complexation and redox reactions. The concentration, molecular weight, and source are the main factors that determine the remediation ability of FA. In addition, the ambient pH, temperature, metal ion concentrations, and competing components in sediment environments have significant effects on the extent and rate of a reaction between metals and FA during the remediation process. Finally, we summarize the challenges that this promising environmental remediation tool may face. The research directions of FA in the field of metals ecological remediation are also prospected. This review can provide new ideas and directions for the research of remediation of metals contaminants in sediments.

Simultaneously pollutant removal and S0 recovery from composite wastewater containing Cr(VI)-S2- based on biofilm enhancement

Bioaugmentation of extracellular polymeric substances-producing bacteria was applied in pollutant removal and S⁰ recovery from composite wastewater in a mixotrophic denitrification system. In the presence of 200 mg·L^-1 S^2- and 50 mg·L^-1 Cr(VI), the removal efficiencies of chemical oxygen demand, NO₃^-, S^2- and Cr(VI) were 86.38%, 91.82%, 95.75%, and 100.00% respectively, while S⁰ recovery efficiency reached 79.17%. Increased contents of protein and polysaccharide, especially the high ratio of protein/polysaccharide verified the structural stability of biofilm was promoted by biofilm enhancement. The widespread distribution of bacteria/extracellular polymeric substance (EPS) revealed the more obvious biofilms formation in biofilm-enhanced group. High-throughput sequencing analysis showed that EPS-producing bacteria (Flavobacterium, Thauera, Thiobacillus and Simplicispira) were dominant bacteria in the biofilm-enhanced group. Moreover, by comprehensive considering of redundancy analysis, the colonization of selected bacteria improved the robustness of the reactor and treatment performance to wastewater contained toxic pollutions.