Bacterial Assisted Phytoremediation

Abstract

Bacterial assisted phytoremediation describes how bacteria, particularly those found in the rhizosphere, can assist plants known as hyperaccumulators in removing heavy metal contamination from the environment. The function of hyperaccumulation is dependent not only on the plant, but also on the interaction of plant roots with rhizosphere microbes and soil bioavailable metal concentrations. Bioremediation is the process of utilizing microorganisms, fungi, green plants, or their enzymes to repair the natural environment that has been harmed by contaminants to its original state. The best plant species for phytoremediation should be hardy, produce a lot of biomass, be resistant to the toxic effects of metals and contaminants, be unappealing to herbivores. Plant growth-promoting bacteria can encourage soil fertility and health, improve plant diseases. They promote the phytoremediation process either by reducing the toxicity of pollutants or increasing the availability of pollutants or promoting the growth of plants. Metal ions bind to the cell wall’s functional groups (amine, carboxyl, hydroxyl, phosphate, sulfate, amine). The effects of stresses, which are brought on whenever changes in metabolism occur, are avoided by plants using a variety of tolerance mechanims and pathways called phytohormone. Heavy metal phytoextraction involves the following steps: Intake of heavy metals by plant roots, translocation of heavy metal ions from roots to aerial parts of plants, and sequestration and compartmentation of heavy metal ions in plant tissue. As bacterial siderophores aid in reducing the stress caused by metal contaminants. Rhizosphere acidification is a common mechanism used by plant with rhizosphere’s bacteria in dealing with low Phosphorus stress, to activate and increase the efficiency of soil Phosphorus utilization. The interaction between bacteria and plants has been found to be helpful in handling various pollutants in various exosystems.

1. Introduction

Heavy metal contamination of soil has resulted from anthropogenic activities such as industrialization and agricultural practices. Heavy metals are typically defined as metals with densities greater than 5 g/cm³ [1]. Metals are classified as essential or nonessential. Essential metals are metals that are required for normal cellular growth, such as zinc, nickel, copper, and so on. These heavy metals are required in low concentrations (nM), but at higher concentrations (M to mM), they are harmful to organisms [2]. Nonessential metals are metals that have no known biological function, such as lead, cadmium, and mercury [3]. At any concentration, these metals are toxic. Heavy metals have severe toxic effects on plants, animals, and human health, making remediation critical. In this context, insitu stabilization has been regarded as an effective method of remediating metal contaminated soil [4]. Among the various techniques used, phytoremediation is regarded as one of the safest, most innovative, and effective tools for heavy metal remediation. Nonessential metals include, for example, lead, cadmium, and mercurypollution has increased dramatically as industries such as mining have expanded, particularly in developing country cities. Different approaches to removing these toxic contaminants from our environment are being considered. This situation could be resolved and accelerated by investigating the plant bacteria partnership, which would improve plant growth in toxic heavy metal polluted environments and, ultimately, improve phytoremediation efficiency [5]. Bacterial assisted phytoremediation describes how bacteria, particularly those found in the rhizosphere, can assist plants known as hyperaccumulators in removing heavy metal contamination from the environment. The function of hyperaccumulation is dependent not only on the plant, but also on the interaction of plant roots with rhizosphere microbes and soil bioavailable metal concentrations [6]. Bacteria aided Phytoremediation is a lowcost and environmentally friendly method of combating heavy metal contamination in soil. Using a plant and beneficial bacteria to remediate heavy metals in soil [7]. Beneficial bacteria help plants absorb metals by mobilizing heavy metals in the soil and increasing plant vigor. Nonetheless, in unsterilized soil, effective interaction between plant and bacteria is not guaranteed, affecting phytoremediation efficiency [7]. Beneficial bacteria help plants absorb metals by mobilizing heavy metals in the soil and increasing plant vigor. Nonetheless, in unsterilized soil, effective interaction between plant and bacteria is not guaranteed, affecting phytoremediation efficiency [8, 9].

Plant-associated bacteria have the potential to promote plant growth and resistance to stress. By controlling growth hormone, ensuring nutrition security, producing siderophore and secondary metabolites, and improving the antioxidant enzyme system, these bacteria have the potential to improve plant growth. Bacteria assisted phytoremediation is a widely used integrated technology that uses bacteria and plants to degrade contaminants while having the least possible impact on the environment.

It is an effective method for cleaning up polluted soil. Sites contaminated with heavy metals, hydrocarbons, and pesticides are examples. Endophytic bacteria and rhizospheric bacteria are two types of bacteria that aid in phytoremediation. Plants and bacteria are both biological agents for heavy metal contamination control [10]. The process of removing heavy metals from soil using only bacteria is known as biosorption, whereas the process of removing heavy metals using plants is known as phytoremediation. These two processes will be discussed in more detail shortly.

1.1 Bioremediation

Bioremediation is a potential and prospective method for the treatment of contaminated environments using microbes and plants for biodegradation or biotransformation of contaminants. Chemical and physical methods of remediation are not environmentally sustainable. Bioremediation, in contrast to these methods, relies on biological processes that are mediated by various groups of living organisms to achieve the permanent removal of these pollutants [11]. The process known as “bioremediation” involves using microorganisms to reduce the amount of hazardous environmental waste that is present at a contaminated site [12]. Bioremediation is the process of utilizing microorganisms, fungi, green plants, or their enzymes to repair the natural environment that has been harmed by contaminants to its original state. Heavy metal bioremediation is the process of removing heavy metals from soil and waste water through metabolically mediated or physico-chemical pathways. The biological potentials of algae, bacteria, fungi, and yeasts are demonstrating their efficacy in the removal of metal from waste waters [13]. With the ability to degrade contaminants using natural microbial activity by various consortia of microbial strains, bioremediation is a very good alternative to conventional methods for removing heavy metal from the environment and media [14]. The most common biosorbents are living and dead microbial cells of bacteria, algae, and fungi. In this context, we’ll concentrate on one type of microorganism, bacteria. The bacterial cell wall is the first component to come into contact with the metal ion during biosorption. Metal ions bind to the cell wall’s functional groups (amine, carboxyl, hydroxyl, phosphate, sulfate, amine) [15]. The general metal uptake process involves metal ions binding to reactive groups on the bacterial cell wall, followed by metal ion incorporation inside the cell [16]. Bacteria have evolved a number of efficient systems for detoxifying metal ions; these resistance mechanisms are developed primarily for survival. Several living and dead bacterial strains have been studied and found to be effective at heavy metal biosorption. Several physiochemical parameters, including pH, temperature, biomass dosage, initial metal concentration, contact time, heavy metal type, and biosorbent type, have been reported to influence heavy metal biosorption by bacterial biomass processes. The functional groups that are present on the surface of the biosorbent and the structure of proteins can both be influenced by temperature. PH has an impact on the active binding sites and solubility of heavy metal ions in biomass. Numerous investigations have been made into the biosorption capacities and efficacies of different bacterial strains, as well as the influence of the process’s influencing factors. According to the data that has been published, there is not a clear focus on the biosorption of heavy metals by bacterial strains that could reveal the best bacterial biosorbent and ideal physio-chemical conditions for the removal of each heavy metal.

1.2 Phytoremediation

Plants, which is both economically advantageous and environmentally beneficial [17]. In order to remove or degrade the pollutants, phytoremediation employs a variety of mechanisms, such as degradation (phytodegradation, rhizodegradation), accumulation (phytoextraction, rhizofiltration), dissipation (phytovolatilization), and immobilization (hydraulic control and phytostabilization). Plants use one or more of these mechanisms to lower the concentrations of contaminants in soil and water, depending on the contaminants. For instance, plants can absorb and store heavy metals in their tissues and degrade organic pollutants to lessen their toxicity in soil and water resources [18]. Depending on the types of contaminants and media, different plants use various techniques or combinations of them to remediate soil and water. Rhizofiltration, phytodegradation, phytovolatilization, rhizodegradation, and phytodegradation are all methods for cleaning up contaminated ground water. Rhizofiltration, phytodegradation, or rhizodegradation are three treatment options for surface and wastewater contamination. Surface and wastewater contaminations can be treated by rhizofiltration, phytodegradation or rhizodegradation. Through phytoextraction, phytodegradations, phytostabilization, rhizodegradation, or phytovolatilization, contamination caused by soil, sediments, or sludge is remedied. The best plant species for phytoremediation should be hardy, produce a lot of biomass, be resistant to the toxic effects of metals and contaminants, be simple to grow, have a high absorption capacity, and be unappealing to herbivores [19]. The harmful heavy metals in the contaminated ecosystem can be detoxified and accumulated in the plant using the efficient, environmentally friendly, and cost-effective bioremediation technique known as phytoremediation. The heavy metals present in the soil are transported and translocated to various plant parts by molecules known as transporters that hyperaccumulators exude. Higher concentrations of harmful heavy metals can be contained in the tissues of plants with hyperaccumulator genes. The effectiveness of phytoremediation depends on a number of factors, including the nature of the rhizosphere, the characteristics of the rhizosphere microflora, the soil properties (pH and soil type), organic matter in the soil, the type of heavy metal, and more. Phytoremediation approach using metal accumulating plants is much convincing in terms of metal removal efficiency, but it has many limitations because of slow plant growth and decreased biomass owing to metal-induced stress. In addition, constrain of metal bioavailability in soils is the prime factor to restrict its applicability.

1.3 Bacteria interactions with plant in detoxifying heavy metal pollutions

The current review covers in great detail the environmental effects of heavy metal toxicity as well as various phytoremediation mechanisms for the transport and accumulation of heavy metals from polluted soil. The assistance of Plant Growth Promoting (PGP) bacteria increases the effectiveness of phytoremediation (i.e rhizobacteria). These bacteria can be found in the soil near plant roots, where they use siderophores, organic acids, biosurfactants, biomethylation, and redox processes to convert toxic metals into bioavailable and soluble forms. Additionally, PGP bacteria produce phytohormones, ACC (1-aminocyclopropane-1-carboxylic acid) deaminase, phosphorus solubilization, nitrogen fixation, iron sequestration, and other growth promoting traits that help plants grow and produce more biomass, aiding in phytoremediation. In many cases, plant species cannot perform well and need aids to enhance phytoremediation. Such aids include soil amendments like biochar [20]. Table 1 shows the important microorganism in bacteria interaction with plant. Ethylene Diamine Tetraacetic Acid (EDTA) [31], endophytic bacteria, Arbuscular mycorrhiza or even transgenic plants [32]. The bacterial rhizosphere population will directly induce root growth and thereby stimulate plant growth, improve metal resistance, and plant health [26, 33]. Plant growth-promotingbacteria have been shown to get a wide ability to increase phytoremediation efficiency. Plant growth-promoting bacteria can encourage soil fertility and health, improve plant diseases, enhance crop resistance to toxic metals, enhance plant nutrient uptake and metal absorption, and efflux. They promote the phytoremediation process either by reducing the toxicity of pollutants or increasing the availability of pollutants or promoting the growth of plants. Factors affecting this processes aids or stimulate or are either specific to soil or water resources. Comprehensive information on the contribution of each phytoremediation mechanism is showned in Table 2. Immunization through plant growth encouraging bacteria to produce growth factors, trigger phytopathogenic resistance, and balance plant hormones and nutrition. Other mechanisms include nutrient mobilization, exopolysaccharide development, and rhizobiotine stimulation of plant growth.

Pseudomonas libanensis, Pseudomonas aeruginosa.; Brevibacterium frigoritolerans and Bacillus paralicheniformis; Bacillus cereus; Brucella sp.; Xanthomonas oryzae, Rhizospus oryzae; Microbactetium sp.

Helianthus annuus; Lens esculenta; L. esculenta; Hibiscus esculentus L.; Oryza sativa; Pisium sativum

production of siderophone, Phosphate solubilization, Phytoremediation of metals in association with phosphate-solubilizing bacteria (PSB) considerably overcomes the practical drawbacks imposed by metal stress on plants; Mobilization of cadmium (Cd) and lead (Pb) in soil. Improve stem and root length and weight of grain number; Enhancement of plant growth, involved in increased uptake of nitrogen, synthesis of phytohormones, solubilization of minerals such as phosphorus, and secretion of siderophores that chelate iron and make it available to the plant root for improved stem and root length; Fortifies plant and decreases plant contamination with Cr(VI),; toxic metal species are accumulated in plant tissues through phytostabilization; in extremely polluted sites; High resistance to disease, mechanism are regulated by gene.; Improves the plant high adsorption capacity and high effectivity to adsorb the metal ions through a process of ion exchange.; Decreases Cr(IV) toxicity. Supply phytohormones and biocontrol the uptake and immobilization of chromium in soil

[21, 22, 23]; [24, 25]; [2, 10, 26, 27]; [17, 18, 28]; [1, 10, 21, 22]; [28, 29, 30]

Table 1.

Some important bacteria interaction with plant and their importance.

Phytovolatilization	Toxicants are Lost to the atmosphere through evapotranspiration from dermal layers of stems and leaves	[29, 32]
Rhizodegradation	The metabolism of toxicants by root enzymes	[13, 32, 34]
Phytodesalination	Uptake of mineral ions	[22, 32, 35]
Phytostabilization	Formation of residue bound molecules and complexes with biomolecules	[2, 36]
Phytoextraction	Metal uptake and formation of ligands and chelation	[21, 28, 37]
Rhizofiltratiom	Uptake from ground water followed by precipitation inside root	[28, 32]
Phytodegradation	metabolism of toxicants by phyto-enzymes	[31, 38]
Rhizobial and soil living microorganisms	Help in mineralization and plant uptake of toxicants	[30, 32, 39]

Table 2.

Some phytoremediation mechanism.

1.4 Some important mechanisms used in bacterial assisted phytoremediation

1.4.1 Biosorption

The removal of metal or metalloid species, compounds, and particulates from a solution by low-cost biological materials is known as biosorption [40]. Biosorption is a physiochemical process that removes heavy metals from contaminated water through ion exchange, surface complexation, chelation, and metal ion coordination using biological adsorbents [41]. In general, biosorption is regarded as an efficient, lowcost, and environmentally friendly remediation method for heavy metal removal from contaminated water. The most common biosorbents are living and dead microbial cells of bacteria, algae, and fungi. In this context, we’ll concentrate on one type of microorganism, bacteria. The bacterial cell wall is the first component to come into contact with the metal ion during biosorption. Metal ions bind to the cell wall’s functional groups (amine, carboxyl, hydroxyl, phosphate, sulfate, amine) [15]. The general metal uptake process involves metal ions binding to reactive groups on the bacterial cell wall, followed by metal ion incorporation inside the cell [16]. Bacteria have evolved a number of efficient systems for detoxifying metal ions; these resistance mechanisms are developed primarily for survival. Several living and dead bacterial strains have been studied and found to be effective at heavy metal biosorption. Several physiochemical parameters, including pH, temperature, biomass dosage, initial metal concentration, contact time, heavy metal type, and biosorbent type, have been reported to influence heavy metal biosorption by bacterial biomass processes. Temperature can change the structure of proteins and the functional groups available on the biosorbent’s surface. The solubility of heavy metal ions as well as active binding sites in biomass are affected by PH. Several studies have been conducted to investigate the biosorption efficiencies and capacities of various bacterial strains, as well as the impact of the above-mentioned influential parameters on the process. On the basis of published data, there is not a clear focus on the biosorption of heavy metals by bacterial strains that could reveal the most effective bacterial biosorbent and ideal physio-chemical conditions for the removal of each heavy metal.

1.4.2 Phytohormones

Abiotic stresses that affect plants include salinity, heavy metals, drought, and extremely high or low temperatures. Abiotic stresses negatively affect a plant’s physiology and morphology due to errors in the genetic control of cellular pathways. The effects of stresses, which are brought on whenever changes in metabolism occur, are avoided by plants using a variety of tolerance mechanisms and pathways. Phytohormones are some of the most significant growth regulators; they are well known for having a significant impact on plant metabolism and also play a significant role in the stimulation of plant defense response mechanisms against stresses. Supplementing with exogenous phytohormones has been used to enhance growth and metabolism in stressful environments [24]. Recent studies have demonstrated that phytohormones made by microbes associated with roots may prove to be significant metabolic engineering targets for promoting host tolerance to abiotic stresses. Several genetic and biochemical techniques have been used to pinpoint the biosynthetic pathways for phytohormones methods.

1.4.3 Phytoextraction

Phytoextraction is the use of plants to translocate and accumulate contaminants in their above ground biomass by absorbing them from soil or water [28, 37]. The most significant phytoremediation method used today to remove heavy metals and metalloids from contaminated soil is phytoextraction [21]. Phytoextraction is a long term method for removing heavy metals from contaminated soil, in contrast to phytostabilization, in which plants only momentarily contain heavy metals that still remain belowground. It is therefore better suited for commercial use. Heavy metal phytoextraction involves the following steps: Intake of heavy metals by plant roots, mobilization of heavy metals in the rhizosphere, translocation of heavy metal ions from roots to aerial parts of plants, and sequestration and compartmentation of heavy metal ions in plant tissues are all examples of heavy metal ion transport in plants. [21]. The efficiency of phytoextraction relies on a few factors such as plant selection, plant performance, heavy metal bioavailability, soil, and rhizosphere properties.

1.4.4 Siderophore secretion

Metal ions are bound by siderophore secretion, which reduces metal bioavailability. Low molecular weight siderophores, which are iron chelators with an exceptionally strong affinity for ferric iron (Fe3+), are secreted by bacteria in iron- limited conditions in order to acquire iron [10]. They can chelate a variety of other metals, including magnesium, manganese, chromium (III), gallium (III), cadmium, zinc, copper, nickel, arsenic, and lead, as well as radionuclides like plutonium (IV), with varying affinities, despite having a preference for Fe3+ [1, 6, 21]. As bacterial siderophores aid in reducing the stress caused by metal contaminants, the supply of iron to growing plants under heavy metal pollution becomes more important.

1.4.5 Rhizosphere acidification

Rhizosphere acidification, a common mechanism by plant with help from rhizosphere’s bacteria to deal with low Phosphorus stress, to activate and increase the efficiency of soil Phosphorus utilization. Due to the release of organic acids and H+, white lupin (Lupinus albus L.) has evolved what are known as proteoid roots to acidify infertile soil when Phosphorus deficiency exists [22, 42, 43, 44, 45]. To solubilize soil Phosphorus, sugar beet roots primarily exude salicylic acid and citramalic acid [5]. Under low-Phosphorus conditions, root-secreted acid phosphatase activity was increased in a Phosphorus efficient genotype of rapeseed (Brassica napus L.), which was advantageous for the increased uptake of Phosphorus [30].

2. Conclusion

Environmental technique that holds promise for purging the environment of heavy metal contamination is bacterial assisted phytoremediation of heavy metals. It is a safe environmental remediation technique. The phytoremediation mechanism from plants can be used by consortia of potential plant growth promoting rhizobacteria, degrading bacteria, as well as endophytic bacteria to reduce heavy metals in soil. The interaction between bacteria and plants has been found to be helpful in handling various pollutants in various exosystems.