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Phytoremediation Mechanisms
Reference: Bioremediation book, Chapter 2
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Description of mechanisms involved in bioremediation
Multiple mechanisms involved in bioremediation of soil, water & air Cleanup of organics and inorganics Mechanisms dealt How contaminants contact with plant system (rhizosphere & transportation processes) Mechanisms interrelated & depends on plant physiological processes driven by solar energy, rhizospheric processes and other available precursors.
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Description of mechanisms involved in bioremediation
Phyto-sequestration Phyto-degradation Phyto-volatilization Phyto-stabilization Phyto-extraction Rhizo-filtration Rhizo-remediation Sequester - 은퇴하다 - isolate or hide away Phyto-sequestration Reduce the contaminant mobility & Prevent migration to soil, water & air 1) Phytochemical complexation 2) Transport protein inhibition 3) Vacuolar storage
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Phytochemical complexation in root zone
Phyto-sequestration 1) Phytochemical complexation 2) Transport protein inhibition 3) Vacuolar storage Phytochemical complexation in root zone Phytochemicals exuded 분출 된 in root zone Complexation Contaminant immobilize 안정화 or precipitation Combination of individual atom groups, ions or molecules to create one large ion or molecule Reduce the fraction of contaminant
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F1- Soluble & exchangeable
Soil Particle Soil Particles
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아미노산
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Phyto-sequestration 1) Phytochemical complexation
2) Transport protein inhibition 3) Vacuolar storage Transport proteins associated with root irreversibly 비가 역적 bind (not able to altered) & stabilize 안정화 contaminants Transport protein inhibition on root membrane Prevent contaminants enter plant
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Vacuolar storage in root cells
Phyto-sequestration 1) Phytochemical complexation 2) Transport protein inhibition 3) Vacuolar storage Vacuolar storage in root cells Transport proteins transfer contaminants between cells But, plant cells compartment (“vacuole”) Preventing translocation to xylem Contaminants sequestered in vacuoles of root cells Storage
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Phyto-degradation or Phyto-transformation
Description of mechanisms involved in bioremediation Phyto-sequestration Phyto-degradation Phyto-volatilization Phyto-stabilization Phyto-extraction Rhizo-filtration Rhizo-remediation Phyto-degradation or Phyto-transformation
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Phyto-degradation or Phytotransformation
Uptake of contaminants by plant root 1) Contaminant breakdown by plant root 3) Metabolization 2) Mineralization 광물 Through various internal enzymatic reactions & metabolic processes
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Phyto-degradation or Phytotransformation
Contaminant Impeded 방해 (delay or prevent) by phytosequestration and/or rhizodegradation Then contaminants (partially or negligibly (less)) pass through rhizosphere Then contaminant subject to biological processes within plant Depends on Concentration & composition of contaminants Plant species Soil conditions Dissolved in transpiration stream & phytoextracted
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Plant internal reactions by producing enzymes
Plants Catalyze 촉매 Oxygenases in plants Nitroreductases Degrade hydrocarbons Reduce & breakdown Explosives 폭발물 trinitrotoluene (TNT), 1, 3, 5-trinitroperhydro -1, 3, 5- triazine (RDX) 1,3,5,7-tetranitro -1,3,5,7-tetrazocine (HMX High melting explosive)
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Plant enzymes metabolize or mineralize contaminants
Converted to CO2 & water Poplar plant Endophytic symbiotic 공생 bacteria Methyl bacterium populum lives within poplar plant Mineralize RDX & HMX Further, oxidation & reduction cycle operating during photosynthesis offers additional contaminant breakdown potential
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Sequester - 은퇴하다 - isolate or hide away
Phytosequestration
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Phyto-volatilization
Reference: Bioremediation book
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Phytovolatilization Volatilization of contaminants Leaf stomata
Plant stems Chemical characteristics Dictate the ability of organic contaminants to volatilize Henry’s constant & vapor pressure Henry’s Law At a constant temperature, the amount of gas that dissolves in a liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
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Henry’s constant C = kPgas
C - solubility of a gas at a fixed temperature in a particular solvent (mL gas/L) K - Henry's law constant Pgas - partial pressure of the gas (Atm)
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Water & mineral transport from roots to aerial parts
Transportation of food & nutrients (Sugars & amino acids) from leaves to storage organs Water & mineral transport from roots to aerial parts
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Rhizodegradation and/or phytodegradation
Volatilization of contaminants Trichloroethene (TCE) Henry’s constant & vapor pressure Some cases Parent contaminant in soil Along the transpiration pathway Rhizodegradation and/or phytodegradation Transpiration is the process of water movement through a plant and its evaporation from aerial parts Phytovolatilized
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Poplars Uptake and phytovolatilization of trichloroethene (TCE) or its breakdown products in poplars
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Mercury Inorganics Similar to above
Volatilized by tobacco plants Mercury Alter the chemical speciation Take up highly toxic methyl-mercury Once volatilized Phytovolatilize safe levels of less toxic elemental mercury to atmosphere Many chemicals recalcitrant 고집 불통 (not control) in Subsurface environment React rapidly with atmospheric hydroxyl radicals, an oxidant formed during the photochemical cycle
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Growing trees & other plants take up water & contaminants.
Phytovolatilization Se by Brassica plants Growing trees & other plants take up water & contaminants. Converted (by methylation to the volatile dimethyl Se) into nontoxic forms & volatilized Contaminants pass Plants leaves & volatilize into atmosphere at low concentrations Used only for contaminants that highly volatile Plants genetically altered due to mercury movement through a cell into air Elemental Hg in air less risk than Hg forms in the soil Hg & Se, taken by roots, converted to non-toxic forms & volatilized from roots, shoots, or leaves
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안정
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Plants control movement of toxins from the site either by
PHYTOSTABILIZATION— Plants control movement of toxins from the site either by Controlling erosion 부식 Movement by water by binding them tightly to their roots or or Rendering them unavailable The contaminants are not removed from the site Thus harmless
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Phytostabilization Holding of contaminated soils and sediments in place by vegetation & immobilizing toxic contaminants in soils
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Establishment of Rooted Vegetation
Prevents windblown dust by human from hazardous waste sites Hydraulic control (Phytohydraulics) Large volume of ground water transpired 나오다 by plants. Prevents migration of leachate towards groundwater or receiving waters.
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Phytoremediation groundcovers Soil/Sediment Stabilization
Applicable to metal contaminants at waste sites where metals do not degrade Phytostabilization Low level or high level of contaminants Capturing in situ at sites Phytoremediation groundcovers Soil/Sediment Stabilization Infiltration Control Phytostabilization
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Blowing wind a) Soil/Sediment Stabilization:
Soil & sediment mobilized by Uncontrolled water flows Erosion” or Leaching 침출 Blowing wind Migrate of contaminants non-point source (NPS) pollution Grasses Infusion 주입 mechanism of plants roots into soil Phytostabilization Natural barrier 장벽 & resistance to erosion & leaching + minimize NPS pollution Herbaceous species Fibrous rooted plants depths cm for upland species & < 30 cm for wetland species Halophytes (dry land plants) & hyperaccumulators also used
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b) Infiltration Control:
Mechanism used in Landfill covers Surface water recharge of groundwater plumes Water interacting with waste Infiltration Control prevent this mixing
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Phytostabilization covers for infiltration control know as
Ground water Evapotranspiration or or water-balance Vegetative covers Rain intercept 차단 by Plants Prevent infiltration Take up & remove rain water from soil Minimize the percolation 여과 (runoff) with waste Called as Phytohydraulics mechanism Time-dependent or climate-dependent processes - successfully remove water from the system Infiltration
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Seed mixes or mixed communities of plants/trees Vegetation cover
They access the stored water Create intercepting canopy (upper layer of mature tree)
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Not good for Phytostabilization due to landfill gas
When minimizing infiltration Create an anaerobic zone underneath the phytostabilization cover Some cases, subsurface conditions Methanogenic (methane-producing) conditions Waste Methanogenic Condition Anaerobic zone Not good for Phytostabilization due to landfill gas While methane may or may not toxic to plants, gas in vadose zone restrict the oxygen transport needed for root cell respiration Also landfill gas diffusion to surface stopped due to landfill cover So, these gases controlled through other methods
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Phytoremediation groundcovers
Densely rooted groundcover Applied to surface soils Groundcovers is cms below ground surface; however, depths down to 1.5 meters Recalcitrant compounds PAHs, PCBs, & persistent organic pollutants (less mobile, soluble, biodegradable, & bioavailable) Used for metals, salts & radionuclides
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Phyto-extraction 추출
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Ability of plants to take up contaminants into the roots and translocate them to the aboveground shoots or leaves
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Or For extraction Come in to contact with roots by transpiration
Toxicant must be dissolved in the soil water For extraction Come in to contact with roots by transpiration Uptake through vapor adsorption by root in vadose zone Or
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Contaminant dissolve in transpiration water or actively taken up by plant transport mechanisms
Contaminant adsorbed
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Absorption Storage via Lignification
Contaminant metabolized through phytodegradation mechanisms or phytovolatilized in transpiration stream existing in plant Absorption Storage (Or) via Lignification (Or) Sequester (isolate) into cell vacuoles of aboveground tissues (as opposed to in root cells) as part of phytosequestration. Converted to by-products of plant biomass (Covalent bonding of chemical or its by-products into lignin of plant)
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For organic chemicals Factors affect Plant uptake Hydro-phobicity
Polarity Sorption properties Solubility Octanol-water partition coefficient, log Kow Quantify molecule’s hydrophobicity Defined as ratio of chemical's concentration in the octanol phase to its concentration in the aqueous phase of a two-phase octanol/water system.
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1-octanol 2-Octanol is a colorless liquid used in the manufacture of perfumes, disinfectant soaps and to prevent foaming
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Octanol-water partition coefficient, log Kow
Kow values between 1 and 3.5 Enter into plants Organic membrane plant root consisting lipid bilayer Lipids make root partially hydrophobic while the bilayering aspects make it also nonpolar Therefore, hydrophobic chemicals
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log Kow > 3.5 Chemicals not soluble in water
Bound strongly to surface of roots Not sufficiently soluble in transpiration stream (or) Not easily translocate into plant xylem
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Chemicals soluble in water
Log Kow <1.0 Not sufficiently sorbed by roots Actively transported through plant membranes due to their high polarity or
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Susceptible to phytoextraction
Short chain aliphatic chemicals Benzene Log Kow = 2.13 Toluene Log Kow = 2.73 Chlorinated solvents Xylene (BTEX) Log Kow = 3.15 Ethylbenzene Log Kow = 3.15 Vapor uptake pathway into plants identified for chlorinated solvents such as perchloroethene (PCE, “tetrachloroethene”).
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Root concentration factor (RCF)
Ability of plant to take chemical from the soil or groundwater into its roots RCF = Ratio of concentration in root (mg/kg): concentration in external solution (mg/L) Transpiration stream concentration factor (TSCF) Chemical translocating from soil to shoots Ratio of Xylem concentration (mg/L): concentration in external solution (mg/L)
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RCF & TSCF Values depend on
Soil properties Plant species Chemical partitioning RCF & TSCF Values Higher values Indication of enhanced contaminant uptake by plants & vary directly with log Kow of chemical Contaminants in solution with the highest TSCF contained a log Kow in the range of 1–3.5
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For inorganics Uptake & translocation to above ground tissues depends on Salts, metals & radionuclides Redox state Chemical speciation in soil, sediment or groundwater Plant species
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Readily bioavailable inorganics As, Cd, Cu, Ni, Se & Zn
Moderately bioavailable Co, Fe & Mn Easily bioavailable Cr, Pb & U Several of these constituents, often considered as environmental contaminants in sufficient concentration, are also essential plant nutrients.
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Hyperaccumulators Plant absorb unusually large amounts of metals compare to other plants & ambient metal concentration Able to accumulate 1,000 mg/kg (dry weight) of a specific metal Some metals or metalloids, concentration 10,000 mg/kg Halophytes (dry land plants) Tolerate & accumulate large quantities of salt (NaCl, Ca & Mg chlorides) Take up & exude the excess salt through stomata Hyperaccumulators & halophytes selected to grow at site based on the metals or salts naturally present, forming their own niche through evolution.
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Plant harvested using conventional agricultural methods
Remediation aspects Plant harvested using conventional agricultural methods Enhance phytoextraction Chelating agents EDTA (for Pb & radionuclides)
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Availability of uranium & 137Cs enhanced by
Citric acid & ammonium nitrate Mobilize target contaminants to deeper soil or groundwater Enhancing risks
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Alter chemistry & speciation of chemicals
Take up saline water & exude excess salt through stomata back onto the ground as a means to create the niche Halophytes 염생 식물 Some plants produce & exude specific phytochemicals directly into the soil environment Alter chemistry & speciation of chemicals Promote mobilization & uptake into plant Planting known crop to accumulate metals, metalloids or, radionuclides and then harvesting the crop contaminant is recovered. Enhancing the uptake of essential nutrients through the release of acidic phytochemicals
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Rhizofiltration
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heavy metals or radionuclides
Rhizofiltration Use of plant roots Absorb Concentrate Precipitate Hazardous compounds heavy metals or radionuclides Aqueous solutions
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Hydroponic cultivation 수경법 Plants rapidly remove HM from water
Rhizofiltration Hydroponic cultivation 수경법 Effective in contaminated wetlands 습지 Water allowed to contact with roots Plants rapidly remove HM from water Contaminants (Pb, Cr(III) U, Ar) sorb strongly by roots Concentrate in roots & shoots
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dense vegetation Densely growing vegetation plants roots sorbe large quantities of Pb & Cr from soil water or from water
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Groundwater or wastewater pumped through this system
Shallow lagoons 얕은 석호 engineered as wetlands & maintained as facultative microbial systems with low dissolved oxygen in the sediment Groundwater or wastewater pumped through this system remove contaminants by rhizofiltration. Success in wetlands.
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Bamboo 대나무 successful with Acid Mine Drainage
Long-term utilization of wetland 습지 plants & sulfate-reducing conditions Increase in pH & decrease in toxic metals Bamboo 대나무 successful with Acid Mine Drainage
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Root systems & sediments in wetlands
Facultative (aerobic & anaerobic zones) facilitates sorption and precipitation of toxic metals. Facultative : bacterium live in absence as well as in the presence of atmospheric oxygen. Ion exchange
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Harvested plants containing heavy metals disposed of or treated to recycle the metal
Plants with high biomass production Wide variety of metal removal capacity Rhizofiltration many benefits than other phytoextraction techniques Including low cost & minimal environmental disruption
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Periodically, older plants harvested & replaced
Continuous flow system Circulates contaminated water Through specially designed plant containment units Periodically, older plants harvested & replaced
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Experimental evidence showing nonlinear kinetics 비선형 동역학 of disappearance of metals from solution suggests that several different mechanisms, of differing speeds, operate simultaneously.
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Surface absorption by roots
Fastest & prevalent mechanism depends on physicochemical processes (ion exchange, chelation) and can even take place on dead roots. 탄산
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Chelation 킬레이트 : ions & molecules bind metal ions
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Primary mechanism for removing metals from waste streams by roots - Biosorption
Surface absorption Absorb large quantities of heavy metals. Plant root used living or dead Microbial, fungal or other biomass
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Other slower mechanisms also occur in rhizofiltration
Surface absorption Biological processes Precipitation of metal from solution by plant exudates Intracellular uptake Deposition in vacuoles Translocation to shoot Rhizofiltration effective for dilute concentrations of contaminants in large volumes of water (radionuclide decontamination)
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Rhizoremediation
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Well established rhizoremediation processes
(a) Sequestration or immobilization or retention of toxicants within a confined area (b) Removal of contaminants from soil/waste water (c) Destruction/degradation of organic pollutants by plant-microbial association Soil at the site of their release (or) in contaminated soil placed in a landfill Used a, b and c - individually (or) in combination
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Soil Root Stem Leaf evaporated by transportation process
Partial immobilization of water soluble contaminants removed by plant transpiration 증발. Soil Root Stem Leaf evaporated by transportation process This process removes soil water contaminants otherwise contaminant leaching & move to groundwater. Removal of toxic metals from contaminated soil occurs when inorganic ions are taken up by plant roots and translocated 이동시키다 through the stem to aboveground plant parts. Soil microflora of plant roots (rhizosphere zone) is involved in xenobiotic metabolism.
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Organic chemicals released from both living and dead roots
Catabolic activity within rhizosphere by bacteria & fungi using enzymatic 효소 expression Organic chemicals released from both living and dead roots Contaminants direct & indirect degradation by root physiology & biosynthetic (or anabolism) pathways Potentially occur at the lowest depth of root penetration, a special feature of plant remediation.
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Rhizodeposition & Root exudates
Deposit High amounts of photosynthetically derived hydrocarbons into surrounding soil Roots Annually, plants transfer 40–90 % of the net fixed carbon (as primary and secondary metabolites) to roots.
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Organic compounds as rhizodeposits
Exudates Secretions Mucigel Plant mucilages 식물 점액 Root lysates All organic substances Stimulation of microbial degradation of contaminants in root zone by maintain gas exchange & soil moisture Mucigel - slimy substance that covers the root cap
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No single plant or microbe
Contaminants immobilization, removal & destruction Maximum uptake of all toxic metals or faster degradation of all organic contaminants Combination of plant species with appropriate remediation properties Successful treatment Rhizosphere communities (bacteria and fungi) active against specific contaminants Rhizosphere microorganisms, closely associated with roots, termed Plant Growth Promoting Rhizobacteria (PGPR).
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Stimulate plant growth & reduce metal toxicity in plants
Microbes help to plant Rhizosphere microbes play significant roles in recycling of plant nutrients, maintenance of soil structure, detoxification of noxious chemicals, and control of plant pests. Plant help to microbes Plant root exudates provide nutrition to rhizosphere microbes for increasing microbiological activity Stimulate plant growth & reduce metal toxicity in plants
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Plant Growth Promoting Rhizobacteria (PGPR) and Arbuscular Mycorrhizal Fungi (AMF) have gained prominence all over the world to treat soil (Figure). Mycorrhizal fungal networks connect the roots of the same or different plant species, provide pathway for nutrient transfer. Associated plant growth promoting rhizobacteria foster rhizoremediation of inorganic and organic pollutants.
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Bacteria, yeast and fungi Naturally select contaminants
Several orders of magnitude Specific microbial populations Food & energy
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Contaminant Degradation, metabolization, or mineralization rate depends on bioactivity of proteins & enzymes from soil organisms. However, contaminant breakdown is often limited by the availability of electron acceptors or donors, cometabolites, inorganic nutrients, plant vitamins and hormones, pH, and/or water. Co-metabolism Simultaneous degradation of two compounds Degradation of second compound (the secondary substrate) depends on the presence of the first compound (the primary substrate).
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Plants & soil microbes in the rhizosphere Symbiotic relationship
Nutrients Plants microbes Healthier soil environment Specifically, plants loosen soil and transport oxygen and water into the rhizosphere. Endosymbiont bacterium or fungus
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Plants exude phytochemicals
Alcohols Carbohydrates Sugars Primary food sources (carbon) for microbes Providing the healthier soil environment. But, plant exuded allelopathic phytochemical suppress other plants growth in the same soil. Plants protected from competition, soil pathogens, toxins, and other chemicals
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Microbial populations higher in vegetated soil compared to unvegetated soil
Rhizodegradation (phytostimulation, rhizosphere biodegradation, or plant assisted bioremediation/degradation) Breakdown of contaminant by increasing bioactivity using plant rhizosphere environment to stimulate the microbial populations Organic contaminants can be remediated converted to source of food and energy for the plants or soil organisms. Specific proteins & enzymes produced by soil organism break down the contaminant.
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