Phytoremediation
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'''Phytoremediation''' describes the treatment of [[natural environment|environment]]al problems ([[bioremediation]]) through the use of [[plant]]s.
The word's etymology comes from the Greek φυτο (phyto) = plant, and Latin « remedium » = restoring balance, or remediating. Phytoremediation consists in depolluting contaminated [[soil]]s, [[water]] or [[air]] with plants able to contain, degrade or eliminate [[metal]]s, [[pesticide]]s, [[solvent]]s, [[explosive]]s, [[crude oil]] and its derivatives, and various other contaminants, from the mediums that contain them.
It is clean, efficient, inexpensive and non-environmentally disruptive, as opposed to processes that require excavation of soil. The definitive textbook on phytoremediation was published in 2003 with contributed, peer reviewed articles from all major research groups involved in phytoremediation research (Phytoremediation: Transformation and Control of Contaminants, edited by Steven C. McCutcheon and Jerald L. Schnoor).
== Various phytoremediation processes ==
A range of processes mediated by plants are useful in treating environmental problems:
* [[#Phytoextraction|''Phytoextraction'']] - uptake and concentration of substances from the environment into the plant [[biomass]].
* [[#Phytostabilization|''Phytostabilization'']] - reducing the mobility of substances in the environment, for example by limiting the [[leaching]] of substances from the [[soil]].
* [[#Phytotransformation|''Phytotransformation'']] - chemical modification of environmental substances as a direct result of plant [[metabolism]], often resulting in their inactivation, degradation (phytodegradation) or immobilization (phytostabilization).
* ''Phytostimulation'' - enhancement of [[soil life|soil microbial]] activity for the degradation of contaminants, typically by organisms that associate with [[root]]s. This process is also known as ''[[rhizosphere]] degradation''.
* ''Phytovolatilization'' - removal of substances from soil or water with release into the air, sometimes as a result of phytotransformation to more volatile and / or less polluting substances.
* ''[[Rhizofiltration]]'' - filtering water through a mass of roots to remove toxic substances or excess [[nutrients]]. The pollutants remain absorbed in or adsorbed to the roots.
=== Phytoextraction ===
Phytoextraction (or ''phytoaccumulation'') uses plants to remove contaminants from soils, sediments or water into harvestable plant biomass. Phytoextraction has been growing rapidly in popularity world-wide for the last twenty years or so. Generally this process has been tried more often for extracting heavy metals than for organics. At the time of disposal contaminants are typically concentrated in the much smaller volume of the plant matter than in the initially contaminated soil or sediment. 'Mining with plants', or '''phytomining''', is also being experimented with.
The plants absorb contaminants through the root system and store them in the root biomass and/or transport them up into the stems and/or leaves. A living plant may continue to absorb contaminants until it is harvested. After harvest a lower level of the contaminant will remain in the soil, so the growth/harvest cycle must usually be repeated through several crops to achieve a significant cleanup. After the process, the cleaned soil can support other vegetation.
'''Two versions of phytoextraction:'''
*'''natural hyper-accumulation''', where plants naturally take up the contaminants in soil unassisted, and
*'''induced or assisted hyper-accumulation''', in which a conditioning fluid containing a chelator or another agent is added to soil to increase metal solubility or mobilization so that the plants can absorb them more easily.
'''Examples of phytoextraction from soils (see also [[Phytoremediation, Hyperaccumulators|'Table of hyperaccumulators']]):'''
*[[Arsenic]], using the Sunflower (''[[Helianthus annuus]]''), or the [[Brake (fern)|Chinese Brake fern]] ("Pteris spp"], a hyperaccumulator. Chinese Brake fern stores [[arsenic]] in its [[leaf|leaves]].
*[[Cadmium]] and [[zinc]], using [[alpine pennycress]] (''Thlaspi caerulescens''), a hyperaccumulator of these metals at levels that would be [[poison|toxic]] to many plants. On the other hand, the presence of copper seems to impair its growth (see table for reference).
*[[Lead]], using Indian Mustard (''Brassica juncea''), Ragweed (''Ambrosia artemisiifolia''), Hemp Dogbane (''Apocynum cannabinum''), or [[Poplar]] trees, which sequester lead in its biomass.
*Salt-tolerant (moderately [[halophyte|halophytic]]) [[barley]] and/or [[sugar beets]] are commonly used for the extraction of [[Sodium chloride]] (common salt) to reclaim fields that were previously flooded by [[sea water]].
*[[Uranium]], using [[sunflower]]s, as used after the [[Chernobyl accident]].
*[[Mercury (element)|Mercury]], [[selenium]] and organic pollutants such as [[polychlorinated biphenyl]]s (PCBs) have been removed from soils by [[transgenic plant]]s containing [[gene]]s for bacterial enzymes<ref>{{cite journal | author=Meagher, RB | title=Phytoremediation of toxic elemental and organic pollutants | journal=Current Opinion In Plant Biology | volume=3 | issue=2 | year=2000 | pages=153–162 | pmid=10712958 | doi=10.1016/S1369-5266(99)00054-0}}</ref>.
=== Phytostabilization ===
Phytostabilization focuses on long-term stabilization and containment of the pollutant. For example, the plant's presence can reduce wind erosion, or the plant's roots can prevent water erosion, immobilize the pollutants by adsorption or accumulation, and provide a zone around the roots where the pollutant can precipitate and stabilize. Unlike phytoextraction, phytostabilization mainly focuses on sequestering pollutants in soil near the roots but not in plant tissues. Pollutants become less bioavailable and livestock, wildlife, and human exposure is reduced. An example application of this sort is using a vegetative cap to stabilize and contain [[mine tailings]].<ref>{{cite journal |journal= Environ Health Perspect |date=2008 |volume=116 |issue=3 |pages=278–83 |title= Phytostabilization of mine tailings in arid and semiarid environments—an emerging remediation technology |author=Mendez MO, Maier RM |url=http://www.ehponline.org/members/2007/10608/10608.html |doi=10.1289/ehp.10608}}</ref>
=== Phytotransformation ===
In the case of [[organic compounds|organic]] pollutants, such as [[pesticides]], [[explosives]], [[solvents]], industrial chemicals, and other [[xenobiotic]] substances, certain plants, such as [[Canna (plant)|Cannas]], render these substances non-toxic by their [[metabolism]]. In other cases, [[microorganism]]s living in association with plant roots may metabolize these substances in [[soil]] or water. These complex and recalcitrant compounds cannot be broken down to basic molecules (water, carbondioxide etc) by plant molecules, and hence the term phytotransformation represents a change in chemical structure without complete breakdown of the compound.
The term "Green Liver Model" [http://www3.interscience.wiley.com/cgi-bin/summary/106569012/SUMMARY?CRETRY=1&SRETRY=0] is used to describe phytotransformation, as plants behave similar to the human liver when dealing with these [[xenobiotic]] compounds(foreign compound/pollutant). After uptake of the xenobiotics, plant enzymes increase the polarity of the xenobiotics by adding functional groups such as hydroxyl groups (-OH). This is known as Phase I metabolism, similar to the way the human liver increases the polarity of drugs and foreign compounds ([[Drug Metabolism]]. While in the human liver, enzymes like [[Cytochrome P450s]] are responsible for the initial reactions, in plants enzymes such as nitroreductases carry out the same role. In the second stage of phytotransformation, known as Phase II metabolism, plant biomolecules such as glucose and amino acids are added to the polarized xenobiotic to further increase the polarity (known as conjugation). This is again similar to the processes occurring in the human liver wherein glucuronidation (addition of glucose molecules by the UGT (e.g. [[UGT1A1]]) class of enzymes) and glutathione addition reactions occur on reactive centers of the xenobiotic. Phase I and II reactions serve to increase the polarity and reduce the toxicity of the compounds, although many exceptions to the rule are seen at least in the case of the human liver. The increased polarity also allows for easy transport of the xenobiotic along aqueous channels. In the final stage of phytotransformation (Phase III metabolism), a [[sequestration]] of the xenobiotic occurs within the plant. The xenobiotics polymerize in a [[lignin]]-like manner and get a complex structure which is sequestered in the plant. This ensures that the xenobiotic is safely stored in the plant, and does not affect the functioning of the plant. However, preliminary studies have shown that these plants can be toxic to small animals (such as snails) and hence plants involved in phytotransformation may need to be maintained in a closed enclosure. The human liver differs from plants in Phase III metabolism, since the liver can transport the xenobiotics into the bile for eventual excretion. Since plants have no excretory mechanisms, they sequester the modified xenobiotics. Hence, the plants reduce toxicity (with exceptions) and sequester the xenobiotics in phytotransformation. [[Trinitrotoluene]] phytotransformation has been extensively researched a transformation pathway has been proposed <ref name="Subra">Murali Subramanian, David J. Oliver, and Jacqueline V. Shanks. ''TNT Phytotransformation Pathway Characteristics in Arabidopsis: Role of Aromatic Hydroxylamines.'' Biotechnol. Prog., 22 (1), 208 -216, 2006.</ref>.
== The role of genetics ==
Breeding programs and [[genetic engineering]] are powerful methods for enhancing natural phytoremediation capabilities, or for introducing new capabilities into plants. Genes for phytoremediation may originate from a [[micro-organism]] or may be transferred from one plant to another variety better adapted to the environmental conditions at the cleanup site. For example, genes encoding a nitroreductase from a bacterium were inserted into tobacco and showed faster removal of TNT and enhanced resistance to the toxic effects of TNT <ref name="Hannink"> Hannink N, Rosser SJ, French CE, Basran A, Murray JA, Nicklin S, Bruce NC. ''Phytodetoxification of TNT by transgenic plants expressing a bacterial nitroreductase.'' 1: Nat Biotechnol. 2001 Dec;19(12):1168-72.</ref>.
== Advantages and limitations==
*'''Advantages''':
** the cost of the phytoremediation is lower than that of traditional processes both ''in situ'' and ''ex situ''
** the plants can be easily monitored
** the possibility of the recovery and re-use of valuable metals (by companies specializing in “phytomining”)
** it is the least harmful method because it uses naturally occurring organisms and preserves the natural state of the environment.
*'''Limitations''':
** phytoremediation is limited to the surface area and depth occupied by the roots.
** slow growth and low [[biomass]] require a long-term commitment
** with plant-based systems of remediation, it is not possible to completely to prevent the leaching of contaminants into the [[groundwater]] (without the complete removal of the contaminated ground which in itself does not resolve the problem of contamination)
** the survival of the plants is affected by the toxicity of the contaminated land and the general condition of the soil.
** possible bio-accumulation of contaminants which then pass into the [[food chain]], from primary level consumers upwards.
== Hyperaccumulators and biotic interactions ==
This section is for the first four points (Protection, Interferences, Mutualism, and Commensalism) mainly inspired from the article: ''The significance of metal hyperaccumulation for biotic interactions'', by R.S. Boyd and S.N. Martens<ref name="Boyd98">[http://scholar.google.com/url?sa=U&q=http://www.auburn.edu/academic/science_math/biology/faculty/boyd/Boyd%26Martens1998Review.pdf] ''The significance of metal hyperaccumulation for biotic interactions'', by R.S. Boyd and S.N. Martens. Chemoecology 8 (1998) pp.1–7</ref>.
A plant is said to be a hyperaccumulator if it can concentrate the pollutants in a minimum percentage which varies according to the pollutant involved (for example: more than 1000 mg/kg of dry weight for [[nickel]], [[copper]], [[cobalt]], [[chromium]] or [[lead]]; or more than 10,000 mg/kg for [[zinc]] or [[manganese]].<ref name="BB89">A.J.M. Baker, R.R. Brooks. ''Terrestrial higher plants which hyperaccumulate metallic elements – A review of their distribution, ecology and phytochemistry.'' Biorecovery (1989), 1:81–126</ref> Most of the 215 metal-hyperaccumulating species included in their review hyperaccumulate nickel. They listed 145 hyperaccumulators of nickel (around 300 Ni accumulators are known; see [[Hyperaccumulators table – 2 : Nickel]] and its notes), 26 of cobalt, 24 of copper, 14 of zinc, four of Lead, and two of Chromium. This capacity for accumulation is due to '''hypertolerance''', or ''phytotolerance'': the result of adaptative evolution from the plants to hostile environments along multiple generations. Boyd and Martens list 4 biotic interactions that may be affected by metal hyperaccumulation, to which can be added the [[biofilm]] as a particular aspect of micorrhizae:
# protection
# Interferences with neighbour plants of different species
# Mutualism (Mycorrhizal associations or micorrhizae, and Pollen and seed dispersal)
# Commensalism
# The [[biofilm]]
=== Protection ===
More and more evidence show that the metals in hyperaccumulating plants give them some protection from various bacteria, fungi and / or insects. For instance, with foliar Ni concentrations as low as 93 mg/kg, the larval weight of ''Spodoptera exigua'' (''Lepidoptera'': ''Noctuidae'') (beet army worm) is reduced and time to pupation extended (Boyd & Moar, subm.).
{| class="wikitable" style="text-align:center"
|+ Information published supporting the defence hypothesis of metal hyperaccumulation
!Paper || Plant species || Metal || Organism(s) affected
|-
| Ernst 1987 || ''Silene vulgaris'' (Moench) Garke || Cu (400 mg g-¹) || ''Hadena cucubalis'' Schiff. (''Lepidoptera'': ''Noctuidae'')
|-
| Boyd ''et al.'' 1994 || ''Streptanthus polygaloides'' Gray || Ni || ''Xanthomonas campestris'' (Gram-negative bacterium)
|-
| Boyd ''et al.'' 1994 || ''Streptanthus polygaloides'' Gray || Ni || ''Alternaria brassicicola'' (Imperfect fungus)
|-
| Boyd ''et al.'' 1994 || ''Streptanthus polygaloides'' Gray || Ni || ''Erisyphe polygoni'' (Powdery mildew)
|-
| Martens & Boyd 1994 || ''Streptanthus polygaloides'' || Ni || ''[[Pieris rapae]]'' L. (''[[Lepidoptera]]'': ''[[Pieridae]]'')
|-
| Boyd & Martens 1994 || ''[[Thlaspi montanum]]'' L. var. ''montanum'' || Ni || ''[[Pieris rapae]]''
|-
| Pollard & Baker 1997 || ''[[Thlaspi caerulescens]]'' J. and C. Presl. || Zn || ''[[Schistocerca gregaria]]'' (Forsk.) (''Orthoptera'': ''Acrididae'')
|-
| Pollard & Baker 1997 || ''Thlaspi caerulescens'' J. and C. Presl. || Zn || ''Deroceras carvanae'' (Pollonera) (''[[Pulmonata]]'': ''Limacidae'')
|-
| Pollard & Baker 1997 || ''Thlaspi caerulescens'' J. and C. Presl. || Zn || ''[[Pieris brassicae]]'' L. (''[[Lepidoptera]]'': ''[[Pieridae]]'')
|}
The defense against viruses is not always supported. Davis ''et al.'' (2001) have compared two close species ''S. polygaloides Gray'' ([[Ni]] hyperaccumulator) and ''S. insignis Jepson'' (non-accumulator), inoculating them with Turnip mosaic virus. They showed that the presence of nickel weakens the plant's response to the virus.<ref name="Davis01>[http://intl-jeq.scijournals.org/cgi/content/abstract/30/1/85] M.A. Davis, J.F. Murphy, and R.S. Boyd. ''Nickel Increases Susceptibility of a Nickel Hyperaccumulator to Turnip mosaic virus''. J. Environ. Qual., Vol. 30, January–February 2001</ref>
Circumvention of plants' elemental defences by their predators may occur in three ways<ref name="Boyd98"/>: (1) selective feeding on low-metal tissues, (2) use of a varied diet to dilute metal-containing food (likely more efficient in large-sized herbivores), and (3) tolerance of high dietary metal content.
# - Avoidance of an elemental defence via selective feeding:
Mishra & Kar (1974)<ref name="Mishra74">D. Mishra, M. Kar. ''Nickel in plant growth and metabolism.'' Bot Rev (1974), 40:395–452</ref> reported [[nickel]] to be transported through the [[xylem]] of crop plants. Similarly, Kramer ''et al.'' (1996) showed that Ni is transported as a complex with the [[amino-acid]] [[histidine]] in the [[xylem]]. This implies that [[phloem]] fluid may contain little [[nickel]]; thus [[phloem]] fluid may be used by able organisms as a rich source of carbohydrates.
Pea [[aphid]]s (''Acyrthosiphon pisum'' [Harris]; ''Homoptera'': ''Aphididae'') feeding on ''Streptanthus polygaloides'' Gray (''Brassicaceae'') have equal survival and reproduction rates for plants containing ca. 5000 mg/kg nickel amended with NiCl<sub>2</sub>, and those containing 40 mg/kg nickel. This means that either the phloem fluid is poor in nickel even for nickel hyperaccumulators, or that the aphids tolerate nickel. Moreover the aphids feeding on high nickel-content plants only show a small increase of nickel content in their bodies, relatively to the nickel content of aphids feeding on low-nickel plants<ref name="Boyd98"/>. On the other hand, aphids (''Brachycaudus lychnidis'' L.) fed on the zinc-tolerant plant ''Silene vulgaris'' (Moench) Garcke (''Caryophyllaceae'') - which can contain up to 1400 mg/kg zinc in its leaves – were reported showing elevated (9000 mg/kg) zinc in their bodies.
* 3 - Metal tolerance
Hopkin (1989)<ref name="Hopkin89">S.P. Hopkin. ''Ecophysiology of Metals in Terrestrial Invertebrates.'' GB-London: Elsevier Applied Science (1989)</ref> and Klerks (1990)<ref name="Klerks90"> P.L. Klerks. ''Adaptation to metals in animals.'' pp 313–321 in Shaw AJ (ed.) ''Heavy Metal Tolerance in Plants: Evolutionary Aspects.'' Boca Raton:FL: CRC Press (1990)</ref> demonstrated it for animal species; Brown & Hall (1990)<ref name="Brown90">M.T. Brown et I.R. Hall. ''Ecophysiology of metal uptake by tolerant plants.'' Pp 95–104 in Shaw AJ (ed.) ''Heavy Metal Tolerance in Plants: Evolutionary Aspects.'' Boca Raton: FL: CRC Press (1990)</ref> for fungal species; and Schlegel & al. (1992) and Stoppel & Schlegel (1995) for bacterial species.
Plants of ''Streptanthus polygaloides'' (''Brassicaceae'', Ni hyperaccumulator) can be parasited by ''Cuscuta californica'' var. ''breviflora'' Engelm. (''Cuscutaceae''). Metal contents of ''Cuscuta'' ranged from 540–1220 mg/kg Ni, 73-fold higher than the metal contents of ''Cuscuta'' parasitizing a co-occurring non-hyperaccumulator plant species. ''Cuscuta'' plants are therefore very Ni-tolerant<ref name="Boyd98"/> - 10 mg Ni/kg is sufficient for growth to start decreasing in unadapted plants<ref name="McNic85">R.D. MacNicol, P.H.T. Beckett. ''Critical tissue concentrations of potentially toxic elements.'' Plant Soil, 1985. 85:107–129</ref>. According to Boyd & Martens (subm.) this is "the first well-documented instance of the transfer of elemental defences from a hyperaccumulating host to a seed plant parasite".
=== Interferences with neighbour plants of different species ===
Its likelihood between hyperaccumulators and neighbouring plants was suggested but no mechanism was proposed.<ref name="BB89"/> Gabrielli ''et al.'' (1991)<ref name="Gabrielli91">R. Gabrielli, C. Mattioni, O. Vergnano. ''Accumulation mechanisms and heavy metal tolerance of a nickel hyperaccumulator.'' Plant Nutr (1991). 14:1067–1080</ref>, and Wilson & Agnew (1992)<ref name="Wilson92">J.B. Wilson, A.D.Q. Agnew. ''Positive-feedback switches in plant communities.'' Adv Ecol Res (1992), 23:263–336</ref>, suggested a decrease in competition experienced by the hyperaccumulators for the litterfall from hyperaccumulators' canopy.
This mechanism mimics allelopathy in its effects, although technically due to redistribution of an element in the soil rather than to the plant manufacturing an organic compound. Boyd et Martens call it ‘‘elemental allelopathy’’ - without the autoxicity problem met in other types of allelopathy (Newman 1978).
=== Mutualism ===
Two types of mutualism are considered here, '''mycorrhizal associations''' or ''mycorrhizae'', and animal-mediated pollen or seed dispersal.
1 - '''Mycorrhizal associations''' or ''mycorrhizae''
There are two types of mycorrhizal fungi: ectomycorrhizae and endomycorrhizae. Ectomycorrhizae form sheaths around plant roots, endomycorrhizae enter cortex cells in the roots<ref name="Rost98">T.L. Rost, M.G. Barbour, C. R. Stocking and T.M. Murphy, ''The root system''. Plant Biology, 1998. (pp. 68-84). California: Wadsworth Publishing Company. Cited in Westhoff99.</ref>.
[[Mycorrhiza]]e are the symbiotic relationship between a soil-borne fungus and the roots of a plant. Some hyperaccumulators may form mycorrhizae and, in some cases, the latter may have a role in metal treatment.<ref name="Boyd98"/>. In soils with low metal levels, vesicular [[arbuscular mycorrhiza]]e enhance metal uptake of non-hyperaccumulating species<ref name="Brown90"/>. On the other hand, some mycorrhizae increase metal tolerance by decreasing metal uptake in some low-accumulating species. Mycorrhizae thus assists ''Calluna'' in avoiding Cu and Zn toxicity<ref name="Bradley82">R. Bradley, A.J. Burt et D.J. Read, ''The biology of mycorrhizal infection in the Ericaceae. VIII. The role of mycorrhizal infection in heavy metal tolerance.'' New Phytol 1982, 91:197–209.</ref>. Most roots need about 100 times the amount of carbon than do the hyphae of its associated ectomycorrhizae in order to develop across the same amount of soil<ref name= "Harley89">J.L. Harley, ''The significance of mycorrhizae''. Mycological Research 1989. 92: 129-134.</ref>. It is therefore easier for hyphae to acquire elements that have a low mobility than it is for plant roots. Caesium-137 and strontium-90 both have low mobilities<ref name= "Kirk89">G.J.D. Kirk and S. Staunton. ''On predicting the fate of radioactive caesium in soil beneath grassland'', Journal of Soil Science, 1989. 40: 71-84</ref>.
Mycorrhizal fungi depend on host plants for carbon, while enabling host plants to absorb the soil's nutrients and water with more efficiency<ref name="Entry99">[http://www.ingentaconnect.com/content/els/02697491/1999/00000104/00000003/art00163] J.A. Entry, L.S. Watrud and M. Reeves, ''Accumulation of cesium-137 and strontium-90 from contaminated soil by three grass species inoculated with mycorrhizal fungi.'' Environmental Pollution, 1999. 104: 449-457. Cited in Westhoff99.</ref>. In mycorrhizae, nutrient uptake is enhanced for the plants while they provide energy-rich organic compounds to the fungus<ref name="Allen91">M.F. Allen. ''The Ecology of Mycorrhizae.'' New York: Cambridge University Press (1991). Cité dans Boyd 1998.</ref>. Although certain plant species that are normally symbiotic with mycorrhizal fungi can exist without the fungal association, the fungus greatly enhances the plant’s growth. Hosting mycorrhizae is much more energy effective to the plant than producing plant roots<ref name="MP87">Marshall and Perry 1987</ref>.
The ''[[Brassicaceae]]'' family reportedly forms few mycorrhizal associations.<ref name="Allen91"/> But Hopkins (1987)<ref name="Hopkins87">N.A. Hopkins. ''Mycorrhizae in a Californian serpentine grassland community.'' Can Bot 1987, 65:484–487</ref> notes mycorrhizae associated with ''Streptanthus glandulosus'' Hook. (''Brassicaceae''), a non-accumulator. Some fungi tolerate easily the generally elevated metal contents of serpentine soils. Some of these fungal species are mycorrhizal<ref name="Maas69">J.L. Maas et D.E. Stuntz. ''Mycoecology on serpentine soil.'' Mycologia 61:1106–1116 (1969). Cited in Boyd 98.</ref>. High levels of phosphate in the soil inhibit mycorrhizal growth<ref name="Kilham95">K. Killham. ''Ecology of polluted soils.'' Soil Ecology, 1995. (pp. 175-181) Cambridge: Cambridge University Press.</ref>.
The uptake of radionuclides by fungi depends on its nutritional mechanism ([[mycorrhiza]]l or [[saprotrophic]])<ref name="Baeza05">[http://cat.inist.fr/?aModele=afficheN&cpsidt=16709330] A. Baeza, J. Guillen, S. Hernandez, A. Salas, M. Bernedo, J.L. Manjon, G. Moreno. ''Influence of the nutritional mechanism of fungi (mycorrhize/saprotroph) on the uptake of radionuclides by mycelium.'' Radiochimica acta, 2005. vol. 93, no4, pp. 233-238</ref>. ''Pleurotus eryngii'' absorbs Cs best over Sr and Co, while ''Hebeloma cylindrosporum'' favours Co. But increasing the amount of K increases the uptake of Sr (chemical analogue to Ca) but not that of Cs (chemical analogue to K). Moreover, the uptake of Cs decreases with ''Pleurotus eryngii'' (mycorrhizal) and ''Hebeloma cylindrosporum'' (saprotrophic) if the Cs content is increased, but that of Sr increases if its content is increased – this would indicate that the uptake is independent from the nutritional mechanism.
2 - '''Pollen and seed dispersal'''
Some animals obtain food from the plant (nectar, pollen, or fruit pulp - Howe & Westley 1988). Animals feeding from hyperaccumulors high in metal content must either be metal-tolerant or dilute it with a mixed diet. Alternatively hyperaccumulators may rely on abiotic vectors or non-mutualistic animal vectors for pollen or seed transport, but we lack information on seed and pollen dispersal mechanisms for hyperaccumulating plants.
Jaffré & Schmid 1974; Jaffré ''et al.'' 1976; Reeves ''et al.'' 1981; have studied metal contents of entire flowers and/or fruits. They have recorded elevated metal levels in these. We find an exception with ''Walsura monophylla'' Elm. (''Meliaceae''), originating from the Philippines and showing 7000 mg/kg Ni in leaves but only 54 mg/kg in fruits<ref name="Baker92">A.J.M. Baker, J. Proctor, M.M.J. van Balgooy, R.D. Reeves. ''Hyperaccumulation of nickel by the flora of the ultramafics of Palawan, Republic of the Philippines.'' Pp 291–304 in Baker AJM, Proctor J, Reeves RD (eds) The Vegetation of Ultramafic (Serpentine) Soils. GB-Andover: Intercept (1992)</ref>. Some plants may thus have a mechanism by which metal or other contaminants is excluded from their reproductive structures.
=== Commensalism ===
This is an interaction benefiting one organism while being of neutral value to another. The most likely one with hyperaccumulators would be epiphytism. But this is most noticeable in humid habitats, whereas only a few detailed field studies of hyperaccumulators have been conducted in such habitats, and those studies (mostly to do with humid tropical forests on serpentine soils) pay little or no attention to that point (e.g., Proctor ''et al.'' 1989; Baker ''et al.'' 1992). Proctor ''et al.'' (1988) studied the tree ''Shorea tenuiramulosa'', which can accumulate up to 1000 mg Ni/kg [[dry weight]] in leaf material. They estimated covers of epiphytes on the boles of trees in Malaysia, but did not report values for individual species. Boyd ''et al.'' (1999) studied the occurrence of epiphytes on leaves of the Ni hyperaccumulating tropical shrub ''Psychotria douarrei'' (Beauvis.). Epiphyte load increased significantly with increasing leaf age, up to 62% for the oldest leaves. An epiphyte sample of leafy liverworts removed from ''P. douarrei'', was found to contain 400 mg Ni /kg dry weight (far less than the host plant, whose oldest and most heavily epiphytized leaves contained a mean value of 32,000 mg Ni/kg dry weight). High doses of Ni therefore do not prevent colonization of ''Psychotria douarrei'' by epiphytes.
Chemicals that mediate host-epiphyte interactions are most likely to be located in the outermost tissues of the host (Gustafsson & Eriksson 1995). Also, most of the metal accumulates in epidermal or subepidermal cell walls or vacuoles (Ernst & Weinert 1972; Vazquez ''et al.'' 1994; Mesjasz- Rzybylowicz ''et al.'' 1996; Gabrielli ''et al.'' 1997). These findings suggest that epiphytes would experience higher metal levels when growing on hyperaccumulator leaves. But Severne (1974) measured the release of metal via leaching of leaves from the Ni hyperaccumulator ''Hybanthus floribundus'' (Lindl.) F. Muell. (''[[Violaceae]]'') from western Australia; he concluded that its leaves do not easily leach Ni.
In theory another commensal interaction could exist, if the high metal content of the soil under hyperaccumulator plants was needed for another plant species to establish itself. No evidence is known showing such effect.
=== The biofilm ===
This section needs be developed. See relevant articles on [[biofilm]] and ''[[Pseudomonas aeruginosa]]''.
A biofilm is a layer of organic matter and microorganism formed by the attachment and proliferation of bacteria on the surface of the object. [[Biofilm]]s are characterised by the presence of bacterial extracellular polymers [[glycocalyx]] that create a thin visible slimy layer on solid surface.
==Table of hyperaccumulators ==
A comprehensive literature survey of [[Phytoremediation, Hyperaccumulators|hyperaccumulating plants]] and their uses was started by Stevie Famulari for her students at the University of New Mexico. It is now considerably increased in size and has had to be split into 3 sections:
*[[Phytoremediation, Hyperaccumulators|Hyperaccumulators table – 1 : Al, Ag, As, Be, Cr, Cu, Mn, Hg, Mo, Naphtalene, Pb, Pd, Pt, Se, Zn]]
*[[Hyperaccumulators table – 2 : Nickel]]
*[[Hyperaccumulators table – 3|Hyperaccumulators table – 3 : Radionuclides (Cd, Cs, Co, Pu, Ra, Sr, U), Hydrocarbures, Organic Solvents.]]
== References ==
<references/>
==See also==
* [[Biodegradation]]
* [[Bioremediation]]
* [[John Todd (biologist)|John Todd]] - cofounder of the [http://www.vsb.cape.com/~nature/greencenter/newalchemy.html New Alchemy Institute]
==External links==
*[http://arabidopsis.info/students/dom/mainpage.html What is Phytoremediation?] A good overview.
*[http://www.aehs.com/journals/phytoremediation/ International Journal of Phytoremediation] - devoted to the publication of current laboratory and field research describing the use of plant systems to remediate contaminated environments.
*[http://www.ars.usda.gov/is/AR/archive/jun00/soil0600.htm Using Plants To Clean Up Soils] - from [http://www.ars.usda.gov/is/AR/ Agricultural Research magazine]
*[http://www.mobot.org/jwcross/phytoremediation/ Phytoremediation website hosted by the Missouri Botanical Garden] - Review Articles, Conferences, Phytoremediation Links, Research Sponsors, Books and Journals, and Recent Research.
==Bibliography==
* [http://www.mobot.org/jwcross/phytoremediation/ “Phytoremediation Website” - Includes reviews, conference announcements, lists of companies doing phytoremediation, and bibliographies.]
* [http://clu-in.org/download/studentpapers/henry.pdf “An Overview of Phytoremediation of Lead and Mercury” June 6 2000. The Hazardous Waste Clean-Up Information Web Site.]
* [http://www.epa.gov/superfund/programs/aml/tech/news/penhance.htm “Enhanced phytoextraction of arsenic from contaminated soil using sunflower” September 22 2004. U.S. Environmental Protection Agency.]
* [http://www.findarticles.com/p/articles/mi_m0820/is_n214/ai_16930491 “Getting the lead out”, June 1995. Vegetarian Times.]
* [http://www.bnl.gov/erd/Peconic/Factsheet/Phytoextract.pdf “Phytoextraction”, February 2000. Brookhaven National Laboratory 2000.]
* [http://www.engg.k-state.edu/HSRC/JHSR/vol2no5.pdf “Phytoextraction of Metals from Contaminated Soil” April 18, 2001. M.M. Lasat]
* [http://64.233.161.104/search?q=cache:PTxNObb-0X4J:www.bren.ucsb.edu/academics/courses/217/Lectures/Phytoremediation.ppt+advantages+of+phytoextraction&hl=en“Phytoremediation” July 2002. Donald Bren School of Environment Science & Management.]
* [http://www.ce.udel.edu/~sdkim/phytoremediation.htm “Phytoremediation” October 1997. Department of Civil Environmental Engineering.]
* [http://www.envirotools.org/factsheets/phytoremediation.shtml “Phytoremediation” June 2001, Todd Zynda.]
* [http://dcc2.bumc.bu.edu/OTLT/EH807/powerpoints/lead/Slides%20Presentation.htm “Phytoremediation of Lead in Residential Soils in Dorchester, MA” May, 2002. Amy Donovan Palmer, Boston Public Health Commission.]
* [http://www.eba-nys.org/eba/971q/7phytote.html “Technology Profile: Phytoextraction” 1997. Environmental Business Association.]
* [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9625697&dopt=Abstract “The role of EDTA in lead transport and accumulation by Indian mustard” June 1998.]
[[Category:Botany]]
[[Category:Pollution control technologies]]
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[[fr:Phytoremédiation]]
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[[ja:ファイトレメディエーション]]
[[pl:Fitoremediacja]]
[[ru:Фиторемедиация]]