phytoremediation
Phytoremediation technologies use living plants to clean up soil, air and water contaminated with hazardous contaminants. It is defined as "the use of green plants and the associated microorganisms, along with proper soil amendments and agronomi ...
in which plants remove dangerous elements or compounds from soil or water, most usually
heavy metals
upright=1.2, Crystals of osmium, a heavy metal nearly twice as dense as lead">lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead
Heavy metals are generally defined as ...
, metals that have a high density and may be toxic to organisms even at relatively low concentrations. The heavy metals that plants extract are toxic to the plants as well, and the plants used for phytoextraction are known
hyperaccumulator A hyperaccumulator is a plant capable of growing in soil or water with very high concentrations of metals, absorbing these metals through their roots, and concentrating extremely high levels of metals in their tissues. The metals are concentrated a ...
s that sequester extremely large amounts of heavy metals in their tissues. Phytoextraction can also be performed by plants that uptake lower levels of pollutants, but due to their high growth rate and biomass production, may remove a considerable amount of contaminants from the soil.
Heavy metals and biological systems
Heavy metals can be a major problem for any biological organism as they may be reactive with a number of chemicals essential to biological processes.
They can also break apart other molecules into even more reactive species (such as
reactive oxygen species
In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen.
The reduction of molecular oxygen () p ...
), which also disrupt biological processes. These reactions deplete the concentration of important molecules and also produce dangerously reactive molecules such as the radicals O. and OH..
Non-hyperaccumulators also absorb some concentration of heavy metals, as many heavy metals are chemically similar to other metals that are essential to the plants' life.
The process
For a plant to extract a heavy metal from water or soil, five things need to happen.
# The metal must dissolve in something the plant roots can absorb.
# The plant roots must absorb the heavy metal.
# The plant must chelate the metal to both protect itself and make the metal more mobile (this can also happen before the metal is absorbed).
Chelation
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a Denticity, polydentate (multiple bonded) ligand and a single central metal atom. These l ...
is a process by which a metal is surrounded and chemically bonded to an organic compound.
# The plant moves the chelated metal to a place to safely store it.
# Finally, the plant must adapt to any damage the metals cause during transportation and storage.
Dissolution
In their normal states, metals cannot be taken into any organism. They must be dissolved as an ion in solution to be mobile in an organism. Once the metal is mobile, it can either be directly transported over the root
cell wall
A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
by a specific metal transporter or carried over by a specific agent. The plant roots mediate this process by secreting things that will capture the metal in the
rhizosphere
The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Soil pores in the rhizosphere can contain many bacteria and other microor ...
and then transport the metal over the cell wall. Some examples are: phytosiderophores,
organic acids
An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are rel ...
, or
carboxylates
In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an ion with negative charge.
Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,...; ''carboxylat ...
If the metal is chelated at this point, then the plant does not need to chelate it later and the chelater serves as a case to conceal the metal from the rest of the plant. This is a way for a hyper-accumulator to protect itself from the toxic effects of poisonous metals.
Root absorption
The first thing that happens when a metal is absorbed is it binds to the root cell wall. The metal is then transported into the root. Some plants then store the metal through chelation or sequestration. Many specific transition metal ligands contributing to metal detoxification and transport are up-regulated in plants when metals are available in the rhizosphere. At this point the metal can be alone or already sequestered by a chelating agent or other compound. To get to the
xylem
Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
, the metal must then pass through the root symplasm.
Root-to-shoot transport
The systems that transport and store heavy metals are the most critical systems in a hyper-accumulator because the heavy metals will damage the plant before they are stored.
The root-to-shoot transport of heavy metals is strongly regulated by gene expression. The genes that code for metal transport systems in plants have been identified. These genes are expressed in both hyper-accumulating and non-hyper-accumulating plants. There is a large body of evidence that genes known to code for the transport systems of heavy metals are constantly over-expressed in hyper-accumulating plants when they are exposed to heavy metals.Rascio, N., and F. Navari-Izzo. "Heavy Metal Hyper-accumulating Plants: How and Why do they do it? and what Makes them so Interesting?" Plant Science 180.2 (2011): 169-81. SCOPUS. Web. 16 October 2011. This genetic evidence suggests that hyper-accumulators overdevelop their metal transport systems. This may be to speed up the root-to-shoot process limiting the amount of time the metal is exposed to the plant systems before it is stored. Cadmium accumulation has been reviewed.
These transporters are known as heavy metal transporting ATPases (HMAs).
One of the most well-documented HMAs is HMA4, which belongs to the Zn/Co/Cd/Pb HMA subclass and is localized at xylem parenchyma plasma membranes. HMA4 is upregulated when plants are exposed to high levels of Cd and Zn, but it is downregulated in its non-hyperaccumulating relatives. Also, when the expression of HMA4 is increased there is a correlated increase in the expression of genes belonging to the ZIP (Zinc regulated transporter Iron regulated transporter Proteins) family. This suggests that the root-to-shoot transport system acts as a driving force of the hyper-accumulation by creating a metal deficiency response in roots.
Storage
Systems that transport and store heavy metals are the most critical systems in a hyper-accumulator, because heavy metals damage the plant before they are stored. Often in hyperaccumulators the heavy metals are stored in the leaves.
How phytoextraction can be useful
For plants
There are several theories to explain why it would be beneficial for a plant to do this. For example, the "elemental defence" hypothesis assumes that maybe predators will avoid eating hyperaccumulators because of the heavy metals. So far, scientists have not been able to determine a correlation.
In 2002 a study was done by the Department of Pharmacology at Bangabandhu Sheikh Mujib Medical University in Bangladesh that used
water hyacinth
''Pontederia crassipes'' (formerly ''Eichhornia crassipes''), commonly known as common water hyacinth is an aquatic plant native to South America, naturalized throughout the world, and often invasive outside its native range. This study proved that water could be completely purified of arsenic in a few hours and that the plant then could be used as animal feed, fire wood and many other practical purposes. Since water hyacinth is invasive, it is inexpensive to grow and extremely practical for this purpose.
See also
*
Biomining
Biomining is the technique of extracting metals from ores and other solid materials typically using prokaryotes, fungi or plants (phytoextraction also known as phytomining or biomining). These organisms secrete different organic compounds that ...