Acidification, heavy metal mobility and nutrient accumulation in the soil–plant system of a revegetated acid mine wasteland

Sheng-Xiang Yang, Bin Liao, Jin-tian Li, Tao Guo, Wen-Sheng Shu *

School of Life Sciences and State Key Laboratory of Biocontrol, Sun Yat-sen University, Guangzhou 510275, PR China

a b s t r a c t

A revegetation program was established at an extreme acidic and metal-toxic pyrite/copper mine wasteland in Guangdong Province, PR China using a combination of four native grass species and one nonnative woody species. It was continued and monitored for 2 y. The emphasis was on acidification, metal mobility and nutrient accumulation in the soil–plant system. Our results showed the following: (i) the acid-forming potential of the mine soils decreased steadily with time, which might be due to plant root-induced changes inhibiting the oxidization of sulphide minerals; (ii) heavy metal extractability (diethylene-triamine-pentaacetic acid-extractable Pb and Zn) in the soils increased with time despite an increase in soil pH, which might be attributed to soil disturbance and plant rhizospheric processes, as well as a consequence of the enhanced metal accumulation in plants over time; and (iii) the vegetation cover increased rapidly with time, and plant development accelerated the accumulation of major nutrients (organic matter, total and ammonium-N, and available P and K). The 2-y field experiment demonstrates that direct seeding/planting of native plant species in combination with lime and manure amelioration is a practical approach to the initial establishment of a self-sustaining vegetation cover on this metalliferous and sulphide-bearing mine wasteland. However, heavy metal accumulation in the soil–plant system should be of great concern, and long-term monitoring of ecological risk must be an integral part of such a restoration scheme.

Get The Full Acces Here

Genetics and Phytoremediation Strategies

Genetic Engineering Possibilities

To determine possible mechanisms for genetically engineering a plant to increase its remediation ability, it is best to begin with the mechanism for arsenic uptake and storage. It is believed that arsenate is taken up from the soil by the phosphate transporter. This has been suggested by exclusion experiments in which inorganic arsenate competed with inorganic phosphate for transport by a phosphate transporter. (Csanaky and Gregus 2001) It is believed that the high affinity phosphate uptake system is involved, because it has a lower selectivity than the low affinity system, and suppression of the high affinity system results in smaller arsenic uptake and accumulation. (Fitz and Wenzel 2002) One mechanism that could be useful would be to overexpress the phosphate ion transporter gene in plant roots. This would allow for more arsenic uptake by the plant, and could therefore lead to greater arsenic sensitivity, due to increased concentration of arsenate in the plant, and accumulation.

Once inside the plant, the arsenate travels with the transpiration flow through the xylem and into the stems and leaves. (Tu et.al 2002) Here it is reduced, possibly by an arsenate reductase enzyme (ArsC) into its more toxic form, arsenite. (Dhankher et.al 2002) Another possible mechanism would be to overexpress the arsenate reductase gene in plant shoots. Studies have coupled the arsenate reductase gene to a soybean ribulose bisphosphate carboxylase small-subunit gene promoter, which will give it strong light-induced expression, insuring that the arsenate is reduced in the shoots, not roots. (Dhankher et.al 2002) Since arsenite is more toxic than arsenate, all the transgenic plants were more sensitive to arsenic in the growing medium. (Dhankher et.al 2002)

Arsenite has a great affinity for thiol groups and consequently binds to peptide-thiol molecules, also known as phytochelatins. (Dhankher et.al 2002) Phytochelatin production appears to be activated by the presence of heavy metals and arsenic. (Sauge-Merle et.al 2003) One enzyme critical in the pathway for the formation of these peptide-thiol molecules is y-glutamylcysteine synthetase (y-ECS). (Dhankher et.al 2002) This enzyme produces the first, and possibly limiting enzyme in the phytochelatin producing pathway. (Dhankher et.al 2002) Studies have coupled this gene to a constitutively expressed actin promoter, giving a constant high level of phytochelatins in the plant and providing a sink for reduced arsenite within the cells. (Dhankher et.al 2002) Plants transformed with both ArsC and y-ECS produced four times more biomass than the control when grown four weeks on a 200 micromolar arsenate medium, showing greater tolerance as well as accumulation. (Dhankher et.al 2002)

This complex is then transported via a specific transport mechanism into a vacuole or into the cell wall. (Pickering et.al 2000) Another possible genetic approach would be to overexpress the gene for the transport protein into the vacuole or cell wall. This would remove the arsenite more quickly from the cells, to reduce the toxicity to the cell, and allow for more arsenic to be taken up.

Arsenic has been shown to have deleterious effects on seed germination, as well as root length and mass, which can detract from the effectiveness of a phytoremediation treatment. (Nie et.al 2002) Some bacteria, such as Enterobacter cloacae promote plant growth and minimize arsenic stress on the roots. (Nie et.al 2002) The Enterobacter cloacae UW4 1-aminocyclopropane-1-carboxylate deaminase gene in plants has been shown to increase the interaction between plants and these growth promoting bacteria. (Nie et.al 2002) In addition, transformed plants accumulated four times as much arsenic as non-transformed plants. (Nie et.al 2002) Overexpression of this gene is another possible mechanism to increase plant growth and phytoremediation of arsenic-contaminated soils.

A combination of these transformations and others could create a plant that was capable of high levels of arsenic accumulation and tolerance, when used to transform a species with high biomass that is adapted to living in the environment in which the arsenic-contaminated site exists. A tree such as cottonwood would be ideal for this purpose, and researchers are concentrating their efforts on performing this transformation. (www.edie.net) Further studies must be performed on the unique hyperaccumulating species, such as Pteris vittata and Pityrogramma calomelanos, to determine if they possess a novel mechanism or just an abnormally high expression of a given enzyme in the accumulation pathway. These hyperaccumulators or the transgenic accumulators can then be used with various phytoremediation strategies.

Phytoremediation Strategies

Phytostabilization is a possible strategy for control of arsenic, where arsenic tolerant plants, such as Spergularia grandis and Agrostis catellana are used to keep Arsenic contaminated soil in place to prevent erosion and leaching of the contaminant. (Fitz and Wenzel 2002)

Phytovolatilization is the use of plants to turn a pollutant into a gas to remove it from a site. Although this does occur to some extent in nature, volatile arsenic is still toxic, so phytovolatilization is not really a feasible option. (Fitz and Wenzel 2002)

One positive phytoremediation option is rhizofiltration, the removal of pollutants from water using a hydroponic system. Arsenic contaminated runoff could be run through a set-up with Lepidium sativum, which has been shown to remove arsenic from anaerobic flooded environments. (Robinson et.al 2003) This can also be used in conjunction with bioleaching, which would produce a heavily arsenic-contaminated runoff from soil with initially low arsenic bioavailability. (Seidel et.al 2002)

One positive phytoremediation option is rhizofiltration, the removal of pollutants from water using a hydroponic system. Arsenic contaminated runoff could be run through a set-up with Lepidium sativum, which has been shown to remove arsenic from anaerobic flooded environments. (Robinson et.al 2003) This can also be used in conjunction with bioleaching, which would produce a heavily arsenic-contaminated runoff from soil with initially low arsenic bioavailability. (Seidel et.al 2002)

Source

Phytoremediation: Cleaning Up With Plants

Have you ever heard of locoweed? As a child, I watched a lot of Bgrade movie Westerns and one thing I learned was that, if you are a rancher, you don’t want your cattle grazing on locoweed! Locoweed is the common name given to several species of Astragalus, a genus in the Fabaceae or pea family. Also known as milk vetch or poison vetch, many species of Astragalus take up unusually large quantities of selenium from the alkaline soils of the western plains.

The high selenium content contributes to a disease known as alkali poisoning or “blind staggers” in cattle unfortunate enough to graze on this plant—the cattle literally behave as though they are crazy. There are many regions where natural geochemical processes have produced soils that are rich in metals such as nickel, chromium, gold, cadmium, selenium, and arsenic. Normally high levels of heavy metals would be toxic to plants, just as they are to humans, yet many plants actually thrive on soils rich in such metals. For some plants, the metals are not a problem simply because the cell membranes surrounding the root cells prevent the metals from entering the root. Other plants actually take up the metals and accumulate them to levels that would be toxic to most other plants.

In Astragalus, for example, selenium may account for as much as 10% of the dry weight of the seeds. In soils that are rich in nickel, some plants may contain 200,000 times more nickel than plants growing in normal soils. Many years ago, such plants were known as “indicator species,” and prospectors would take the presence of such plants as an early indication that the soils may have contained a mineral of interest, such as gold. This was called phytoprospecting. We now call these plants accumulator species, which are not injured by high concentrations of heavy metals because they sequester (isolate) the metals with small proteins called phytochelatins.

The sequestered metals are then stored in the large central vacuole of the plant cell, where they cannot interfere with the cell’s metabolism. There has recently been a renewed interest in accumulator species because these plants may have the potential to assist in cleaning up soils contaminated with heavy metals as a result of twentieth-century industrial activities. Using plants to clean up soils is called phytoremediation (phyto meaning “plant” and remediation meaning “to correct a fault”).

The idea is to grow accumulator species on mine tailings and wastes from paper mills, for example, where they would extract the heavy metals. Plants will naturally take longer to do the job, but plants are much more cost-effective and would not create even more ecological problems as engineering-based technologies often do.

Phytoremediation would also help to stabilize contaminated sites because the plants help to control erosion. An additional benefit of accumulator species is that they begin the revegetation of barren industrial sites and assist in the recovery of useful metals. Phytomining, as it is called, has proven effective in the recovery of both nickel and thallium in demonstrations. Inother trials, various species of willows (Salix) have shown promise for extraction of heavy metals from soils treated with sewage sludge. The advantage of using plants is that they can be harvested and burned. The heavy metals remain concentrated in the ash, which makes their disposal much easier.

Heavy metal hyperaccumulating plants: How and why do they do it? And what makes them so interesting?

Nicoletta Rascio^{a, ,} and Flavia Navari-Izzo^b

^a Department of Biology, University of Padova, via U. Bassi 58/B, I-35121 Padova, Italy

^b Department of Chemistry and Agricultural Biotechnologies, University of Pisa, via San Michele degli Scalzi 2, I-56124 Pisa, Italy

Received 26 May 2010;

revised 25 August 2010;

accepted 26 August 2010.

Available online 15 September 2010.

Abstract

The term “hyperaccumulator” describes a number of plants that belong to distantly related families, but share the ability to grow on metalliferous soils and to accumulate extraordinarily high amounts of heavy metals in the aerial organs, far in excess of the levels found in the majority of species, without suffering phytotoxic effects. Three basic hallmarks distinguish hyperaccumulators from related non-hyperaccumulating taxa: a strongly enhanced rate of heavy metal uptake, a faster root-to-shoot translocation and a greater ability to detoxify and sequester heavy metals in leaves. An interesting breakthrough that has emerged from comparative physiological and molecular analyses of hyperaccumulators and related non-hyperaccumulators is that most key steps of hyperaccumulation rely on different regulation and expression of genes found in both kinds of plants. In particular, a determinant role in driving the uptake, translocation to leaves and, finally, sequestration in vacuoles or cell walls of great amounts of heavy metals, is played in hyperaccumulators by constitutive overexpression of genes encoding transmembrane transporters, such as members of ZIP, HMA, MATE, YSL and MTP families. Among the hypotheses proposed to explain the function of hyperaccumulation, most evidence has supported the “elemental defence” hypothesis, which states that plants hyperaccumulate heavy metals as a defence mechanism against natural enemies, such as herbivores. According to the more recent hypothesis of “joint effects”, heavy metals can operate in concert with organic defensive compounds leading to enhanced plant defence overall.

Heavy metal contaminated soils pose an increasing problem to human and animal health. Using plants that hyperaccumulate specific metals in cleanup efforts appeared over the last 20 years. Metal accumulating species can be used for phytoremediation (removal of contaminant from soils) or phytomining (growing plants to harvest the metals). In addition, as many of the metals that can be hyperaccumulated are also essential nutrients, food fortification and phytoremediation might be considered two sides of the same coin. An overview of literature discussing the phytoremediation capacity of hyperaccumulators to clean up soils contaminated with heavy metals and the possibility of using these plants in phytomining is presented.

Get Fulltext Here